SMS Controlled Home Automation Systems

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ABSTRACT
GSM technology can provide a sophisticated theft alert system for bank locker system. The embedded I/O unit automates the inner door and entry door. The inner door always kept open. There are two modes in this project one is normal mode and another one is security mode. In normal mode an authorized person can open the locker key and he can close the entry door. At that time GSM never send the message to the required person. If any person tries to open the locker key in security mode, the inner door will be closed automatically and SMS is transferred to the required person’s hand phone. After identifying the theft an authorized person can automate the inner door through the SMS. The GSM module is connected with the microcontroller through serial port. Using ‘AT’ commands the SMS is transferred to the GSM module. The GSM module converts the digital information into airborne signals. Through GSM network the SMS is transferred to the required person’s hand phone. This system offers better solution for the Bank security system and also it will help you to track the intruder. The system includes a ATMEGA8 microcontroller which is interfaced with GSM module and relays which can allow human to switch on the devices by sending a message and also gives the feedback message about the device status.

BLOCK DIAGRAM:

2. INTRODUCTION TO EMBEDDED SYSTEM

An embedded system is a special-purpose computer system designed to perform one or a few dedicated functions, sometimes with real-time computing constraints. It is usually embedded as part of a complete device including hardware and mechanical parts. In contrast, a generalpurpose computer, such as a personal computer, can do many different tasks depending on programming. Embedded systems have become very important today as they control many of the common devices we use. Since the embedded system is dedicated to specific tasks, design engineers can optimize it, reducing the size and cost of the product, or increasing the reliability and performance. Some embedded systems are mass-produced, benefiting from economies of scale. Physically, embedded systems range from portable devices such as digital watches and MP3 players, to large stationary installations like traffic lights, factory controllers, or the systems controlling nuclear power plants. Complexity varies from low, with a single microcontroller chip, to very high with multiple units, peripherals and networks mounted inside a large chassis or enclosure. In general, "embedded system" is not an exactly defined term, as many systems have some element of programmability. For example, Handheld computers share some elements with embedded systems — such as the operating systems and microprocessors which power them — but are not truly embedded systems, because they allow different applications to be loaded and peripherals to be connected. An embedded system is some combination of computer hardware and software, either fixed in capability or programmable, that is specifically designed for a particular kind of application device. Industrial machines, automobiles, medical equipment, cameras, household appliances, airplanes, vending machines, and toys (as well as the more obvious cellular phone and PDA) are among the myriad possible hosts of an embedded system. Embedded systems that are programmable are provided with a programming interface, and embedded systems programming is a specialized occupation.

Certain operating systems or language platforms are tailored for the embedded market, such as Embedded Java and Windows XP Embedded. However, some low-end consumer products use very inexpensive microprocessors and limited storage, with the application and operating system both part of a single program. The program is written permanently into the system's memory in this case, rather than being loaded into RAM (random access memory), as programs on a personal computer are. 2.1 APPLICATIONS OF EMBEDDED SYSTEM We are living in the Embedded World. You are surrounded with many embedded products and your daily life largely depends on the proper functioning of these gadgets. Television, Radio, CD player of your living room, Washing Machine or Microwave Oven in your kitchen, Card readers, Access Controllers, Palm devices of your work space enable you to do many of your tasks very effectively. Apart from all these, many controllers embedded in your car take care of car operations between the bumpers and most of the times you tend to ignore all these controllers. In recent days, you are showered with variety of information about these embedded controllers in many places. All kinds of magazines and journals regularly dish out details about latest technologies, new devices; fast applications which make you believe that your basic survival is controlled by these embedded products. Now you can agree to the fact that these embedded products have successfully invaded into our world. You must be wondering about these embedded controllers or systems. What is this Embedded System? The computer you use to compose your mails, or create a document or analyze the database is known as the standard desktop computer. These desktop computers are manufactured to serve many purposes and applications. You need to install the relevant software to get the required processing facility. So, these desktop computers can do many things. In contrast, embedded controllers carryout a specific work for which they are designed. Most of the time, engineers design these embedded controllers with a specific goal in mind. So these controllers cannot be used in any other place. Theoretically, an embedded controller is a combination of a piece of microprocessor based hardware and the suitable software to undertake a specific task.

These days designers have many choices in microprocessors/microcontrollers. Especially, in 8 bit and 32 bit, the available variety really may overwhelm even an experienced designer. Selecting a right microprocessor may turn out as a most difficult first step and it is getting complicated as new devices continue to pop-up very often. In the 8 bit segment, the most popular and used architecture is Intel's 8031. Market acceptance of this particular family has driven many semiconductor manufacturers to develop something new based on this particular architecture. Even after 25 years of existence, semiconductor manufacturers still come out with some kind of device using this 8031 core.  Military and aerospace software applications From in-orbit embedded systems to jumbo jets to vital battlefield networks, designers of mission-critical aerospace and defense systems requiring real-time performance, scalability, and high-availability facilities consistently turn to the LynxOS® RTOS and the LynxOS-178 RTOS for software certification to DO-178B. Rich in system resources and networking services, LynxOS provides an off-the-shelf software platform with hard real-time response backed by powerful distributed computing (CORBA), high reliability, software certification, and long-term support options.The LynxOS-178 RTOS for software certification, based on the RTCA DO-178B standard, assists developers in gaining certification for their mission- and safety-critical systems. Real-time systems programmers get a boost with LynuxWorks' DO-178B RTOS training courses.LynxOS-178 is the first DO-178B and EUROCAE/ED-12B certifiable, POSIX®-compatible RTOS solution.
 Communications applications

"Five-nines" availability, CompactPCI hot swap support, and hard real-time response— LynxOS delivers on these key requirements and more for today's carrier-class systems. Scalable kernel configurations, distributed computing capabilities, integrated communications stacks, and fault-management facilities make LynxOS the ideal choice for companies looking for a single operating system for all embedded telecommunications applications—from complex central controllers to simple line/trunk cards.

