Project Report on Electronic Stethoscope

CHAPTER 1:INTRODUCTION 1.1 History:
Assessing the sounds of the human body was reported in the ancient medical literature. Amongst the earliest known medical manuscripts are the medical papyruses of ancient Egypt dating to the seventeenth century B.C., which referred to audible signs of disease within the body. Hippocrates, the Father of Medicine, advocated for the search of philosophical and practical instruments to improve medicine in 350 B.C. He discussed a procedure for shaking a patient by the shoulders and listening for sounds evoked by the chest. Hippocrates also used the method of applying the ear directly to the chest and found it useful in order to detect the accumulation of fluid within the chest. The French physician Jean-Nicolas Corvisart, who is considered the founder of French clinical medicine, was accustomed to placing his ear over the cardiac region of the chest to listen to the heart. Bayle and Double, who like Laennec were students of Corvisart, used the unaided ear to listen to the heart of their patients. Nevertheless, the evolution from listening with the unaided ear (immediate auscultation) to the aided ear (mediate auscultation) awaited Laennec's invention of the stethoscope in 1816.

Fig 1.1: An engraving of a physician examining a patient by "immediate" ausculatation, in which the doctor placed his ear on the chest of the patient to hear the sounds made by the lungs during breathing. The print shows a group of physicians, medical students and nurses observing the physician performing his exam.

The stethoscope was invented in 1816 when a young French physician named Rene Theophile Hyacinthe Laennec was examining a young female patient. Laennec was embarrassed to place his ear to her chest (Immediate Auscultation), which was the method of auscultation used by physicians at that time. He remembered a trick he learned as a child that sound travels through solids and thus he rolled up 24 sheets of paper, placed one end to his ear and the other end to the woman's chest. He was delighted to discover that the sounds were not only conveyed through the paper cone, but they were also loud and clear. The first recorded manuscript documenting auscultation using the stethoscope 1

(Mediate Auscultation) was in March 8, 1817, when Laennec noted examining a Marie-Melanie Basset, who was 40 years old. Laennec preferred to have his instrument simply called "Le Cylindre," as he thought naming such a fundamental instrument was unnecessary. He became remorse at the names it was being given by his colleagues and decided that if it should be called anything, it should be "Stethoscope," which is derived from the Greek words for 'I see' and 'the chest.' He created a stethoscope from a turned piece of wood with hollow bore in the center. It was made of two pieces. One end had a hole to place against the ear and the other end was hollowed out into a funnel shaped cone. There was a plug that fit into this cone which had a hollow brass tube placed inside it. This plug was put in the funnel shaped end of the stethoscope to listen to the heart, and removed to examine the lungs. Laennec published his classic treatise on mediate auscultation in 1819 in which he discussed mediate auscultation and illustrated the design of the stethoscope. The stethoscope was described as being 12 inches long and 1.5 inches in diameter with a 3/8 inch central bore hole throughout its length.

Fig 1.2: Original version of the Laennec stethoscope made of a turned dense, finely grained, light colored wood, circa 1819.

1.2 About the project:
The project we have undertaken aims to design and simulate an electronic stethoscope which will not only provide us with a better signal but can also be interfaced with computers and other display devices so that it can be further analyzed and stored for future uses. We have made this design comprising of very simple and well known components like microphone, operational amplifier, low pass filter , high pass filter and analog to digital converter and some light emitting diodes. The project is simulated on multisim software. Our project is really cost effective as it doesn’t involves any costly components and this design is undoubtedly better than other acoustic and electronic stethoscopes as it provides with better noise cancellation and provides a good form factor for the signal.

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CHAPTER 2: THEORY:

2.1 Definition of stethoscope:
The stethoscope is an acoustic medical device for auscultation, or listening to the internal sounds of a human body. It is often used to listen to lung and heart sounds. It is also used to listen to intestines and blood flow in arteries and veins. In combination with sphygmomanometer, it is commonly used for measurements of blood pressure.

2.2 Anatomy of stethoscope:
Fig2.1Acoustic Stethoscope

Fig 2.2 Anatomy of an Acoustic Stethoscope 1. Headset :
The headset is the metal part of the stethoscope onto which the tubing is fitted. The headset is made up of the two ear tubes, tension springs, and the ear tips. The wearer can adjust the tension to a comfortable level by pulling the ear tubes apart to loosen the headset or crossing them over to tighten.

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2. Ear tip :
Soft-sealing ear tips offer increased comfort, seal and durability, and feature a surface treatment that increases surface lubricity and reduces lint and dust adhesion.

3. Ear tube :
The ear tube is the part to which the ear tips are attached.

4. Tunable Diaphragm :
A traditional stethoscope consists of a bell and a diaphragm. The bell is used with light skin contact to hear low frequency sounds and the diaphragm is used with firm skin contact to hear high frequency sounds.

5. Stem
The stem connects the stethoscope tubing to the chest piece.

6. Tubing
The tubing consists of two openings. The tubing on all Littmann stethoscopes is manufactured from polyvinyl chloride (PVC). The tubing does not contain either natural rubber latex or dry natural rubber.

7. Chest piece
The chest piece is the part of the stethoscope that is placed on the location where the user wants to hear sound.

2.3 Electronic stethoscope:
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An electronic stethoscope overcomes the low sound levels by electronically amplifying body sounds. However, amplification of stethoscope contact artifacts, and component cutoffs (frequency response thresholds of electronic stethoscope microphones, pre-amps, amps, and speakers) limit electronically amplified stethoscopes' overall utility by amplifying mid-range sounds, while simultaneously attenuating high- and low- frequency range sounds. Currently, a number of companies offer electronic stethoscopes. Electronic stethoscopes require conversion of acoustic sound waves to electrical signals which can then be amplified and processed for optimal listening. Unlike acoustic stethoscopes, which are all based on the same physics, transducers in electronic stethoscopes vary widely. The simplest and least effective method of sound detection is achieved by placing a microphone in the chestpiece. Because the sounds are transmitted electronically, an electronic stethoscope can be a wireless device, can be a recording device, and can provide noise reduction, signal enhancement, and both visual and audio output.

Fig 2.3 Electronic Stethoscope

2.4 The Heart:

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The heart is the pump station of the body and is responsible for circulating blood throughout the body. It is about the size of our clenched fist and sits in the chest cavity between two lungs. Its walls are made up of muscle that can squeeze or pump blood out every time the organ "beats" or contracts. Fresh, oxygen-rich air is brought to the lungs through the trachea or windpipe every time we take a breath. The lungs are responsible for delivering oxygen to the blood, and the heart circulates the blood to the lungs and different parts of the body.

2.5 Functioning of the Heart:
The heart is divided into four chambers made up of a right and a left unit, separated from each other by a partition wall known as a septum. Each chamber is subdivided into an upper and a lower chamber. The upper chamber is known as an atrium while the lower chamber is referred to as a ventricle. The right atrium (RA) sits on top of the right ventricle (RV) on the right side of the heart while the left atrium (LA) sits atop the left ventricle (LV) on the left. The right side of the heart is responsible for sending blood to the lungs, where the red blood cells pick up fresh oxygen. This oxygenated blood is then returned to the left side of the heart. From here the oxygenated blood is transported to the whole body supplying the fuel that the body cells need to function. The blood cells of the body extract or remove oxygen from the blood. The oxygen-poor blood is returned to the right atrium, where the journey begins. This round trip is known as the circulation of blood.

Fig 2.4 functioning of the heart The figure shown above is a section of the heart, as viewed from the front. It demonstrates the four chambers. It is also observed that there is an opening between the right atrium (RA) and the right ventricle (RV). This is actually a valve known as the TRICUSPID valve. It has three flexible thin

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parts, known as leaflets, that open and shut. The figure below shows the mitral and tricuspid valves, as seen from above, in the open and shut position.

Fig 2.5 Heart Valves When shut, the edge of the three leaflets touch each other to close the opening and prevent blood from leaving the RV and going back into the RA. Thus, the tricuspid valve serves as a trapdoor valve that allows blood to move only in one direction - from RA to RV. Similarly, the MITRAL valve allows blood to flow only from the left atrium to the left ventricle. Unlike the tricuspid valve, the mitral valve has only two leaflets. In the top diagram, we also notice thin thread like structures attached to the edges of the mitral and tricuspid valves. These chords or strings are known as chordae tendineae.They connects the edges of the tricuspid and mitral valves to muscle bands or papillary muscles. The papillary muscles shorten and lengthen during different phases of the cardiac cycle and keep the valve leaflets from flopping back into the atrium. The chords are designed to control the movement of the valve leaflets similar to ropes attached to the sail of a boat. Like ropes, they allow the sail to bulge outwards in the direction of a wind but prevent them from helplessly flapping in the breeze. When the three leaflets of the tricuspid bulge upwards during contraction or emptying of the ventricles, their edges touch each other and close off backward flow to the right atrium. This important feature allows blood to flow through the heart in only ONE direction, and prevents it from leaking backwards when the valve is shut. The two leaflets of the mitral valve functions in a similar manner and allows flow of blood from the left atrium to the left ventricle, but closes and cuts off backward leakage into the left atrium when the left ventricle contracts and starts to empty.

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2.6 Electrical conduction system of the Heart:
The normal electrical conduction of the heart allows electrical propagation to be transmitted from the sinoatrial node through both atria and forward to the atrioventricular node. Normal/baseline physiology allows further propagation from the AV node to the Purkinje Fibers and bundle branches. Both the SA and AV nodes stimulate the myocardium. It is the time ordered stimulation of the myocardium that allows efficient contraction of the heart, thereby allowing selective blood perfusion throughout the body.

Electrical conduction system of the heart

2.7 Conduction pathway:
Signals arising in the SA node (and propagating to the left atrium) stimulates the atria to contract. In parallel, action potentials travel to the AV node via internodal pathways. After a delay, the stimulus is conducted through the bundle of His to the bundle branches and then to the purkinje fibers and the endocardium at the apex of the heart, then finally to the ventricular myocardium. Microscopically, the wave of depolarization propagates to adjacent cells via gap junctions located on the intercalated disk. The heart is a functional syncytium. In a functional syncytium, electrical impulses propagate freely between cells in every direction, so that the myocardium functions as a single contractile unit. This property allows rapid, synchronous depolarization of the myocardium. While normally advantageous, this property can be detrimental as it potentially allows the propagation of incorrect electrical signals. These gap junctions can close to isolate damaged or dying tissue, as in a myocardial infarction. Fig 2.7 Heart; conduction system Fig 2.6 Isolated conduction system of the heart

2.8 Electrocardiogram (ECG or EKG):
An EKG is an important part of the initial evaluation of a patient who is suspected to have a heart related problem. Small sticky electrodes are applied to the patient's chest, arms and legs. The electrical activity created by the patient's heart is processed by the EKG machine and then printed on a special graph paper. This is then interpreted by the physician. The EKG can provide important 8

information about the patient's heart rhythm, a previous heart attack, increased thickness of heart muscle, and signs of decreased oxygen delivery to the heart, and problems with conduction of the electrical current from one portion of the heart to another.

Fig 2.8: An example of an EKG of a patient with a heart attack

2.9 Waveform: SA node: P wave
Under normal conditions, electrical activity is spontaneously generated by the SA node, the physiological pacemaker. This electrical impulse is propagated throughout the right atrium, and through Bachmann's bundle to the left atrium, stimulating the myocardium of both atria to contract. The conduction of the electrical impulse throughout the left and right atria is seen on the ECG as the P wave. As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts, from the SA node to the AV node.

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Fig 2.9 The ECG complex. P wave, PR=PR interval, P=QRS=QRS complex, QT=QT interval, ST=ST segment, T=T wave

AV node: PR interval
The AV node functions as a critical delay in the conduction system. Without this delay, the atria and ventricles would contract at the same time, and blood wouldn't flow effectively from the atria to the ventricles. The delay in the AV node forms much of the PR segment on the ECG. Part of atrial repolarization can be represented by PR segment. The distal portion of the AV node is known as the bundle of His.

Purkinje fibers/ventricular myocardium: QRS complex
The two bundle branches taper out to produce numerous purkinje fibers, which stimulate individual groups of myocardial cells to contract. The spread of electrical activity (depolarization) through the ventricular myocardium produces the QRS complex on the ECG.

Ventricular repolarization: T wave
The last event of the cycle is the repolarization of the ventricles. The transthoracically measured PQRS portion of an electrocardiogram is chiefly influenced by the sympathetic nervous system. The T (and occasionally U) waves are chiefly influenced by the parasympathetic nervous system guided by integrated brainstem control from the vagus nerve and the thoracic spinal accessory ganglia.

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2.10 Heart sounds:
The heart sounds are the noises (sound) generated by the beating heart and the resultant flow of blood through it. This is also called a heartbeat. In cardiac auscultation, an examiner uses a stethoscope to listen for these sounds, which provide important information about the condition of the heart. In healthy adults, there are two normal heart sounds often described as a lub and a dub (or dup), that occur in sequence with each heartbeat. These are the first heart sound (S1) and second heart sound (S2), produced by the closing of the AV valves and semilunar valves respectively. In addition to these normal sounds, a variety of other sounds may be present including heart murmurs, adventitious sounds, and gallop rhythms S3 and S4. Heart murmurs are generated by turbulent flow of blood, which may occur inside or outside the heart. Murmurs may be physiological (benign) or pathological (abnormal). Abnormal murmurs can be caused by stenosis restricting the opening of a heart valve, resulting in turbulence as blood flows through it. Abnormal murmurs may also occur with valvular insufficiency (or regurgitation), which allows backflow of blood when the incompetent valve closes with only partial effectiveness. Different murmurs are audible in different parts of the cardiac cycle, depending on the cause of the murmur.

Fig 2.10 Front of thorax, showing surface relations of bones, lungs (purple), Pleura (blue) and heart (red outline). Heart valves are labeled with “M”,”T”,”A” and “P” First heart sound: caused by AV valves - Mitral (M) and Tricuspid (T).

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2.10.1 First heart sound (S1):
The first heart tone, or S1, forms the "lubb" of "lubb-dub" or "lubb-dup" and is composed of components M1 and T1. Normally M1 precedes T1 slightly. It is caused by the sudden block of reverse blood flow due to closure of the atrioventricular valves, i.e. mitral and tricuspid, at the beginning of ventricular contraction, or systole. When the ventricles begin to contract, so do the papillary muscles in each ventricle. The papillary muscles are attached to the tricuspid and mitral valves via chordae tendineae, which bring the cusps of the valve closed (chordae tendineae also prevent the valves from blowing into the atria as ventricular pressure rises due to contraction). The closing of the inlet valves prevents regurgitation of blood from the ventricles back into the atria. The S1 sound results from reverberation within the blood associated with the sudden block of flow reversal by the valves. If T 1 occurs more than slightly after M1, then the patient likely has a dysfunction of conduction of the right side of the heart such as a Right bundle branch block.

2.10.2 Second heart sound (S2):
The second heart tone, or S2, forms the "dub" of "lubb-dub" or "lubb- dup" and is composed of components A2 and P2. Normally A2 precedes P2 especially during inspiration when a split of S2 can be heard. It is caused by the sudden block of reversing blood flow due to closure of the aortic valve and pulmonary valve at the end of ventricular systole, i.e. beginning of ventricular diastole. As the left ventricle empties, its pressure falls below the pressure in the aorta, aortic blood flow quickly reverses back toward the left ventricle, catching the aortic valve leaflets and is stopped by aortic (outlet) valve closure. Similarly, as the pressure in the right ventricle falls below the pressure in the pulmonary artery, the pulmonary (outlet) valve closes. The S2 sound results from reverberation within the blood associated with the sudden block of flow reversal.

2.11 Extra heart sounds:
The rarer extra heart sounds form gallop rhythms and are heard in both normal and abnormal situations.

2.11.1 Third heart sound (S3)
Rarely, there may be a third heart sound also called a protodiastolic gallop or ventricular gallop. It occurs at the beginning of diastole after S2 and is lower in pitch than S1 or S2 as it is not of valvular origin. The third heart sound is benign in youth and some trained athletes, but if it re-emerges later in life it may signal cardiac problems like a failing left ventricle as in dilated congestive heart failure (CHF). S3 is thought to be caused by the oscillation of blood back and forth between the walls of the ventricles initiated by inrushing blood from the atria. The reason the third heart sound does not occur until the middle third of diastole is probably because during the early part of diastole, the ventricles are not filled sufficiently to create enough tension for reverberation.. In other words, an S3 heart sound indicates increased volume of blood within the ventricle. An S3 heart sound is best heard with the bellside of the stethoscope (used for lower frequency sounds).

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2.11.2 Fourth heart sound (S4):
The rare fourth heart sound is sometimes audible in healthy children and again in trained athletes, but when audible in an adult is called a presystolic gallop or atrial gallop. This gallop is produced by the sound of blood being forced into a stiff/hypertrophic ventricle. It is a sign of a pathologic state, usually a failing left ventricle, but can also be heard in other conditions such as restrictive cardiomyopathy. The sound occurs just after atrial contraction ("atrial kick") at the end of diastole and immediately before S1, producing a rhythm sometimes referred to as the "Tennessee" gallop where S4 represents the "tenn-" syllable. It is best heard at the cardiac apex with the patient in the left lateral decubitus position and holding his breath. The combined presence of S3 and S4 is a quadruple gallop. At rapid heart rates, S3 and S4 may merge to produce a summation gallop.

2.12 Murmurs:
Heart murmurs are produced as a result of turbulent flow of blood, turbulence sufficient to produce audible noise. They are usually heard as a whooshing sound. The term murmur only refers to a sound believed to originate within blood flow through or near the heart; rapid blood velocity is necessary to produce a murmur. Yet most heart problems do not produce any murmur and most valve problems also do not produce an audible murmur.

Fig 2.11 Murmurs Produced by the Heart The following paragraphs overview the murmurs most commonly heard in adults who do not have major congenital heart abnormalities.

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







 

Regurgitation through the mitral valve is by far the most commonly heard murmur, sometimes fairly loud to a practiced ear, even though the volume of regurgitant blood flow may be quite small. Yet, though obvious using echocardiography visualization, probably about 20% of cases of mitral regurgitation does not produce an audible murmur. Stenosis of the aortic valve is typically the next most common heart murmur, a systolic ejection murmur. This is more common in older adults or in those individuals having a two, not a three leaflet aortic valve. Regurgitation through the aortic valve, if marked, is sometimes audible to a practiced ear with a high quality, especially electronically amplified, stethoscope. Generally, this is a very rarely heard murmur, even though aortic valve regurgitation is not so rare. Aortic regurgitation, though obvious using echocardiography visualization, usually does not produce an audible murmur. Stenosis of the mitral valve, if severe, also rarely produces an audible, low frequency soft rumbling murmur, best recognized by a practiced ear using a high quality, especially electronically amplified, stethoscope. Either regurgitation through, or stenosis of, the tricuspid or pulmonary valves essentially never produces audible murmurs. Other audible murmurs are associated with abnormal openings between the left ventricle and right heart or from the aortic or pulmonary arteries back into a lower pressure heart chamber.

Gradations of Murmurs Grade Grade 1 Grade 2 Grade 3 Grade 4 Grade 5 Grade 6

(Defined based on use of an acoustic, not a high-fidelity amplified electronic stethoscope Description Very faint, heard only after listener has "tuned in"; may not be heard in all positions. Quiet, but heard immediately after placing the stethoscope on the chest. Moderately loud. Loud, with palpable thrill (i.e., a tremor or vibration felt on palpation) Very loud, with thrill. May be heard when stethoscope is partly off the chest. Very loud, with thrill. May be heard with stethoscope entirely off the chest.

Table 2.1 Description of Murmurs

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CHAPTER 3: EXPERIMENTAL SETUP
The project encompasses the implementation and verification of a fully functional electronic based stethoscope that automatically identifies the four sounds of the heart and systolic and diastolic murmurs related to valvular insufficiency and stenosis. To start with, a rough classification of the first and second heart sound is needed. These are based on the fact that the origin of heart sounds and murmurs are different; heart sounds originates from the closing of the valves while the murmurs are generated from a turbulent blood flow through the valves in response to pathologies in the valve. Thus the murmurs are more chaotic than the heart sounds. Electronic stethoscopes require conversion of acoustic sound waves to electrical signals which can then be amplified and processed for optimal listening. Heart sounds, respiratory sounds and murmurs are heard loud and clear. The faint murmur or abnormalities that are often missed by the regular stethoscopes are easily detected with this stethoscope. It overcomes the low sound levels by electronically amplifying body sounds. In the project undertaken by us we have used a stethoscope head to acquire the heart signal and one end of an acoustically insulated tube is inserted. In this head the acquired signal is passed to transducer present in the tube at another end. This transducer converts the heart sound signal to electrical signal so that it can be interfaced to a computer. We will require an analog to digital converter (ADC) to change this analog signal into digital signal. But for carrying out this conversion, firstly the signal needs to be amplified and a level shifter is also required for level shifting this signal as the ADC operates with the positive part of the signal. The digital data is transferred to a computer through USB port. As the transducer will also pick up the surrounding noises a noise cancellation technique which gives 70% noise reduction is used. This design provides a low price electronic stethoscope. 3.1 Sequence of the project: The project will be accomplished in the following stages: Stage 1: Hardware Implementation. Stage 2: Software Implementation.

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CHAPTER 4 Hardware implementation:
Hardware implementation has been done on a solder less breadboard utilizing the following components which are described as follows.

4.1 Breadboard:
A breadboard is used to build and test circuits quickly before finalizing any circuit design. The breadboard has many holes into which circuit components like ICs and resistors can be inserted. A typical breadboard is shown below:

Fig 4.1 Breadboard The bread board has strips of metal which run underneath the board and connect the holes on the top of the board. The metal strips are laid out as shown below. Note that the top and bottom rows of holes are connected horizontally while the remaining holes are connected vertically. To use the bread board, the legs of components are placed in the holes. Each set of holes connected by a metal strip underneath forms a node. A node is a point in a circuit where two components are connected. Connections between different components are formed by putting their legs in a common node. The long top and bottom row of holes are usually used for power supply connections. The rest of the circuit is built by placing components and connecting them together with jumper wires. ICs are placed in the middle of the board so that half of the legs are on one side of the middle line and half on the other.

4.2 General description:
The basic circuit diagram implemented is as shown:

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Fig 4.2 General Block Diagram

Components U2, U3,U7 U1 C1,C3 C2,C4 R1,R8,R4.R6 R2,R7 R5,R3 R9

Total quantity 3 1 2 2 3 2 2 1
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Description (LM741CN) operational amplifier (0804) ADC (0.5µF) capacitor (0.47µF) capacitor (33Ωk ) resistor (56Ωk) resistor (160Ωk) resistor (47Ωk) resistor

R11 XLV1 XSC1 XLV4

1 1 1 1

(2.2Ωk) resistor Microphone CRO Signal Analyzer

Table 3.1 List of Components Used

4.3 Operational amplifier:
An operational amplifier, which is often called an op-amp, is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. An op-amp produces an output voltage that is typically millions of times larger than the voltage difference between its input terminals.

Fig 4.3 Various op-amp ICs in eight-pin dual in-line packages ("DIPs") Typically the op-amp's very large gain is controlled by negative feedback, which largely determines the magnitude of its output ("closed-loop") voltage gain in amplifier applications, or the transfer function required (in analog computers). Without negative feedback, and perhaps with positive feedback for regeneration, an op-amp essentially acts as a comparator. High input impedance at the input terminals (ideally infinite) and low output impedance at the output terminal(s) (ideally zero) are important typical characteristics. Op-amps are among the most widely used electronic devices today, being used in a vast array of consumer, industrial, and scientific devices.

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4.3.1 Circuit notation:
The circuit symbol for an op-amp is shown, where:      V+: non-inverting input V-: inverting input Vout: output Vs+: positive power supply Vs-: negative power supply Fig 4.4 Circuit diagram symbol for an op-amp

4.3.2 Operation:
Fig 4.5 Op-Amp as a Switch The amplifier's differential inputs consist of a input and a input, and ideally the op-amp amplifies only the difference in voltage between the two, which is called the differential input voltage. The output voltage of the op-amp is given by the equation, With no negative feedback, the op-amp acts as a switch. The inverting input is held at ground (0 V) by the resistor, so if the Vin applied to the non-inverting input is positive, the output will be maximum positive, and if Vin is negative, the output will be maximum negative. Since there is no feedback from the output to either input, this is an open loop circuit.

where is the voltage at the non-inverting terminal, is the voltage at the inverting terminal and Gopen-loop is the open-loop gain of the amplifier. (The term open-loop refers to the absence of a feedback loop from the output to the input.) The magnitude of Gopen-loop is typically very large—seldom less than a million—and therefore even a quite small difference between and (a few microvolts or less) will result in amplifier saturation, where the output voltage goes to either the extreme maximum or minimum end of its range, which is set approximately by the power supply voltages. Finley's law states that "When the inverting and non-inverting inputs of an op-amp are not equal, its output is in saturation." Additionally, the precise magnitude of Gopen-loop is not well controlled by the manufacturing process, 19

Fig 4.6 Op-Amp with feedback Adding negative feedback via Rf reduces the gain. Equilibrium will be established when Vout is just sufficient to reach around and pull the inverting input to the same voltage as Vin. As a simple example, if Vin = 1 V and Rf = Rg, Vout will be 2 V, the amount required to keep V– at 1 V. Because of the feedback provided by Rf, this is a closed loop circuit. Its over-all gain Vout / Vin is called the closed-loop gain Gclosed-loop.

and so it is impractical to use an operational amplifier as a stand-alone differential amplifier. If linear operation is desired, negative feedback must be used, usually achieved by applying a portion of the output voltage to the inverting input. The feedback enables the output of the amplifier to keep the inputs at or near the same voltage so that saturation does not occur. Another benefit is that if much negative feedback is used, the circuit's overall gain and other parameters become determined more by the feedback network than by the op-amp itself. If the feedback network is made of components with relatively constant, predictable, values such as resistors, capacitors and inductors, the unpredictability and inconstancy of the op-amp's parameters (typical of semiconductor devices) do not seriously affect the circuit's performance. If no negative feedback is used, the op-amp functions as a switch or comparator. Positive feedback may be used to introduce hysteresis or oscillation. Returning to a consideration of linear (negative feedback) operation, the high open-loop gain and low input leakage current of the op-amp imply two "golden rules" that are highly useful in analysing linear op-amp circuits.

4.3.3 Golden rules of op-amp negative feedback:
If there is negative feedback and if the output is not saturated, 1. Both inputs are at the same voltage. 2. No current flows in or out of either input. These rules are true of the ideal op-amp and for practical purposes are true of real op-amps unless very high-speed or high-precision performance is being contemplated (in which case account must be taken of things such as input capacitance, input bias currents and voltages, finite speed, and other op-amp imperfections) As a consequence of the first rule, the input impedance of the two inputs will be nearly infinite. That is, even if the open-loop impedance between the two inputs is low, the closed-loop input impedance will be high because the inputs will be held at nearly the same voltage. This impedance is considered as infinite for an ideal op-amp and is about one megaohm in practice.

4.3.4 Ideal and real op-amps:
An ideal op-amp is usually considered to have the following properties, and they are considered to hold for all input voltages:
  

Infinite open-loop gain (when doing theoretical analysis, a limit may be taken as open loop gain G goes to infinity) Infinite voltage range available at the output (vout) (in practice the voltages available from the output are limited by the supply voltages and ) Infinite bandwidth (i.e., the frequency magnitude response is considered to be flat everywhere with zero phase shift). 20

   

   

Infinite input impedance (so, in the diagram, , and zero current flows from to ) Zero input current (i.e., there is assumed to be no leakage or bias current into the device) Zero input offset voltage (i.e., when the input terminals are shorted so that , the output is a virtual ground or vout = 0). Infinite slew rate (i.e., the rate of change of the output voltage is unbounded) and power bandwidth (full output voltage and current available at all frequencies). Zero output impedance (i.e., Rout = 0, so that output voltage does not vary with output current) Zero noise Infinite Common-mode rejection ratio (CMRR) Infinite Power supply rejection ratio for both power supply rails.

In practice, none of these ideals can be realized, and various shortcomings and compromises have to be accepted. Depending on the parameters of interest, a real op-amp may be modeled to take account of some of the non-infinite or nonzero parameters using equivalent resistors and capacitors in the op-amp model. The designer can then include the effects of these undesirable, but real, effects into the overall performance of the final circuit. Some parameters may turn out to have negligible effect on the final design while others represent actual limitations of the final performance that must be evaluated.

Fig 4.7 An equivalent circuit of an operational amplifier that models some resistive nonideal parameters.

4.3.5 Operational amplifier IC (LM741CN):
We have used IC LM741CN operational amplifier in our project.

4.3.5.1 General Description:
The LM741 series are general purpose operational amplifiers which feature improved performance over industry standards like the LM709. They are direct, plug-in replacements for the 709C, LM201, MC1439 and 748 in most applications. The amplifiers offer many features:  Overload protection on the input and output,  No latch-up when the common mode range is exceeded  Freedom from oscillations. The LM741C is identical to the LM741/LM741A except that the LM741C has their performance guaranteed over a 0°C to +70°C temperature range, instead of −55°C to +125°C.

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4.3.5.2 Internal circuitry of 741 type op-amp:
Though designs vary between products and manufacturers, all op-amps have basically the same internal structure, which consists of three stages: 1. Differential amplifier – provides low noise amplification, high input impedance, usually a differential output. 2. Voltage amplifier – provides high voltage gain, a single-pole frequency roll-off, usually singleended output. 3. Output amplifier – provides high current driving capability, low output impedance, current limiting and short circuit protection circuitry.

Fig 4.8 A component level diagram of the common 741 op-amp. Dotted lines outline: current mirrors (red); differential amplifier (blue); class A gain stage (magenta); voltage level shifter (green); output stage (cyan).

4.3.5.3 Input stage:
22

Constant-current stabilization system
The input stage DC conditions are stabilized by a high-gain negative feedback system whose main parts are the two current mirrors on the left of the figure, outlined in red. The main purpose of this negative feedback system—to supply the differential input stage with a stable constant current—is realized as follows. The current through the 39 kΩ resistor acts as a current reference for the other bias currents used in the chip. The voltage across the resistor is equal to the voltage across the supply rails ( ) minus two transistor diode drops (i.e., from Q11 and Q12), and so the current has value . The Widlar current mirror built by Q10, Q11, and the 5 kΩ resistor produces a very small fraction of Iref at the Q10 collector. This small constant current through Q10's collector supplies the base currents for Q3 and Q4 as well as the Q9 collector current. The Q8/Q9 current mirror tries to make Q9's collector current the same as the Q3 and Q4 collector currents. Thus Q3 and Q4's combined base currents (which are of the same order as the overall chip's input currents) will be a small fraction of the already small Q10 current. So, if the input stage current increases for any reason, the Q8/Q9 current mirror will draw current away from the bases of Q3 and Q4, which reduces the input stage current, and vice versa. The feedback loop also isolates the rest of the circuit from common-mode signals by making the base voltage of Q3/Q4 follow tightly 2Vbe below the higher of the two input voltages.

4.3.5.4 Differential amplifier:
The blue outlined section is a differential amplifier. Q1 and Q2 are input emitter followers and together with the common base pair Q3 and Q4 form the differential input stage. In addition, Q3 and Q4 also act as level shifters and provide voltage gain to drive the class A amplifier. They also help to increase the reverse Vbe rating on the input transistors (the emitter-base junctions of the NPN transistors Q1 and Q2 break down at around 7 V but the PNP transistors Q3 and Q4 have breakdown voltages around 50 V. The differential amplifier formed by Q1–Q4 drives a current mirror active load formed by transistors Q5–Q7 (actually, Q6 is the very active load). Q7 increases the accuracy of the current mirror by decreasing the amount of signal current required from Q3 to drive the bases of Q5 and Q6. This configuration provides differential to single ended conversion as follows: The signal current of Q3 is the input to the current mirror while the output of the mirror (the collector of Q6) is connected to the collector of Q4. Here, the signal currents of Q3 and Q4 are summed. For differential input signals, the signal currents of Q3 and Q4 are equal and opposite. Thus, the sum is twice the individual signal currents. This completes the differential to single ended conversion. The open circuit signal voltage appearing at this point is given by the product of the summed signal currents and the paralleled collector resistances of Q4 and Q6. Since the collectors of Q4 and Q6 appear as high resistances to the signal current, the open circuit voltage gain of this stage is very high. The base current at the inputs is not zero and the effective (differential) input impedance of a 741 is about 2 MΩ. The "offset null" pins may be used to place external resistors in parallel with the two 1 kΩ resistors (typically in the form of the two ends of a potentiometer) to adjust the balancing of the Q5/Q6 current mirror and thus indirectly control the output of the op-amp when zero signal is applied between the inputs. 23

4.3.5.5 Class A gain stage:
The section outlined in magenta is the class A gain stage. The top-right current mirror Q12/Q13 supplies this stage by a constant current load, via the collector of Q13 that is largely independent of the output voltage. The stage consists of two NPN transistors in a Darlington configuration and uses the output side of a current mirror as its collector load to achieve high gain. The 30 pF capacitor provides frequency selective negative feedback around the class A gain stage as a means of frequency compensation to stabilise the amplifier in feedback configurations. This technique is called Miller compensation and functions in a similar manner to an op-amp integrator circuit. It is also known as 'dominant pole compensation' because it introduces a dominant pole (one which masks the effects of other poles) into the open loop frequency response. This pole can be as low as 10 Hz in a 741 amplifier and it introduces a −3 dB loss into the open loop response at this frequency. This internal compensation is provided to achieve unconditional stability of the amplifier in negative feedback configurations where the feedback network is non-reactive and the closed loop gain is unity or higher. Hence, the use of the operational amplifier is simplified because no external compensation is required for unity gain stability; amplifiers without this internal compensation may require external compensation or closed loop gains significantly higher than unity.

4.3.5.6 Output bias circuitry:
The green outlined section (based on Q16) is a voltage level shifter or rubber diode (i.e., a VBE multiplier); a type of voltage source. In the circuit as shown, Q16 provides a constant voltage drop between its collector and emitter regardless of the current through the circuit. If the base current to the transistor is assumed to be zero, and the voltage between base and emitter (and across the 7.5 kΩ resistor) is 0.625 V (a typical value for a BJT in the active region), then the current through the 4.5 kΩ resistor will be the same as that through the 7.5 kΩ, and will produce a voltage of 0.375 V across it. This keeps the voltage across the transistor, and the two resistors at 0.625 + 0.375 = 1 V. This serves to bias the two output transistors slightly into conduction reducing crossover distortion. In some discrete component amplifiers this function is achieved with (usually two) silicon diodes.

4.3.5.7 Output stage:
The output stage (outlined in cyan) is a Class AB push-pull emitter follower (Q14, Q20) amplifier with the bias set by the Vbe multiplier voltage source Q16 and its base resistors. This stage is effectively driven by the collectors of Q13 and Q19. Variations in the bias with temperature, or between parts with the same type number, are common so crossover distortion and quiescent current may be subject to significant variation. The output range of the amplifier is about one volt less than the supply voltage, owing in part to Vbe of the output transistors Q14 and Q20. The 25 Ω resistor in the output stage acts as a current sense to provide the output current-limiting function which limits the current in the emitter follower Q14 to about 25 mA for the 741. Current limiting for the negative output is done by sensing the voltage across Q19's emitter resistor and using this to reduce the drive into Q15's base. Later versions of this amplifier schematic may show a slightly different method of output current limiting. The output resistance is not zero, as it would be in an ideal op-amp, but with negative feedback it approaches zero at low frequencies.

24

The "741" has come to often mean a generic op-amp IC (such as uA741, LM301, 558, LM324, TBA221 - or a more modern replacement such as the TL071). The description of the 741 output stage is qualitatively similar for many other designs (that may have quite different input stages), except:  Some devices (uA748, LM301, LM308) are not internally compensated (require an external capacitor from output to some point within the operational amplifier, if used in low closed-loop gain applications).  Some modern devices have rail-to-rail output capability (output can be taken to positive or negative power supply rail within a few millivolts).

4.3.5.8 Absolute Maximum Ratings (741C):
Supply Voltage Power Dissipation Differential Input Voltage Input Voltage Output Short Circuit Duration Operating Temperature Range Storage Temperature Range Junction Temperature Soldering Information N-Package (10 seconds) J- or H-Package (10 seconds) M-Package Vapor Phase (60 seconds) Infrared (15 seconds) ±18V 500 mW ±30V ±15V Continuous 0°C to +70°C −65°C to +150°C 100°C 260°C 300°C 215°C 215°C

Table 4.2 Maximum Ratings of IC LM 741C

4.4 Analog-to-digital converter:
An analog-to-digital converter (abbreviated ADC, A/D or A to D) is a device which converts continuous signals to discrete digital numbers. Typically, an ADC is an electronic device that converts an input analog voltage (or current) to a digital number proportional to the magnitude of the voltage or current. However, some non-electronic or only partially electronic devices, such as rotary encoders, can also be considered ADCs. The digital output may use different coding schemes, such as binary, Gray code or two's complement binary. 25

4.4.1 Electrical Symbol

Fig 4.9 Electrical Symbol of ADC

4.4.2 ADC structures:
The most common ways of implementing an electronic ADC are : A direct conversion ADC or flash ADC has a bank of comparators sampling the input signal in parallel, each firing for their decoded voltage range. The comparator bank feeds a logic circuit that generates a code for each voltage range. Direct conversion is very fast, capable of gigahertz sampling rates, but usually has only 8 bits of resolution or fewer, since the number of comparators needed, 2N 1, doubles with each additional bit, requiring a large expensive circuit. A successive-approximation ADC uses a comparator to reject ranges of voltages, eventually settling on a final voltage range. Successive approximation works by constantly comparing the input voltage to the output of an internal digital to analog converter (DAC, fed by the current value of the approximation) until the best approximation is achieved. At each step in this process, a binary value of the approximation is stored in a successive approximation register (SAR). The SAR uses a reference voltage (which is the largest signal the ADC is to convert) for comparisons. For example if the input voltage is 60 V and the reference voltage is 100 V, in the 1st clock cycle, 60 V is compared to 50 V (the reference, divided by two. This is the voltage at the output of the internal DAC when the input is a '1' followed by zeros), and the voltage from the comparator is positive (or '1') (because 60 V is greater than 50 V). At this point the first binary digit (MSB) is set to a '1'. In the 2nd clock cycle the input voltage is compared to 75 V (being halfway between 100 and 50 V: This is the output of the internal DAC when its input is '11' followed by zeros) because 60 V is less than 75 V, the comparator output is now negative (or '0'). The second binary digit is therefore set to a '0'. In the 3rd clock cycle, the input voltage is compared with 62.5 V (halfway between 50 V and 75 V: This is the output of the internal DAC when its input is '101' followed by zeros). The output of the comparator is negative or '0' (because 60 V is less than 62.5 V) so the third binary digit is set to a 0. The fourth clock cycle similarly results in the fourth digit being a '1' (60 V is greater than 56.25 V, the DAC output for '1001' followed by zeros). The result of this would be in the binary form 1001. This is also called bitweighting conversion, and is similar to a binary search. The analogue value is rounded to the nearest binary value below, meaning this converter type is mid-rise. Because the approximations are successive (not simultaneous), the conversion takes one clock-cycle for each bit of resolution desired. The clock frequency must be equal to the sampling frequency multiplied by the number of bits of resolution desired. 26

A ramp-compare ADC produces a saw-tooth signal that ramps up or down then quickly returns to zero. When the ramp starts, a timer starts counting. When the ramp voltage matches the input, a comparator fires, and the timer's value is recorded. Timed ramp converters require the least number of transistors. The ramp time is sensitive to temperature because the circuit generating the ramp is often just some simple oscillator. A special advantage of the ramp-compare system is that comparing a second signal just requires another comparator, and another register to store the voltage value. A very simple (non-linear) ramp-converter can be implemented with a microcontroller and one resistor and capacitor The Wilkinson ADC was designed by D. H. Wilkinson in 1950. The Wilkinson ADC is based on the comparison of an input voltage with that produced by a charging capacitor. The capacitor is allowed to charge until its voltage is equal to the amplitude of the input pulse. (A comparator determines when this condition has been reached.) Then, the capacitor is allowed to discharge linearly, which produces a ramp voltage. At the point when the capacitor begins to discharge, a gate pulse is initiated. The gate pulse remains on until the capacitor is completely discharged. Thus the duration of the gate pulse is directly proportional to the amplitude of the input pulse. This gate pulse operates a linear gate which receives pulses from a high-frequency oscillator clock. While the gate is open, a discrete number of clock pulses pass through the linear gate and are counted by the address register. The time the linear gate is open is proportional to the amplitude of the input pulse, thus the number of clock pulses recorded in the address register is proportional also. Alternatively, the charging of the capacitor could be monitored, rather than the discharge An integrating ADC (also dual-slope or multi-slope ADC) applies the unknown input voltage to the input of an integrator and allows the voltage to ramp for a fixed time period (the run-up period). Then a known reference voltage of opposite polarity is applied to the integrator and is allowed to ramp until the integrator output returns to zero (the run-down period). The input voltage is computed as a function of the reference voltage, the constant run-up time period, and the measured run-down time period. The run-down time measurement is usually made in units of the converter's clock, so longer integration times allow for higher resolutions. Likewise, the speed of the converter can be improved by sacrificing resolution. Converters of this type are used in most digital voltmeters for their linearity and flexibility. A delta-encoded ADC or Counter-ramp has an up-down counter that feeds a digital to analog converter (DAC). The input signal and the DAC both go to a comparator. The comparator controls the counter. The circuit uses negative feedback from the comparator to adjust the counter until the DAC's output is close enough to the input signal. The number is read from the counter. Delta converters have very wide ranges, and high resolution, but the conversion time is dependent on the input signal level, though it will always have a guaranteed worst-case. Delta converters are often very good choices to read real-world signals. Most signals from physical systems do not change abruptly. Some converters combine the delta and successive approximation approaches; this works especially well when high frequencies are known to be small in magnitude.

4.4.3 ADC 0804
We have used IC ADC0804 in our project: 27

Fig 4.10 IC ADC 0804

4.4.3.1 Features:
         Compatible with 8080 Microprocessors Easy interface to all microprocessors, or operates 'stand alone' Differential analog voltage inputs Logic inputs and outputs meet both MOS and TTL voltage level specifications Works with 2.5V (LM336) Voltage Reference N-chip clock generator 0V to 5V analog input voltage range with single 5V supply No zero adjust required Operates ratiometrically or with 5 Vdc, 2.5Vdc, or analog span adjusted voltage reference

4.4.3.2 Pin Layout:

28

Fig 4.11 Pin Layout of ADC 0804

4.4.3.3 Pin Description:
Pin Number Description

1 2 3 4 5 6 7 8 9 10 11 12 13

CS - Chip Select (Active Low) RD - Read (Active Low) WR - Write (Active Low) CLK IN - Clock IN INTR - Interrupt (Active Low) Vin+ - Analog Voltage Input Vin- - Analog Voltage Input AGND - Analog Ground Vref/2 - Voltage Reference / 2 DGND - Digital Ground DB7 - Data Bit 7 (MSB) DB6 - Data Bit 6 DB5 - Data Bit 5

29

14 15 16 17 18 19 20

DB4 - Data Bit 4 DB3 - Data Bit 3 DB2 - Data Bit 2 DB1 - Data Bit 1 DB0 - Data Bit 0 (LSB) CLKR - Clock Reset Vcc - Positive Supply or Vref

Table 4.3 Description of Pins of ADC 0804

4.4.3.4 Pin configuration of ADC 0804:

Fig 4.12 pin configuration of ADC 0804

4.4.3.5 Description Of pins of ADC :

30

 

CS(chip select):- Chip select is an active low input used to activate the ADC0804 chip. To access the ADC0804, this pin must be low. RD(read):- This is an input signal and is active low. The ADC converts the analog input to its binary equivalent and holds it in an internal register. RD is used to get the converted data out of the ADC0804 chip. When CS=0, if a high to low pulse is applied to the RD pin, the 8-bit digital output shows up at the D0-D7 data pins. The RD pin is also referred to as output enable (OE). WR (write; a better name might be “start conversion”): This is an active low input used to inform the ADC804 to start the conversion process. If CS = 0 then WR makes low to high transition the ADC 0804 starts converting the analog input value of Vin to an 8- bit digital number. The amount of time it takes to convert varies depending on the CLK IN and CLK R values explained below. When the data conversion is complete, the INTR pin is forced low by the ADC 0804. CLK IN and CLK R: This is an input pin connected to an external clock source when an external clock is used for timing. However, the 0804 has an internal clock generator. To use this CLK IN and CLK R pins are connected to a capacitor and a resistor as shown in figure. In that case the clock frequency is determined by the equation f = 1/ (1.1) RC Typical values or R = 33K Ω and C = 5µ farad. Substituting in above equation we get 1 KHz. INTR (interrupt; a better name might be “end of conversion”): this is an output pin and is active low. It is abnormally high pin and when the conversion is finished it goes low to signal the CPU that the converted data is ready to be picked up. After INTR goes low, we make CS =0 and send a high to low pulse to the RD pin to get the data out of the ADC 0804 chip. Vin(+) and Vin(-): these are the differential analog inputs where Vin = Vin(+) – Vin(-). Often the Vin(-) pin is connected to ground and the Vin(+) is used as the analog input to be converted to digital









 

Vcc : this is the +5 volt power supply. It is also used as a reference voltage when the Vref/2 input (pin 9) is open. Vref/2: pin 9 is an input voltage used for the reference voltage. If this pin is open analog input voltage for the ADC 0804 is in the range of 0 to 5 volts( same as the Vcc pin). However, there are many applications where the analog input applied to Vin needs to be other than 0 to 5 volt range. Vref/2 is used to implement the analog input voltage other than 0 to 5 volts.

Vref/2 Not connected

Vin(V) 0 to 5

step size(mV) 5/256 = 19.53

31

2.0 1.5 1.28

0 to 4 0 to 3 0 to 2.56

4/255 = 15.62 3/256 = 11.71 2.56/256 = 10

Table 4.4 Vref/2 relation to Vin range (ADC 0804)  D0 to D7: These are the digital data output pins since ADC 0804 is a parallel ADC chip. These are tri state buffered and the converted data is accesd only when CS =0 and is forced low. To calculate the output voltage use the following formula

Dout = Vin/ step size Where, Dout = digital data output in decimal Vin = analog input voltage Step size (resolution) = smallest change which (2* Vref/2)/256 for ADC 0804.

Absolute Maximum Ratings Thermal Information:
Supply Voltage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.5V Voltage at Any Input. . . . . . . . . . . . . . . . . . . . . . -0.3V to (V+ +0.3V)

Operating Conditions:
Temperature Range . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0oC to 70oC

Thermal Information:
Thermal Resistance (Typical, Note 1) θJA (C/W) PDIP Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 Maximum Junction Temperature Plastic Package . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .150oC Maximum Storage Temperature Range. . . . . . . . . . -65oC to 150oC Maximum Lead Temperature (Soldering, 10s). . . . . . . . . . . . .300oC

4.4.3.6 Timing Diagrams:

32

Fig 4.13: Start conversion

Fig 4.14: Output enable and reset INTR

The above timing diagrams are from ADC0804 datasheet. The first diagram (figure 4.13) shows how to start a conversion. We can also see which signals are to be asserted and at what time to start a conversion. So looking into the timing diagram of figure 4.13. We note down the steps or say the order in which signals are to be asserted to start a conversion of ADC. As we have decided to make Chip select pin as low so we need not bother about the CS signal in the timing diagram. Steps for starting an ADC conversion are:  Make chip select (CS) signal low.  Make write (WR) signal low.  Make chip select (CS) high.  Wait for INTR pin to go low (means conversion ends). Once the conversion in ADC is done, the data is available in the output latch of the ADC. Figure 4.14 shows the timing diagram of how to read the converted value from the output latch of the ADC. Data of the new conversion is only available for reading after ADC0804 made INTR pin low or say when the conversion is over. The steps to read output from the ADC0804 are:  Make chip select (CS) pin low. 33

   

Make read (RD) signal low. Read the data from port where ADC is connected. Make read (RD) signal high. Make chip select (CS) high.

4.5 Microphone:
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1876, Emile Berliner invented the first microphone used as a telephone voice transmitter. Microphones are used in many applications such as telephones, tape recorders, karaoke systems, hearing aids, motion picture production, live and recorded audio engineering, FRS radios, megaphones, in radio and television broadcasting and in computers for recording voice, speech recognition, VoIP, and for non-acoustic purposes such as ultrasonic checking or knock sensors. Most microphones today use electromagnetic induction (dynamic microphone), capacitance change (condenser microphone, pictured right), piezoelectric generation, or modulation to produce an electrical voltage signal from mechanical vibration.

light

4.5.1 Omnidirectional:
An omnidirectional (or nondirectional) microphone's response is generally considered to be a perfect sphere in three dimensions. In the real world, this is not the case. As with directional microphones, the polar pattern for an Fig 4.15 A Neumann U87 "omnidirectional" microphone is a function of condenser microphone frequency. The body of the microphone is not infinitely small with shock mount. and, as a consequence, it tends to get in its own way with respect to sounds arriving from the rear, causing a slight flattening of the polar response. This flattening increases as the diameter of the microphone (assuming it's cylindrical) reaches the wavelength of the frequency in question. Therefore, the smallest diameter microphone gives the best omnidirectional characteristics at high frequencies. The wavelength of sound at 10 kHz is little over an inch (3.4 cm) so the smallest measuring microphones are often 1/4" (6 mm) in diameter, which practically eliminates directionality even up to the highest frequencies. Omnidirectional microphones, unlike cardioids, do not employ resonant cavities as delays, and so can be considered the "purest" microphones in terms of low coloration; they add very little to the original sound. Being pressure-sensitive they can also have a very flat low-

34

frequency response down to 20 Hz or below. Pressure-sensitive microphones also respond much less to wind noise than directional (velocity sensitive) microphones. An example of a nondirectional microphone is the round black eight ball.

4.5.2 Unidirectional
A unidirectional microphone is sensitive to sounds from only one direction.The sound intensity for a particular frequency is plotted for angles radially from 0 to 360°.

4.5.3 Condenser/Capacitor or Electrostatic Microphone:
We have used a condenser type of microphone in our project. In a condenser microphone also called a capacitor or electrostatic microphone, the diaphragm acts as one plate of a capacitor, and the vibrations produce changes in the distance between the plates. There are two methods of extracting an audio output from the transducer thus formed: DC-biased and radio frequency (RF) or high frequency (HF) condenser microphones. With a DC-biased microphone, the plates are biased with a fixed charge (Q). The voltage maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation (C = Q / V), where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor.

Fig 4.16 Condenser Microphone

4.5.3.1 Principle:
Sound pressure changes the spacing between a thin metallic membrane and the stationary back plate. The plates are charged to a total charge 35

Where C is the capacitance, V the voltage of the biasing battery, A the area of each plate and d the separation of the plates.

4.5.3.2 Advantages:


Best overall frequency response makes this the microphone of choice for many recording applications.

4.5.3.3 Disadvantages:
  

Expensive May pop and crack when close miked Requires a battery or external power supply to bias the plates.

4.5.3.4 Condenser Microphone Signal:

Fig 4.17 Condenser Microphone Signal The flat, faithful frequency response of the condenser microphone arises from its mechanism. The charge on the membrane depends only upon the spacing and shows no appreciable resonances to skew the frequency response. The capacitance of the parallel plate membrane structure is given by 36

When the spacing changes, the charge changes, giving an electric current through the resistor R.

The voltage measured across the resistor is an electrical image of the sound pressure which moves the membrane. Because the sensing element of a condenser microphone is a light membrane, it is capable of excellent transient response. The fact that the condenser has excellent high frequency response implies good transient response, since sharp transients have more high frequency content than the sustained sounds which follow them. Because the condenser microphone must have a continuous, stable DC voltage to bias the membrane, it is common practice to supply that voltage from the sound mixing board. The voltage is applied via one of the microphone leads, typically 48 volts, and is commonly referred to as "phantom power". Since the alternative is a battery supplied bias, with the risk that a battery can go out in mid performance, the phantom power provision from mixing boards is useful.

4.6 Low-Pass Filters:
A low-pass filter is a filter that passes low-frequency signals but attenuates (reduces the amplitude of) signals with frequencies higher than the cutoff frequency. The actual amount of attenuation for each frequency varies from filter to filter. It is sometimes called a high-cut filter, or treble cut filter when used in audio applications. A low-pass filter is the opposite of a high-pass filter, and a band-pass filter is a combination of a low-pass and a high-pass. The concept of a low-pass filter exists in many different forms, including electronic circuits (like a hiss filter used in audio), digital algorithms for smoothing sets of data, acoustic barriers, blurring of images, and so on. Low-pass filters play the same role in signal processing that moving averages do in some other fields, such as finance; both tools provide a smoother form of a signal which removes the shortterm oscillations, leaving only the long-term trend.

4.6.1 Electronic low-pass filters: Passive electronic realization:

37

Fig 4.18 Passive, first order low-pass RC filter One simple electrical circuit that will serve as a low-pass filter consists of a resistor in series with a load, and a capacitor in parallel with the load. The capacitor exhibits reactance, and blocks lowfrequency signals, causing them to go through the load instead. At higher frequencies the reactance drops, and the capacitor effectively functions as a short circuit. The combination of resistance and capacitance gives us the time constant of the filter τ = RC. The break frequency, also called the turnover frequency or cutoff frequency (in hertz), is determined by the time constant:

or equivalently (in radians per second):

One way to understand this circuit is to focus on the time the capacitor takes to charge. It takes time to charge or discharge the capacitor through that resistor:
 

At low frequencies, there is plenty of time for the capacitor to charge up to practically the same voltage as the input voltage. At high frequencies, the capacitor only has time to charge up a small amount before the input switches direction. The output goes up and down only a small fraction of the amount the input goes up and down. At double the frequency, there's only time for it to charge up half the amount.

Another way to understand this circuit is with the idea of reactance at a particular frequency:
 

Since DC cannot flow through the capacitor, DC input must "flow out" the path marked Vout. Since AC flows very well through the capacitor — almost as well as it flows through solid wire — AC input "flows out" through the capacitor, effectively short circuiting to ground (analogous to replacing the capacitor with just a wire).

38

4.6.2 Active electronic realization

Fig 4.19 An active low-pass filter Another type of electrical circuit is an active low-pass filter. In the operational amplifier circuit shown in the figure, the cutoff frequency (in hertz) is defined as:

or equivalently (in radians per second):

The gain in the pass band is
and the stopband drops off at −6 dB per octave as it is a first-order filter.

,

Sometimes, a simple gain amplifier (as opposed to the very-high-gain operation amplifier) is turned into a low-pass filter by simply adding a feedback capacitor C. This feedback decreases the frequency response at high frequencies via the Miller effect, and helps to avoid oscillation in the amplifier. For example, an audio amplifier can be made into a low-pass filter with cutoff frequency 100 kHz to reduce gain at frequencies which would otherwise oscillate. Since the audio band (what we can hear) only goes up to 20 kHz or so, the frequencies of interest fall entirely in the passband, and the amplifier behaves the same way as far as audio is concerned.

4.6.3 Ideal and real filters:

39

Fig 4.20 The sinc function, the impulse response of an ideal low-pass filter. An ideal low-pass filter completely eliminates all frequencies above the cutoff frequency while passing those below unchanged: its frequency response is a rectangular function, and is a brick-wall filter. The transition region present in practical filters does not exist in an ideal filter. An ideal low-pass filter can be realized mathematically (theoretically) by multiplying a signal by the rectangular function in the frequency domain or, equivalently, convolution with its impulse response, a sinc function, in the time domain. However, the ideal filter is impossible to realize without also having signals of infinite extent, and so generally needs to be approximated for real ongoing signals, because the sinc function's support region extends to all past and future times. The filter would therefore need to have infinite delay, or knowledge of the infinite future and past, in order to perform the convolution. It is effectively realizable for pre-recorded digital signals by assuming extensions of zero into the past and future, or more typically by making the signal repetitive and using Fourier analysis. Real filters for real-time applications approximate the ideal filter by truncating and windowing the infinite impulse response to make a finite impulse response; applying that filter requires delaying the signal for a moderate period of time, allowing the computation to "see" a little bit into the future. This delay is manifested as phase shift. Greater accuracy in approximation requires a longer delay. An ideal low-pass filter results in ringing artifacts via the Gibbs phenomenon. These can be reduced or worsened by choice of windowing function, and the design and choice of real filters involves understanding and minimizing these artifacts. For example, "simple truncation [of sinc] causes severe ringing artifacts," in signal reconstruction, and to reduce these artifacts one uses window functions "which drop off more smoothly at the edges." The Whittaker–Shannon interpolation formula describes how to use a perfect low-pass filter to reconstruct a continuous signal from a sampled digital signal. Real digital-to-analog converters use real filter approximations.

4.6.4 Types of low pass filter:

40

There are many different types of filter circuits, with different responses to changing frequency. The frequency response of a filter is generally represented using a Bode plot, and the filter is characterized by its cutoff frequency and rate of frequency roll off. In all cases, at the cutoff frequency, the filter attenuates the input power by half or 3 dB. So the order of the filter determines the amount of additional attenuation for frequencies higher than the cutoff frequency.


A first-order filter, for example, will reduce the signal amplitude by half (so power reduces by 6 dB) every time the frequency doubles (goes up one octave); more precisely, the power rolloff approaches 20 dB per decade in the limit of high frequency. The magnitude Bode plot for a first-order filter looks like a horizontal line below the cutoff frequency, and a diagonal line above the cutoff frequency. There is also a "knee curve" at the boundary between the two, which smoothly transitions between the two straight line regions. If the transfer function of a first-order low-pass filter has a zero as well as a pole, the Bode plot will flatten out again, at some maximum attenuation of high frequencies; such an effect is caused for example by a little bit of the input leaking around the one-pole filter; this one-pole–one-zero filter is still a firstorder low-pass. A second-order filter attenuates higher frequencies more steeply. The Bode plot for this type of filter resembles that of a first-order filter, except that it falls off more quickly. For example, a second-order Butterworth filter will reduce the signal amplitude to one fourth its original level every time the frequency doubles (so power decreases by 12 dB per octave, or 40 dB per decade). Other all-pole second-order filters may roll off at different rates initially depending on their Q factor, but approach the same final rate of 12 dB per octave; as with the first-order filters, zeroes in the transfer function can change the high-frequency asymptote. Third- and higher-order filters are defined similarly. In general, the final rate of power rolloff for an order-n all-pole filter is 6n dB per octave (i.e., 20n dB per decade).





On any Butterworth filter, if one extends the horizontal line to the right and the diagonal line to the upper-left (the asymptotes of the function), they will intersect at exactly the "cutoff frequency". The frequency response at the cutoff frequency in a first-order filter is 3 dB below the horizontal line. The various types of filters – Butterworth filter, Chebyshev filter, Bessel filter, etc. – all have differentlooking "knee curves". Many second-order filters are designed to have "peaking" or resonance, causing their frequency response at the cutoff frequency to be above the horizontal line.

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Fig 4.21 The gain-magnitude frequency response of a first-order (one-pole) low-pass filter. Power gain is shown in decibels. Angular frequency is shown on a logarithmic scale in units of radians per second.

4.6.5 Phase Response:
For a first-order low-pass filter, vOUT always lags vIN by some phase angle betweeen 0 and 90°. The vector diagrams to the right show why. The RL vector diagram shows that vL leads vR by 90°. However, the input voltage is applied to the series combination of the two components. Thus, vL leads vIN, while vR lags vIN. Since the output is taken from across R, vOUT = vR and lags behind vIN by some phase angle that depends on frequency. In the case of the RC low-pass filter, vC lags vR, and again vIN is applied to the series combination of the two components. This time, however, the output is taken from across C, so vOUT = vC, which again lags vIN by some angle in the range 0 to 90°, according to the signal frequency.

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Fig 4.22 phase and frequency curve Most of the variation in phase angle occurs within one decade of the cutoff frequency, as shown in the figure to the right. Note that at frequencies well below cutoff, there is essentially no phase shift. The phase lag begins to become significant about a decade below co, and reaches 45° (or /4 radians) at co itself. Above co, the output phase continues to change rapidly during the first decade, by which time the phase lag is close to 90° ( /2). Because of this shifting phase lag, non-sinusoidal signals with frequency components near co will be distorted by this filter. Such distortion must be taken into account when designing filters of this type for some kinds of applications. Fig4.24 An active high-pass filter

4.7 High pass filters:
A high-pass filter, or HPF, is a filter that passes high frequencies well but attenuates (i.e., reduces the amplitude of) frequencies lower than the filter's cutoff frequency. The actual amount of attenuation for each frequency is a design parameter of the filter. The simple first-order electronic high-pass filter shown in Figure is implemented by placing an input voltage across the series combination of a capacitor and a resistor and using the voltage across the resistor as an output. The product of the resistance and capacitance (R×C) is the time constant (τ); it is inversely proportional to the cutoff frequency fc, at which the output power is half the input power. That is,

where fc is in hertz, τ is in seconds, R is in ohms, and C is in farads. Figure 4.24 shows an active electronic implementation of a first-order high-pass filter using an operational amplifier. In this case, the filter has a pass band gain of -R2/R1 and has a corner frequency of Fig 4.23 A passive, analog, first order highpass filter, realized by an RC circuit

Because this filter is active, it may have non-unity pass band gain. That is, high-frequency signals are inverted and 43

amplified by R2/R1.

4.7.1 The Frequency Response Curve:

Fig 4.25 Frequency Response Curve The frequency response of a basic high-pass filter is actually a mirror image of its low-pass counterpart. At the cutoff frequency where R = XL or R = XC, the attenuation is only 3 db, so the signal voltage is still 70.7% of its higher-frequency value. Below the cutoff frequency, attenuation increases at the rate of 20 db per decade, which is the same roll-off as for the low-pass filter. Above the cutoff frequency, attenuation rapidly decreases to nothing, and all higher frequencies pass with ease. The green line in the graph is the straight line extension of the constant slopes of the actual frequency response. As with the low-pass filter, the intersection point is the cutoff frequency. This straight-line approximation of the real frequency response curve is very easy to draw, and is sufficiently accurate for some kinds of applications. Of course, the actual curve near the cutoff frequency is understood.

4.7.2 Phase Response:
As we have already seen, in a first-order low-pass filter, vOUT always lags vIN by some phase angle betweeen 0 and 90°. Exactly the reverse is true for a first-order high-pass filter: as shown in the vector diagrams, vOUT is always taken from across the component whose voltage leads v IN by some phase angle, .

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For the RL filter, vOUT is taken from across L, so its phase angle necessarily leads v IN as shown in the upper vector diagram. For the RC filter, vOUT is taken from across R, which again leads vIN as shown in the lower vector diagram.

Fig 4.27 Cathode Ray Oscilloscope (CRO)

Fig 4.26 Phase Response Of course, the actual phase angle by which vOUT leads vIN depends on the specific frequency of the signal, as compared to the cutoff frequency of the filter. As shown in the phase diagram signals more than 10 times the cutoff frequency show little or no appreciable phase shift, while signals less than 0.1 times the cutoff frequency are shifted close to 90°. Most of the change in phase occurs within a factor of 0.1 to 10 times CO. As with the low-pass filter, non-sinusoidal signals with frequency components at or near distorted when passing through the high-pass filter.
CO

will be

4.8 Cathode ray oscilloscope:
An oscilloscope (abbreviated sometimes as scope or O-scope) is a type of electronic test instrument that allows signal voltages to be viewed, usually as a two-dimensional graph of one or more electrical potential differences (vertical axis) plotted as a function of time or of some other voltage (horizontal axis). Although an oscilloscope displays voltage on its vertical axis, any other quantity that can be converted to a voltage can be displayed as well. In most instances, oscilloscopes show events that repeat with either no change, or change slowly. The oscilloscope is one of the most versatile and widely-used electronic instruments. 45

Oscilloscopes are commonly used when it is desired to observe the exact wave shape of an electrical signal. In addition to the amplitude of the signal, an oscilloscope can show distortion and measure frequency, time between two events (such as pulse width or pulse rise time), and relative timing of two related signals. Some modern digital oscilloscopes can analyze and display the spectrum of a repetitive event. Special-purpose oscilloscopes, called spectrum analyzers, have sensitive inputs and can display spectra well into the GHz range. A few oscilloscopes that accept plug-ins can display spectra in the audio range. Oscilloscopes are used in the sciences, medicine, engineering, telecommunications, and industry. General-purpose instruments are used for maintenance of electronic equipment and laboratory work. Special-purpose oscilloscopes may be used for such purposes as analyzing an automotive ignition system, or to display the waveform of the heartbeat as an electrocardiogram. Originally all oscilloscopes used cathode ray tubes as their display element and linear amplifiers for signal processing, but modern oscilloscopes can have LCD or LED screens, fast analog-to-digital converters and digital signal processors. Although not as commonplace, some oscilloscopes used storage CRTs to display single events for a limited time. Oscilloscope peripheral modules for general purpose laptop or desktop personal computers use the computer's display, and can convert them into useful and flexible test instruments.

4.8.1 Description:
4.8.2 Display and general external appearance: The basic oscilloscope, as shown, is typically divided into four sections: the display, vertical controls, horizontal controls and trigger controls. The display is usually a CRT or LCD panel which is laid out with both horizontal and vertical reference lines referred to as the graticule. In addition to the screen, most display sections are equipped with three basic controls, a focus knob, an intensity knob and a beam finder button. The vertical section controls the amplitude of the displayed signal. This section carries a Volts-perDivision (Volts/Div) selector knob, an AC/DC/Ground selector switch and the vertical (primary) input for the instrument. Additionally, this section is typically equipped with the vertical beam position knob. The horizontal section controls the time base or ―sweep‖ of the instrument. The primary control is the Seconds-per-Division (Sec/Div) selector switch. Also included is a horizontal input for plotting dual X-Y axis signals. The horizontal beam position knob is generally located in this section. The trigger section controls the start event of the sweep. The trigger can be set to automatically restart after each sweep or it can be configured to respond to an internal or external event. The principal controls of this section will be the source and coupling selector switches. An external trigger input (EXT Input) and level adjustment will also be included. In addition to the basic instrument, most oscilloscopes are supplied with a probe as shown. The probe will connect to any input on the instrument and typically has a resistor of ten times the 'scope's input 46

impedance. This results in a .1 (-10X) attenuation factor, but helps to isolate the capacitive load presented by the probe cable from the signal being measured. Some probes have a switch allowing the operator to bypass the resistor when appropriate.

4.8.3 Size and Portability:
Most modern oscilloscopes are lightweight, portable instruments that are compact enough to be easily carried by a single person. In addition to the portable units, the market offers a number of miniature battery-powered instruments for field service applications. Laboratory grade oscilloscopes, especially older units which use vacuum tubes, are generally bench-top devices or may be mounted into dedicated carts. Special-purpose oscilloscopes may be rack-mounted or permanently mounted into a custom instrument housing.

4.8.4 Inputs
The signal to be measured is fed to one of the input connectors, which is usually a coaxial connector such as a BNC or UHF type. Binding posts or banana plugs may be used for lower frequencies. If the signal source has its own coaxial connector, then a simple coaxial cable is used; otherwise, a specialised cable called a "scope probe", supplied with the oscilloscope, is used. In general, for routine use, an open wire test lead for connecting to the point being observed is not satisfactory, and a probe is generally necessary. General-purpose oscilloscopes have a standardised input resistance of 1 megohm in parallel with a capacitance of around 20 picofarads. This allows the use of standard oscilloscope probes. Scopes for use with very high frequencies may have 50-ohm inputs, which must be either connected directly to a 50-ohm signal source or used with Z0 or active probes. Less-frequently-used inputs include one (or two) for triggering the sweep, horizontal deflection for XY mode displays, and trace brightening/darkening, sometimes called "Z-axis" inputs.

4.8.5 Probes:
Open wire test leads are likely to pick up interference, and their capacitance at the probing end is likely to disturb the circuit/device being examined. They are appropriate only for low frequencies and lowimpedance devices. Nearly always, probes made for 'scope use are the ordinary means of connecting to the device being examined. The probe cable is a special coaxial type (with a resistive center conductor to damp out ringing), with quite-effective shielding. Its capacitance is greater than that of an open wire, and in some cases, such a probe is satisfactory. However, a typical 'scope probe contains a 9-megohm series resistor shunted by a low-value capacitor; combined with the input resistance and capacitance of a standard 'scope input, the probe and the 'scope input form a fairly-accurate 10:1 attenuator that (up to a certain bandwidth) is frequency-independent. This degrades the 'scope's sensitivity by a factor of 10, but the capacitance at the probe tip as only a few pF (picofarads), which is not enough to disturb many typical circuits. (Nevertheless, the reactance of even that few pF is significantly low at high frequencies within the probe and 'scope's bandwidth.) In the great majority of cases, the loss of sensitivity in order to gain less disturbance to the circuit being observed is very worth while.

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Attenuator probes do not necessarily match the input of a given 'scope, and their capacitance needs to be adjusted if they are connected to a different 'scope. As well, they should be checked periodically even when not moved. They are checked and if necessary adjusted by looking at a square wave with a quite-flat top and bottom. When properly adjusted, the horizontal trace of the square wave does not tilt either upward or downward. Because the probe, combined with the 'scope input, forms a frequencycompensated attenuator, this procedure is often called "compensating" a probe. Any decent 'scope has an output jack that provides a known-amplitude square wave with excellent shape for checking and adjusting probes. Probes with 10:1 attenuation are by far the most common; for large signals (and slightly-less capacitive loading), 100:1 probes are not rare. There are also probes that contain switches to select 10:1 or direct (1:1) ratios, but one must be aware that the 1:1 setting has significant capacitance (tens of pF) at the probe tip, because the whole cable's capacitance is now directly connected. Good 'scopes allow for probe attenuation, easily showing effective sensitivity at the probe tip. Some of the best ones have indicator lamps behind translucent windows in the panel to prompt the user to read effective sensitivity. The probe connectors (modified BNC's) have an extra contact to define the probe's attenuation. (A certain value of resistor, connected to ground, "encodes" the attenuation.) There are special high-voltage probes which also form compensated attenuators with the 'scope input; the probe body is physically large, and one made by Tektronix requires partly filling a canister surrounding the series resistor with volatile liquid fluorocarbon to displace air. At the 'scope end is a box with several waveform-trimming adjustments. For safety, a barrier disc keeps one's fingers distant from the point being examined. Maximum voltage is in the low tens of kV. (Observing a high-voltage ramp can create a staircase waveform with steps at different points every repetition, until the probe tip is in contact. Until then, a tiny arc charges the probe tip, and its capacitance holds the voltage (open circuit). As the voltage continues to climb, another tiny arc charges the tip further.) There are also current probes, with cores that surround the conductor carrying current to be examined. One type has a hole for the conductor, and requires that the wire be passed through the hole; it's for semi-permanent or permanent mounting. However, other types, for testing, have a two-part cores that permit them to be placed around a wire. Inside the probe, a coil wound around the core provides a current into an appropriate load, and the voltage across that load is proportional to current. However, this type of probe can sense AC, only. A more-sophisticated probe (originally made by Tektronix) includes a magnetic flux sensor in the magnetic circuit. The probe connects to an amplifier, which feeds (low frequency) current into the coil to cancel the sensed field; the magnitude of that current provides the low-frequency part of the current waveform, right down to DC. The coil still picks up high frequencies. There is a combining network akin to a loudspeaker crossover network.

4.8.6 The trace:
In its simplest mode, the oscilloscope repeatedly draws a horizontal line called the trace or sweep across the middle of the screen from left to right. One of the controls, the timebase control, sets the speed at which the line is drawn, and is calibrated in seconds or decimal fractions of a second per 48

division. If the input voltage departs from zero, the trace is deflected either upwards (normally for positive polarity) or downwards (negative). Another control, the vertical control, sets the scale of the vertical deflection, and is calibrated in volts per division. The resulting trace is a plot of voltage against time, with the more distant past on the left and the more recent past on the right.

4.8.7 Front panel controls:
4.8.7.1 Focus control This control adjusts CRT focus to obtain the sharpest, most-detailed trace. In practice, focus needs to be adjusted slightly when observing quite-different signals, which means that it needs to be an external control. Flat-panel displays do not need a focus control; their sharpness is always optimum. 4.8.7.2 Intensity control This adjusts trace brightness. Slow traces on CRT 'scopes need less, and fast ones, especially if they don't repeat very often, require more. On flat panels, however, trace brightness is essentially independent of sweep speed, because the internal signal processing effectively synthesizes the display from the digitized data.

4.8.7.3 Beam finder Modern oscilloscopes have direct-coupled deflection amplifiers, which means the trace could be deflected off-screen. They also might have their CRT beam blanked without the operator knowing it. In such cases, the screen is blank. To help in restoring the display quickly and without experimentation, the beam finder circuit overrides any blanking and ensures that the beam will not be deflected off-screen; it limits the deflection. With a display, it's usually very easy to restore a normal display. (While active, beam-finder circuits might temporarily distort the trace severely, however this is acceptable.) 4.8.7.4 Graticule The graticule is a grid of squares that serve as reference marks for measuring the displayed trace. These markings, whether located directly on the screen or on a removable plastic filter, usually consist of a 1 cm grid with closer tick marks (often at 2 mm) on the centre vertical and horizontal axis. One expects to see ten major divisions across the screen; the number of vertical major divisions varies. Comparing the grid markings with the waveform permits one to measure both voltage (vertical axis) and time (horizontal axis). Frequency can also be determined by measuring the waveform period and calculating its reciprocal.

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Accuracy and resolution of measurements using a graticule is relatively limited; better 'scopes sometimes have movable bright markers on the trace that permit internal circuits to make more refined measurements. Both calibrated vertical sensitivity and calibrated horizontal time are set in 1 - 2 - 5 - 10 steps. This leads, however, to some awkward interpretations of minor divisions. At 2, each of the five minor divisions is 0.4, so one has to think 0.4, 0.8, 1.2, and 1.6, which is rather awkward. One Tektronix plug-in used a 1 - 2.5 - 5 - 10 sequence, which simplified estimating. The "2.5" didn't look as "neat", but was very welcome. 4.8.7.5 Timebase Controls These select the horizontal speed of the CRT's spot as it creates the trace; this process is commonly referred to as the sweep. In all but the least-costly modern 'scopes, the sweep speed is selectable and calibrated in units of time per major graticule division. Quite a wide range of sweep speeds is generally provided, from seconds to as fast as picoseconds (in the fastest 'scopes) per division. Usually, a continuously-variable control (often a knob in front of the calibrated selector knob) offers uncalibrated speeds, typically slower than calibrated. This control provides a range somewhat greater than that of consecutive calibrated steps, making any speed available between the extremes. 4.8.7.6 Holdoff control Found on some better analog oscilloscopes, this varies the time (holdoff) during which the sweep circuit ignores triggers. It provides a stable display of some repetitive events in which some triggers would create confusing displays. It is usually set to minimum, because a longer time decreases the number of sweeps per second, resulting in a dimmer trace. 4.8.7.7 Vertical sensitivity, coupling, and polarity controls To accommodate a wide range of input amplitudes, a switch selects calibrated sensitivity of the vertical deflection. Another control, often in front of the calibrated-selector knob, offers a continuously-variable sensitivity over a limited range from calibrated to less-sensitive settings. Often the observed signal is offset by a steady component, and only the changes are of interest. A switch (AC position) connects a capacitor in series with the input that passes only the changes (provided that they are not too slow -- "slow" would mean visible). However, when the signal has a fixed offset of interest, or changes quite slowly, the input is connected directly (DC switch position). Most oscilloscopes offer the DC input option. For convenience, to see where zero volts input currently shows on the screen, many oscilloscopes have a third switch position (GND) that disconnects the input and grounds it. Often, in this case, the user centers the trace with the Vertical Position control. Better oscilloscopes have a polarity selector. Normally, a positive input moves the trace upward, but this permits inverting -- positive deflects the trace downward. 4.8.7.8 Horizontal sensitivity control

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This control is found only on more elaborate oscilloscopes; it offers adjustable sensitivity for external horizontal inputs. 4.8.7.9 Vertical position control The vertical position control moves the whole displayed trace up and down. It is used to set the noinput trace exactly on the center line of the graticule, but also permits offsetting vertically by a limited amount. With direct coupling, adjustment of this control can compensate for a limited DC component of an input. 4.8.7.10 Horizontal position control The horizontal position control moves the display sidewise. It usually sets the left end of the trace at the left edge of the graticule, but it can displace the whole trace when desired. This control also moves the X-Y mode traces sidewise in some 'scopes, and can compensate for a limited DC component as for vertical position. 4.8.7.11 Dual-trace controls Each input channel usually has its own set of sensitivity, coupling, and position controls, although some four-trace 'scopes have only mimimal controls for their third and fourth channels. Dual-trace 'scopes have a mode switch to select either channel alone, both channels, or (in some 'scopes) an X-Y display, which uses the second channel for X deflection. When both channels are displayed, the type of channel switching can be selected on some 'scopes; on others, the type depends upon timebase setting. If manually selectable, channel switching can be free-running (asynchronous), or between consecutive sweeps. Some Philips dual-trace analog 'scopes had a fast analog multiplier, and provided a display of the product of the input channels. Multiple-trace 'scopes have a switch for each channel to enable or disable display of that trace's signal. 4.8.7.12 Delayed-sweep controls These include controls for the delayed-sweep timebase, which is calibrated, and often also variable. The slowest speed is several steps faster than the slowest main sweep speed, although the fastest is generally the same. A calibrated multiturn delay time control offers wide range, high resolution delay settings; it spans the full duration of the main sweep, and its reading corresponds to graticule divisions (but with much finer precision). Its accuracy is also superior to that of the display. A switch selects display modes: Main sweep only, with a brightened region showing when the delayed sweep is advancing, delayed sweep only, or (on some 'scopes) a combination mode. Good CRT 'scopes include a delayed-sweep intensity control, to allow for the dimmer trace of a much-faster delayed sweep that nevertheless occurs only once per main sweep. Such 'scopes also are likely to have a trace separation control for multiplexed display of both the main and delayed sweeps together. 51

4.8.7.13 Sweep trigger controls A switch selects the Trigger Source. It can be an external input, one of the vertical channels of a dual or multiple-trace 'scope, or the AC line (mains) frequency. Another switch enables or disables Auto trigger mode, or selects single sweep, if provided in the 'scope. Either a spring-return switch position or a pushbutton arms single sweeps. A Level control varies the voltage on the waveform which generates a trigger, and the Slope switch selects positive-going or negative-going polarity at the selected trigger level.

4.8.8 Waveforms obtained from the CRO:

Fig 4.28 Input waveform

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Fig 4.29 Waveform with noise

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Fig 4.30 Output waveform

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4.9 Capacitors:
A capacitor or condenser is a passive electronic component consisting of a pair of conductors separated by a dielectric (insulator). When a potential difference (voltage) exists across the conductors, an electric field is present in the dielectric. This field stores energy and produces a mechanical force between the conductors. The effect is greatest when there is a narrow separation between large areas of conductor, hence capacitor conductors are often called plates. An ideal capacitor is characterized by a single constant value, capacitance, which is measured in farads. This is the ratio of the electric charge on each conductor to the potential difference between them. In practice, the dielectric between the plates passes a small amount of leakage current. The conductors and leads introduce an equivalent series resistance and the dielectric has an electric field strength limit resulting in a breakdown voltage. Capacitors are widely used in electronic circuits to block the flow of direct current while allowing alternating current to pass, to filter out interference, to smooth the output of power supplies, and for many other purposes. They are used in resonant circuits in radio frequency equipment to select particular frequencies from a signal with many frequencies.

4.9.1 Theory of operation:
A capacitor consists of two conductors separated by a non-conductive region.The non-conductive substance is called the dielectric medium, although this may also mean a vacuum or a semiconductor depletion region chemically identical to the conductors. A capacitor is assumed to be self-contained and isolated, with no net electric charge and no influence from an external electric field. The conductors thus contain equal and opposite charges on their facing surfaces and the dielectric contains an electric field. The capacitor is a reasonably general model for electric fields within electric circuits. An ideal capacitor is wholly characterized by a constant capacitance C, defined as the ratio of charge ±Q on each conductor to the voltage V between them:

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Sometimes charge buildup affects the mechanics of the capacitor, causing the capacitance to vary. In this case, capacitance is defined in terms of incremental changes:

In SI units, a capacitance of one farad means that one coulomb of charge on each conductor causes a voltage of one volt across the device.

4.9.2 Energy storage:
Work must be done by an external influence to move charge between the conductors in a capacitor. When the external influence is removed, the charge separation persists and energy is stored in the electric field. If charge is later allowed to return to its equilibrium position, the energy is released. The work done in establishing the electric field, and hence the amount of energy stored, is given by:

4.9.3Parallel plate model:

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Fig 4.32 Dielectric is placed between two conducting plates, each of area A and with a separation of d. The simplest capacitor consists of two parallel conductive plates separated by a dielectric with permittivity ε (such as air). The model may also be used to make qualitative predictions for other device geometries. The plates are Fig 4.33 Several considered to extend uniformly over an area A and a capacitors in parallel. charge density ±ρ = ±Q/A exists on their surface. Assuming that the width of the plates is much greater than their separation d, the electric field near the centre of the device will be uniform with the magnitude E = ρ/ε. The voltage is defined as the line integral of the electric field between the plates

Solving this for C = Q/V reveals that capacitance increases with area and decreases with separation

. The capacitance is therefore greatest in devices made from materials with a high permittivity.

4.9.4 Networks: 4.9.4.1 Capacitors Connected in parallel:
Capacitors in a parallel configuration each have the same applied voltage. Their capacitances add up. Charge is apportioned among them by size. Using the schematic diagram to visualize parallel plates, it is apparent that each capacitor contributes to the total surface area.

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4.9.4.2 Capacitors Capacitors in series
Connected in series, the schematic diagram reveals that the separation distance, not the plate area, adds up. The capacitors each store instantaneous charge build-up equal to that of every other capacitor in the series. The total voltage difference from end to end is apportioned to each capacitor according to the inverse of its capacitance. The entire series acts as a capacitor smaller than any of its components.

Capacitors are combined in series to achieve a higher working voltage, for example for smoothing a high voltage power supply. The voltage ratings, which are based on plate separation, add up. In such an application, several series connections may in turn be connected in parallel, forming a matrix. The goal is to maximize the energy storage utility of each capacitor without overloading it. Series connection is also used to adapt electrolytic capacitors for AC use.

4.9.5 Ceramic capacitor:
In electronics, a ceramic capacitor is a capacitor constructed of alternating layers of metal and ceramic, with the ceramic material acting as the dielectric. The temperature coefficient depends on whether the dielectric is Class 1 or Class 2. A ceramic capacitor (especially the class 2) often has high dissipation factor, high frequency coefficient of dissipation.

Fig 4.34 Several capacitors in series

4.9.5.1 Construction:
A ceramic capacitor is a two-terminal, non-polar device. The classical ceramic capacitor is the "disc capacitor".Ceramic disc capacitors are in widespread use in electronic equipment, providing high capacity and small size at low price compared to other low value capacitor types. Ceramic capacitors come in various shapes and styles, including:
   

disc, resin coated, with through-hole leads multilayer rectangular block, surface mount bare leadless disc, sits in a slot in the PCB and is soldered in place, used for UHF applications tube shape, not popular now 58

4.9.5.2 Classes of ceramic capacitors
Three classes of ceramic capacitors are commonly available:








Class I capacitors: accurate, temperaturecompensating capacitors. They are the most stable over voltage, temperature, and to some extent, frequency. They also have the lowest losses. Class II capacitors: better volumetric efficiency, but lower accuracy and stability. A typical class II capacitor may change capacitance by 15% over a −55 °C to 85 °C temperature range. A typical class II capacitor will have a dissipation factor of 2.5%. It will have average to poor accuracy (from 10% down to +20/-80%). Class III capacitors: high volumetric efficiency, poor accuracy and stability. A typical class III capacitor will change capacitance by -22% to +56% over a temperature range of 10 °C to 55 °C. will have a dissipation factor of 4%. It will have fairly poor accuracy (commonly, 20%, or +80/20%). At one point, Class IV capacitors were also available, with worse electrical characteristics than Class III, but even better volumetric efficiency. They are now rather rare and considered obsolete.

Fig 4.35 An electrolytic capacitor

but

It

Fig 4.36 Axial lead (top) and radial lead (bottom) electrolytic capacitors

4.9.6 Electrolytic capacitor:
An electrolytic capacitor is a type of capacitor that uses an ionic conducting liquid as one of its plates with a larger capacitance per unit volume than other types. They are often referred to in electronics usage simply as "electrolytics". They are valuable in relatively high-current and low-frequency electrical circuits. This is especially the case in power-supply filters, where they store charge needed to moderate output voltage and current fluctuations in rectifier output. They are also widely used as coupling capacitors in circuits where AC should be conducted but DC should not. Electrolytic capacitors can have a very high capacitance, allowing filters made with them to have very low corner frequencies.

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4.9.6.1 Construction:
Aluminum electrolytic capacitors are constructed from two conducting aluminum foils, one of which is coated with an insulating oxide layer, and a paper spacer soaked in electrolyte. The foil insulated by the oxide layer is the anode while the liquid electrolyte and the second foil acts as the cathode. This stack is then rolled up, fitted with pin connectors and placed in a cylindrical aluminum casing. The two most popular geometries are axial leads coming from the center of each circular face of the cylinder, or two radial leads or lugs on one of the circular faces.

4.10 Resistors:
A resistor is a two-terminal electronic component that produces a voltage across its terminals that is proportional to the electric current passing through it in accordance with Ohm's law: V = IR Fig 4.37 Resistors Resistors are elements of electrical networks and electronic circuits and are ubiquitous in most electronic equipment. Practical resistors can be made of various compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as nickel/chrome). The primary characteristics of a resistor are the resistance, the tolerance, maximum working voltage and the power rating. Other characteristics include temperature coefficient, noise, and inductance. Less well-known is critical resistance, the value below which power dissipation limits the maximum permitted current flow, and above which the limit is applied voltage. Critical resistance is determined by the design, materials and dimensions of the resistor. Resistors can be integrated into hybrid and printed circuits, as well as integrated circuits. Size, and position of leads (or terminals) are relevant to equipment designers; resistors must be physically large enough not to overheat when dissipating their power.

4.10.1 Electronic symbol:

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4.10.2 Units:
The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm. Commonly used multiples and submultiples in electrical and electronic usage are the milliohm (1x10−3), kilohm (1x103), and megohm (1x106).

4.10.3 Theory of operation:
Ohm's law The behavior of an ideal resistor is dictated by the relationship specified in Ohm's law:

Ohm's law states that the voltage (V) across a resistor is proportional to the current (I) through it where the constant of proportionality is the resistance (R). Equivalently, Ohm's law can be stated:

This formulation of Ohm's law states that, when a voltage (V) is maintained across a resistance (R), a current (I) will flow through the resistance.

4.10.4 Series and parallel resistors:
Resistors in a parallel configuration each have the same potential difference (voltage). To find their total equivalent resistance (Req):

Fig 4.38 Parallel Combination of Resistors

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The parallel property can be represented in equations by two vertical lines "||" to simplify equations. For two resistors,

The current through resistors in series stays the same, but the voltage across each resistor can be different. The sum of the potential differences (voltage) is equal to the total voltage. To find their total resistance:

Fig 4.39 Series of Resistors

4.10.5 Power dissipation:
The power dissipated by a resistor (or the equivalent resistance of a resistor network) is calculated using the following: All three equations are equivalent. The first is derived from Joule's first law. Ohm’s Law derives the other two from that. The total amount of heat energy released is the integral of the power over time:

If the average power dissipated is more than the resistor can safely dissipate, the resistor may depart from its nominal resistance and may become damaged by overheating. Excessive power dissipation may raise the temperature of the resistor to a point where it burns out, which could cause a fire in adjacent components and materials. 62

4.11 Light emitting diodes:
A light-emitting diode (LED) is a semiconductor light source. LEDs are used as indicator lamps in many devices, and are increasingly used for lighting. Introduced as a practical electronic component in 1962, early LEDs emitted low-intensity red light, but modern versions are available across the visible, ultraviolet and infrared wavelengths, with very high brightness. The LED is based on the semiconductor diode. When a diode is forward biased (switched on), electrons are able to recombine with holes within the device, releasing energy in the form of photons. This effect is called electroluminescence and the colour of the light (corresponding to the energy of the photon) is determined by the energy gap of the semiconductor. An LED is usually small in area (less than 1 mm2), and integrated optical components are used to shape its radiation pattern and assist in reflection. LEDs present many advantages over incandescent light sources including lower energy consumption, longer lifetime, improved robustness, smaller size, faster switching, and greater durability and reliability. However, they are relatively expensive and require more precise current and heat management than traditional light sources. The compact size of LEDs has allowed new text and video displays and sensors to be developed, while their high switching rates are useful in advanced communications technology. The basic mechanism of electro- magnetic radiations from the device is injection luminescence. This occurs in two steps 1. Injection of minority carriers across the junction. 2. The radiative recombination of minority carriers. When the diode has zero bias under thermal equilibrium condition, the space charge ( or depletion) layer potential prevents the cross-over of large concentrations of mobile conduction band electrons and the valance band holes across the junction. When a forward bias is applied to the device the magnitude of this barrier potential is reduced making it possible for diffusion of electrons and holes across the junction. The minority carrier concentrations on both sides of the junction are considerably increased which increases the rate of recombination. This is responsible for electromagnetic radiation. Two type of radiative recombination mechanism are commonly encountered in LED’s depending upon the band gap characteristics of the semi-conductor material used. These are: 1. Direct recombination 2. Indirect recombination The emission of photons as a result of recombination of electrons and holes is possible only when energy and momentum are conserved. A photon can have considerable energy but its momentum hf/c is very small. Therefore the simplest and the most probable recombination process will be that where the electrons and holes have the same value of momentum. The situation exists in many group 3 to 5 compounds semiconductors which are of the direct band gap type. With the conduction band minimum and valance band maximum being at zero momentum position. 63

In indirect band gap semi-conductors the conduction band minimum and the valance band maximum occur at different values of momentum therefore, in this case it is necessary to involve a third particle to conserve a momentum. Therefore this process is known as indirect type recombination. Phonons and chemical impurities are used for this operation

4.11.1 BasicTechnology:

Fig. 4.40 Parts of an LED

Fig.4.41 Cross section of LED

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Fig. 4.42 I-V diagram for a diode an LED will begin to emit light when the on-voltage is exceeded. Typical on voltages are 2-3 Volt

4.11.2 Physics of LEDs:
Like a normal diode, the LED consists of a chip of semiconducting material doped with impurities to create a p-n junction. As in other diodes, current flows easily from the p-side, or anode, to the n-side, or cathode, but not in the reverse direction. Charge-carriers—electrons and holes—flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon. The wavelength of the light emitted, and therefore its color, depends on the band gap energy of the materials forming the p-n junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect band gap materials. The materials used for the LED have a direct band gap with energies corresponding to nearinfrared, visible or near-ultraviolet light. LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever-shorter wavelengths, producing light in a variety of colors. LEDs are usually built on an n-type substrate, with an electrode attached to the p-type layer deposited on its surface. P-type substrates, while less common, occur as well. Many commercial LEDs, especially GaN/InGaN, also use sapphire substrate. Most materials used for LED production have very high refractive indices. This means that much light will be reflected back into the material at the material/air surface interface. Therefore Light extraction in LEDs is an important aspect of LED production, subject to much research and development.

4.11.3 Colours and materials:
Conventional LEDs are made from a variety of inorganic semiconductor materials, the following table shows the available colours with wavelength range, voltage drop and material: Color Wavelength Voltage (V) 65 Semiconductor Material

(nm) Infrared λ > 760 ΔV < 1.9 1.63 < ΔV < 2.03 2.03 < ΔV < 2.10 2.10 < ΔV < 2.18 Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs) Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP)

Red

610 < λ < 760

Orange

590 < λ < 610

Yellow

570 < λ < 590

Green

500 < λ < 570

Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) 1.9[42] < ΔV < Gallium(III) phosphide (GaP) 4.0 Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP) 2.48 < ΔV < 3.7 2.76 < ΔV < 4.0 2.48 < ΔV < 3.7 Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate — (under development) Indium gallium nitride (InGaN) Dual blue/red LEDs, blue with red phosphor, or white with purple plastic

Blue

450 < λ < 500

Violet Purple

400 < λ < 450 multiple types

Ultraviolet λ < 400

Diamond (235 nm)[43] Boron nitride (215 nm)[44][45] Aluminium nitride (AlN) (210 nm)[46] 3.1 < ΔV < 4.4 Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm)[47] Blue/UV diode with yellow phosphor

White

Broad spectrum ΔV = 3.5

Table 4.5 Colour Coding of Resistors

4.11.4 Types:

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Fig 4.43 Types of LEDs LEDs are produced in a variety of shapes and sizes. The 5 mm cylindrical package (red, fifth from the left) is the most common, estimated at 80% of world production. The colour of the plastic lens is often the same as the actual colour of light emitted, but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have clear housings. There are also LEDs in SMT packages, such as those found on blinkies and on cell phone keypads (not shown). The main types of LEDs are miniature, high power devices and custom designs such as alphanumeric or multi-colour.

4.11.5 Miniature LEDs:

Fig.4.44 Different sized LEDs. 8 mm, 5 mm and 3 mm, with a wooden match-stick for scale. Main article: Miniature light-emitting diode These are mostly single-die LEDs used as indicators, and they come in various-sizes from 2 mm to 8 mm, through-hole and surface mount packages. They are usually simple in design, not requiring any separate cooling body. Typical current ratings range from around 1 mA to above 20 mA. The small 67

scale sets a natural upper boundary on power consumption due to heat caused by the high current density and need for heat sinking.

4.11.6 High power LEDs:
High power LEDs from Philips Lumileds Lighting Company mounted on a 21 mm star shaped base metal core PCB High power LEDs (HPLED) can be driven at currents from hundreds of mA to more than an ampere, compared with the tens of mA for other LEDs. Some can produce over a thousand lumens. Since overheating is destructive, the HPLEDs must be mounted on a heat sink to allow for heat dissipation. If the heat from a HPLED is not removed, the device will burn out in seconds. A single HPLED can often replace an incandescent bulb in a flashlight, or be set in an array to form a powerful LED lamp.

4.11.7 Mid-range LEDs:
Medium power LEDs are often through-hole mounted and used when a output of a few lumen is needed. They sometimes have the diode mounted to four leads (two cathode leads, two anode leads) for better heat conduction and carry an integrated lens. An example of this is the Super flux package, from Philips Lumileds. These LEDs are most commonly used in light panels, emergency lighting and automotive tail-lights. Due to the larger amount of metal in the LED, they are able to handle higher currents (around 100 mA). The higher current allows for the higher light output required for tail-lights and emergency lighting.

4.11.8 Application-specific variations:


Flashing LEDs are used as attention seeking indicators without requiring external electronics. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit which causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of a single colour, but more sophisticated devices can flash between multiple colours and even fade through a colour sequence using RGB colour mixing. Bi-colour LEDs are actually two different LEDs in one case. They consist of two dies connected to the same two leads antiparallel to each other. Current flow in one direction produces one colour, and current in the opposite direction produces the other colour. Alternating the two colours with sufficient frequency causes the appearance of a blended third colour. For example, a red/green LED operated in this fashion will colour blend to produce a yellow appearance.



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

Tri-color LEDs are two LEDs in one case, but the two LEDs are connected to separate leads
so that the two LEDs can be controlled independently and lit simultaneously. A three-lead arrangement is typical with one common lead (anode or cathode).



RGB LEDs contain red, green and blue emitters, generally using a four-wire connection with
one common lead (anode or cathode). These LEDs can have either common positive or common negative leads. Others however, have only two leads (positive and negative) and have a built in tiny electronic control unit.



Alphanumeric LED displays are available in seven-segment and starburst format. Sevensegment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Seven-segment LED displays were in widespread use in the 1970s and 1980s, but increasing use of liquid crystal displays, with their lower power consumption and greater display flexibility, has reduced the popularity of numeric and alphanumeric LED displays.

4.12 Power sources:
The current/voltage characteristic of an LED is similar to other diodes, in that the current is dependent exponentially on the voltage (see Shockley diode equation). This means that a small change in voltage can lead to a large change in current. If the maximum voltage rating is exceeded by a small amount the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is therefore to use constant current power supplies, or driving the LED at a voltage much below the maximum rating. Since most household power sources (batteries, mains) are not constant current sources, most LED fixtures must include a power converter. However, the I/V curve of nitride-based LEDs is quite steep above the knee and gives an If of a few mill amperes at a Vf of 3 V, making it possible to power a nitride-based LED from a 3 V battery such as a coin cell without the need for a current limiting resistor.

4.12.1 Electrical polarity:
As with all diodes, current flows easily from p-type to n-type material. However, no current flows and no light is produced if a small voltage is applied in the reverse direction. If the reverse voltage becomes large enough to exceed the breakdown voltage, a large current flows and the LED may be damaged. If the reverse current is sufficiently limited to avoid damage, the reverse-conducting LED is a useful noise diode.

4.12.2 Advantages:


Efficiency: LEDs produce more light per watt than incandescent bulbs. Their efficiency is
not affected by shape and size, unlike Fluorescent light bulbs or tubes.



Colour: LEDs can emit light of an intended colour without the use of the colour filters that
traditional lighting methods require. This is more efficient and can lower initial costs. 69



Size: LEDs can be very small (smaller than 2 mm2) and are easily populated onto printed
circuit boards.



On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full
brightness in microseconds. LEDs used in communications devices can have even faster response times.



Cycling: LEDs are ideal for use in applications that are subject to frequent on-off cycling,
unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.



Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering
the forward current.



Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR
that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.



Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt burn-out of
incandescent bulbs.



Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000
hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000–2,000 hours.

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

Shock resistance: LEDs, being solid state components, are difficult to damage with
external shock, unlike fluorescent and incandescent bulbs which are fragile.



Focus: The solid package of the LED can be designed to focus its light. Incandescent and
fluorescent sources often require an external reflector to collect light and direct it in a usable manner.



Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.

4.12.3 Disadvantages:


Some Fluorescent lamps can be more efficient.



High initial price: LEDs are currently more expensive, price per lumen, on an initial capital
cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed.



Temperature dependence: LED performance largely depends on the ambient
temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life. This is especially important when considering automotive, medical, and military applications where the device must operate over a large range of temperatures, and is required to have a low failure rate.



Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a
current below the rating. This can involve series resistors or current-regulated power supplies.



Light quality: Most cool-white LEDs have spectra that differ significantly from a black
body radiator like the sun or an incandescent light. The spike at 460 nm and dip at 500 nm can cause the colour of objects to be perceived differently under cool-white LED illumination than sunlight or incandescent sources, due to metamerism, red surfaces being rendered particularly 71

badly by typical phosphor based cool-white LEDs. However, the colour rendering properties of common fluorescent lamps are often inferior to what is now available in state-of-art white LEDs.



Area light source: LEDs do not approximate a ―point source‖ of light, but rather a
lambertian distribution. So LEDs are difficult to use in applications requiring a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.



Blue hazard: There is a concern that blue LEDs and cool-white LEDs are now capable of
exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photo biological Safety for Lamp and Lamp Systems.



Blue pollution: Because cool-white LEDs (i.e., LEDs with high colour temperature) emit
proportionally more blue light than conventional outdoor light sources such as high-pressure sodium lamps, the strong wavelength dependence of Rayleigh scattering means that cool-white LEDs can cause more light pollution than other light sources. The International Dark-Sky Association discourages the use of white light sources with correlated colour temperature above 3,000 K.

4.12.4 Applications:
LED panel light source used in an experiment on plant growth. The findings of such experiments may be used to grow food in space on long duration missions. Application of LEDs falls into four major categories:
   

Visual signal application where the light goes more or less directly from the LED to the human eye, to convey a message or meaning. Illumination where LED light is reflected from object to give visual response of these objects. Generate light for measuring and interacting with processes that do not involve the human visual system. Narrow band light sensors where the LED is operated in a reverse-bias mode and is responsive to incident light instead of emitting light.

4.12.5 Indicators and signs:

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The low energy consumption, low maintenance and small size of modern LEDs has led to applications as status indicators and displays on a variety of equipment and installations. Large area LED displays are used as stadium displays and as dynamic decorative displays. Thin, lightweight message displays are used at airports and railway stations, and as destination displays for trains, buses, trams, and ferries. The single colour light is well suited for traffic lights and signals, exit signs, emergency vehicle lighting, ships' lanterns and LED-based Christmas lights. In cold climates, LED traffic lights may remain snow covered. Red or yellow LEDs are used in indicator and alphanumeric displays in environments where night vision must be retained: aircraft cockpits, submarine and ship bridges, astronomy observatories, and in the field, e.g. night time animal watching and military field use. Because of their long life and fast switching times, LEDs have been used for automotive high-mounted brake lights and truck and bus brake lights and turn signals for some time, but many vehicles now use LEDs for their rear light clusters. The use of LEDs also has styling advantages because LEDs are capable of forming much thinner lights than incandescent lamps with parabolic reflectors. The significant improvement in the time taken to light up (perhaps 0.5 s faster than an incandescent bulb) improves safety by giving drivers more time to react. It has been reported that at normal highway speeds this equal’s one car length increased reaction time for the car behind. White LED headlamps are beginning to make an appearance. In an dual intensity circuit(i.e. rear markers and brakes) if the LEDs are not pulsed at a fast enough frequency, they can create a phantom array, where ghost images of the LED will appear if the eyes quickly scan across the array. Due to the relative cheapness of low output LEDs, they are also used in many temporary applications such as glow sticks, throwies, and the photonic textile Lumalive. Artists have also used LEDs for LED art. Weather/all-hazards radio receivers with Specific Area Message Encoding (SAME) have three LEDs: red for warnings, orange for watches, and yellow for advisories & statements whenever issued.

With the development of high efficiency and high power LEDs it has become possible to incorporate LEDs in lighting and illumination. Replacement light bulbs have been made as well as dedicated fixtures and LED lamps. LEDs are used as street lights and in other architectural lighting where colour changing is used. The mechanical robustness and long lifetime is used in automotive lighting on cars, motorcycles and on bicycle lights. LEDs have been used for lighting of streets and of parking garages. In 2007, the Italian village Torraca was the first place to convert its entire illumination system to LEDs. LEDs are used in aviation lighting. Airbus has used LED lighting in their Airbus A320 Enhanced since 2007, and Boeing plans its use in the 787. LEDs are also being used now in airport and heliport lighting. LED airport fixtures currently include medium intensity runway lights, runway centre line lights and obstruction lighting.

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LEDs are also suitable for backlighting for LCD televisions and lightweight laptop displays and light source for DLP projectors (See LED TV). RGB LEDs increase the colour gamut by as much as 45%. Screens for TV and computer displays can be made increasingly thin using LEDs for backlighting. LEDs are being used increasingly commonly for aquarium lighting. Particular for reef aquariums, LED lights provide an efficient light source with less heat output to help maintain optimal aquarium temperatures. LED-based aquarium fixtures also have the advantage of being manually adjustable to produce a specific colour-spectrum for ideal coloration of corals, fish, and invertebrates while optimizing photo synethically active radiation (PAR) which increases growth and sustainability of photosynthetic life such as corals, anemones, clams. These fixtures can be electronically programmed in order to simulate various lighting conditions throughout the day, reflecting phases of the sun and moon for a dynamic reef experience. LED fixtures typically cost up to five times as much as similarly rated fluorescent or high-intensity discharge lighting designed for reef aquariums and is not as high output to date. The lack of IR/heat radiation makes LEDs ideal for stage lights using banks of RGB LEDs that can easily change colour and decrease heating from traditional stage lighting, as well as medical lighting where IR-radiation can be harmful. Since LEDs are small, durable and require little power they are used in hand held devices such as flashlights. LED strobe lights or camera flashes operate at a safe, low voltage, as opposed to the 250+ volts commonly found in xenon flash lamp-based lighting. This is particularly applicable to cameras on mobile phones, where space is at a premium and bulky voltage-increasing circuitry is undesirable. LEDs are used for infrared illumination in night vision applications including security cameras. A ring of LEDs around a video camera, aimed forward into a retro reflective background, allows Chroma keying in video productions. LEDs are used for decorative lighting as well. Uses include but are not limited to indoor/outdoor decor, limousines, cargo trailers, conversion vans, cruise ships, RVs, boats, automobiles, and utility trucks. Decorative LED lighting can also come in the form of lighted company signage and step and aisle lighting in theatres and auditoriums.

4.12.6 Smart lighting:
Light can be used to transmit broadband data, which is already implemented in IrDA standards using infrared LEDs. Because LEDs can cycle on and off millions of times per second, they can, in effect, become wireless routers for data transport. Lasers can also be modulated in this manner.

4.12.7 Sustainable lighting:
Efficient lighting is needed for sustainable architecture. A 13 watt LED lamp produces 450 to 650 lumens. This is equivalent to a standard 40 watt incandescent bulb. A standard 40 W incandescent bulb has an expected lifespan of 1,000 hours while an LED can continue to operate with reduced efficiency for more than 50,000 hours, 50 times longer than the incandescent bulb.

4.12.8 Environmentally friendly options:
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A single kilowatt-hour of electricity will generate 1.34 pounds (610 g) of CO2 emissions.[92] Assuming the average light bulb is on for 10 hours a day, a single 40-watt incandescent bulb will generate 196 pounds (89 kg) of CO2 every year. The 13-watt LED equivalent will only be responsible for 63 pounds (29 kg) of CO2 over the same time span. A building’s carbon footprint from lighting can be reduced by 68% by exchanging all incandescent bulbs for new LEDs in warm climates. In cold climates, the energy saving may be lower, since more heating would be needed to compensate for the lower temperature. LEDs are also non-toxic unlike the more popular energy efficient bulb option: the compact fluorescent a.k.a. CFL which contains traces of harmful mercury. While the amount of mercury in a CFL is small, introducing less into the environment is preferable.

4.12.9 Economically sustainable:
LED light bulbs could be a cost-effective option for lighting a home or office space because of their very long lifetimes. Consumer use of LEDs as a replacement for conventional lighting system is currently hampered by the high cost and low efficiency of available products. 2009 DOE testing results showed an average efficacy of 35 lm/W, below that of typical CFLs, and as low as 9 lm/W, worse than standard incandescent. The high initial cost of the commercial LED bulb is due to the expensive sapphire substrate which is key to the production process. The sapphire apparatus must be coupled with a mirror-like collector to reflect light that would otherwise be wasted. In 2008, a materials science research team at Purdue University succeeded in producing LED bulbs with a substitute for the sapphire components.. The team used metal-coated silicon wafers with a builtin reflective layer of zirconium nitride to lessen the overall production cost of the LED. They predict that within a few years, LEDs produced with their revolutionary, new technique will be competitively priced with CFLs. The less expensive LED would not only be the best energy saver, but also a very economical bulb.

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CHAPTER 5: SOFTWARE IMPLEMENTATION
The evolution of stethoscopes over the last two centuries has assisted the medical community in solving common medical problems more effectively. Heart murmurs, lung diseases and other internal conditions were once considered difficult to diagnose. The stethoscope has gone from a simple, audio device made of inexpensive materials to an electronic device that allows hospital staff to focus their attention to important symptoms. The software implementation of our project has been done using National Instruments designed software Multisim. National Instruments, or NI (NASDAQ: NATI), is an American company with over 5,000 employees and direct operations in 41 countries. Headquartered in Austin, Texas, it is a producer of automated test equipment and virtual instrumentation software. Their software products include Lab VIEW, a graphical development environment, Lab Windows/CVI, which provides VI tools for C, Test Stand, a test sequencing and management environment, and Multisim (formerly Electronics Workbench), an electrical circuit analysis program. Their hardware products include VXI, VMEbus, and PXI frames and modules, as well as interfaces for GPIB, I²C, and other industrial automation standards. They also sell real-time embedded controllers, including Compact Field Point and CompactRIO. Common applications include data acquisition, instrument control and machine vision. Multisim provides the user with a virtual breadboard with a varied number of components option to be used for easy designing of circuits. Multisim equips educators, students, and professionals with the tools to analyze circuit behavior. The intuitive and easy-to-use software platform combines schematic capture and industry-standard SPICE simulation into a single integrated environment. Multisim abstracts the complexities and difficulties of traditional syntax-based simulation, so you no longer need to be an expert in SPICE to simulate and analyze circuits. Multisim is available in two distinct versions to meet the teaching needs of educators or the design needs of professionals. Multisim provides with a lot of options of components that are already a part of the software so that little lefts to be done on the part of the designer. All other components that are needed can be easily simulated on the virtual bread board provided in it. We simulated the following circuit diagram as per the requirement of the project:

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Fig. 5.1 Simulated Diagrams on Multisim

The components used sequentially are: 1. Microphone 2. Operational amplifier 3. Low pass filter. 4. High pass filter. 5. Analog to digital converter. 6. Cathode ray oscilloscope. 7. LED Using this simulation we can record a heart sound, analyze it and view the waveform on the oscilloscope. The maximum amount of time for which heart sound can be recorded in multisim 11.0 is 9 seconds.

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Fig.5.2 Input Signal with Microphone

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Fig.5.3 Signal after Passing through Low Pass Filter

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Fig.5.4 Signal after Passing through Band Pass Filter

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CHAPTER 6: FUTURE SCOPE
We are not just doing a project; we are developing a useful product. This provides an opportunity for students to find out what really motivates good engineers: the thrill of creating something that other people will be excited about and want to use. Nearly all medical personnel actively involved in the treatment and diagnosis of patients use stethoscopes on a daily basis. Stethoscopes are used for pulse measuring, blood pressure monitoring, and diagnosis of cardiovascular, respiratory, and digestive diseases. The majority of stethoscopes currently on the market are acoustic devices that use purely passive mechanical parts to isolate and focus sound generated by the body. Though these methods have been used for years, the simplicity of such devices is overshadowed by poor sound quality, discomfort, and high cost. These devices are also difficult to interface with modern technologies such as computers to record and analyze body sounds. Therefore electronic stethoscopes need to be designed that are comparable in cost, has better acoustic response, and can interface with modern technologies better than the current acoustic stethoscope. Electronic stethoscopes have been used for the last couple of decades, although it is only recently that they have gained any acceptance in everyday medical practice. This is because historical electronic stethoscopes were typically bulky and non-portable, requiring large separate cases to house the electronics. Because of this, electronic stethoscopes were only used in research and advanced diagnostic settings. Recent advances in microelectronics have led to smaller, more portable devices, and a subsequent rise in electronic stethoscope usage in everyday medicine. This project is our effort towards designing such an electronic stethoscope which not only interfaces with computers and other display devices easily but is also cost effective and easy to use. We have used the simplest of components known so that the designing of this stethoscope can be universal and have simulated it through multisim software which is rather simple software to work on. So considering the widespread use of stethoscopes for diagnostic purposes we hope the stethoscope we designed to be a success keeping in mind the its advantages over the acoustic and other bulky stethoscopes now being used. The various advantages which this stethoscope has over others are: 1. This provides for better noise cancellation so a better signal is obtained. 2. It is very easy to be implemented both in terms of software and hardware. 3. It is rather compact and portable. 4. It is very cost effective. 5. A heart beat can be recorded and analyzed later on. 6. This provides for the viewing and storing of the waveform which is not possible in an acoustic stethoscope. This design can be further improved and made a lot better with slight variations in the components used. More advanced components if used can result in a more compact and error free device.

REFERENCES

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LIST OF FIGURES Fig 1.1....................................................................................................................................1 Fig 1.2....................................................................................................................................2 Fig 2.1....................................................................................................................................3 Fig 2.2....................................................................................................................................3 Fig 2.3....................................................................................................................................5 Fig 2.4....................................................................................................................................6 Fig 2.5....................................................................................................................................7 Fig 2.6....................................................................................................................................8 Fig 2.7....................................................................................................................................8 Fig 2.8....................................................................................................................................9 Fig 2.9....................................................................................................................................10 Fig 2.10..................................................................................................................................11 Fig 2.11..................................................................................................................................13 Fig 4.1....................................................................................................................................16 83

Fig 4.2....................................................................................................................................17 Fig 4.3....................................................................................................................................18 Fig 4.4....................................................................................................................................19 Fig 4.5....................................................................................................................................19 Fig 4.6....................................................................................................................................19 Fig 4.7....................................................................................................................................21 Fig 4.8....................................................................................................................................22 Fig 4.9....................................................................................................................................26 Fig 4.10..................................................................................................................................27 Fig 4.11..................................................................................................................................28 Fig 4.12..................................................................................................................................30 Fig 4.13..................................................................................................................................33 Fig 4.14..................................................................................................................................33 Fig 4.15..................................................................................................................................34 Fig 4.16..................................................................................................................................35 Fig 4.17..................................................................................................................................36 Fig 4.18..................................................................................................................................38 Fig 4.19..................................................................................................................................39 Fig 4.20..................................................................................................................................40 Fig 4.21..................................................................................................................................42 Fig 4.22..................................................................................................................................43 Fig 4.23..................................................................................................................................43 Fig 4.24..................................................................................................................................44 Fig 4.25..................................................................................................................................44 Fig 4.26..................................................................................................................................45 Fig 4.27..................................................................................................................................46 Fig 4.28..................................................................................................................................53 Fig 4.29..................................................................................................................................54 Fig 4.30..................................................................................................................................55 Fig 4.31..................................................................................................................................56 Fig 4.32..................................................................................................................................57 Fig 4.33..................................................................................................................................58 Fig 4.34..................................................................................................................................58 Fig 4.35..................................................................................................................................60 Fig 4.36..................................................................................................................................60 Fig 4.37..................................................................................................................................61 Fig 4.38..................................................................................................................................62 Fig 4.39..................................................................................................................................62 Fig 4.40..................................................................................................................................64 Fig 4.41..................................................................................................................................65 Fig 4.42..................................................................................................................................65 Fig 4.43..................................................................................................................................67 Fig 4.44..................................................................................................................................68 Fig 5.1....................................................................................................................................78 Fig 5.2....................................................................................................................................79 Fig 5.3....................................................................................................................................80 84

Fig 5.4....................................................................................................................................81

LIST OF TABLES Table 2.1...................................................................................................................................14 Table 3.1...................................................................................................................................17 Table 4.2...................................................................................................................................25 Table 4.3...................................................................................................................................28 Table 4.4...................................................................................................................................32 Table 4.5...................................................................................................................................66

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