LynuxWorks Jumpstarts for Communications package enables OEMs to rapidly develop mission-critical communications equipment, with pre-integrated, state-of-the-art, data networking and porting software components—including source code for easy customization. The Lynx Certifiable Stack (LCS) is a secure TCP/IP protocol stack designed especially for applications where standards certification is required.
 Electronics applications and consumer devices

As the number of powerful embedded processors in consumer devices continues to rise, the Blue Cat® Linux® operating system provides a highly reliable and royalty-free option for systems designers. And as the wireless appliance revolution rolls on, web-enabled navigation systems, radios, personal communication devices, phones and PDAs all benefit from the cost-effective dependability, proven stability and full product life-cycle support opportunities associated with Blue Cat embedded Linux. Blue Cat has teamed up with industry leaders to make it easier to build Linux mobile phones with Java integration. For makers of low-cost consumer electronic devices who wish to integrate the LynxOS realtime operating system into their products, we offer special MSRP-based pricing to reduce royalty fees to a negligible portion of the device's MSRP.
 Industrial automation and process control software

Designers of industrial and process control systems know from experience that LynuxWorks operating systems provide the security and reliability that their industrial applications require.From ISO 9001 certification to fault-tolerance, POSIX conformance, secure partitioning and high availability, we've got it all. Take advantage of our 20 years of experience.

3. MICROCONTROLLER DETAILS

What is the difference between a Microprocessor and Microcontroller? By microprocessor is meant the general purpose Microprocessors such as Intel's X86 family (8086, 80286, 80386, 80486, and the Pentium) or Motorola's 680X0 family (68000, 68010, 68020, 68030, 68040, etc). These microprocessors contain no RAM, no ROM, and no I/O ports on the chip itself. For this reason, they are commonly referred to as general-purpose Microprocessors. A system designer using a general-purpose microprocessor such as the Pentium or the 68040 must add RAM, ROM, I/O ports, and timers externally to make them functional. Although the addition of external RAM, ROM, and I/O ports makes these systems bulkier and much more expensive, they have the advantage of versatility such that the designer can decide on the amount of RAM, ROM and I/O ports needed to fit the task at hand. This is not the case with Microcontrollers. A Microcontroller has a CPU (a microprocessor) in addition to a fixed amount of RAM, ROM, I/O ports, and a timer all on a single chip. In other words, the processor, the RAM, ROM, I/O ports and the timer are all embedded together on one chip. In many applications, for example a TV remote control, there is no need for the computing power of a 486 or even an 8086 microprocessor. These applications most often require some I/O operations to read signals and turn on and off certain bits. 3.1 MICROCONTROLLERS FOR EMBEDDED SYSTEMS In the Literature discussing microprocessors, we often see the term Embedded System. Microprocessors and Microcontrollers are widely used in embedded system products. An embedded system product uses a microprocessor (or Microcontroller) to do one task only. A printer is an example of embedded system since the processor inside it performs one task only; namely getting the data and printing it. Contrast this with a Pentium based PC. A PC can be used for any number of applications such as word processor, print-server, bank teller terminal, Video game, network server, or Internet terminal. Software for a variety of applications can be loaded and run. Of course the reason a pc can perform myriad tasks is that it has RAM memory and an operating system that loads the application software into RAM memory and lets the CPU run it. In an Embedded system, there is only one application software that is typically burned into ROM. An x86 PC contains or is connected to various embedded products such as keyboard,

printer, modem, disk controller, sound card, CD-ROM drives, mouse, and so on. Each one of these peripherals has a Microcontroller inside it that performs only one task. For example, inside every mouse there is a Microcontroller to perform the task of finding the mouse position and sending it to the PC. Table 1-1 lists some embedded products.

3.2 Typical Microcontroller Architecture and Features The basic internal designs of microcontrollers are pretty similar. Figure1 shows the block diagram of a typical microcontroller. All components are connected via an internal bus and are all integrated on one chip. The modules are connected to the outside world via I/O pins.

Fig 3.2.1: Basic Layout of Microcontroller The following list contains the modules typically found in a microcontroller. You can find a more detailed description of these components in later sections. Processor Core: The CPU of the controller. It contains the arithmetic logic unit, the control unit, and the registers (stack pointer, program counter, accumulator register, register file . . .). Memory: The memory is sometimes split into program memory and data memory. In larger controllers, a DMA controller handles data transfers between peripheral components and the memory.

Interrupt Controller: Interrupts are useful for interrupting the normal program flow in case of (important) external or internal events. In conjunction with sleep modes, they help to conserve power. Timer/Counter: Most controllers have at least one and more likely 2-3 Timer/Counters, which can be used to timestamp events, measure intervals, or count events. Many controllers also contain PWM (pulse width modulation) outputs, which can be used to drive motors or for safe breaking (antilock brake system, ABS). Furthermore the PWM output can, in conjunction with an external filter, be used to realize a cheap digital/analog converter. Digital I/O: Parallel digital I/O ports are one of the main features of microcontrollers. The number of I/O pins varies from 3-4 to over 90, depending on the controller family and the controller type. Analog I/O: Apart from a few small controllers, most microcontrollers have integrated analog/digital converters, which differ in the number of channels (2-16) and their resolution (8-12 bits). The analog module also generally features an analog comparator. In some cases, the microcontroller includes digital/analog converters. 3.3 The UART: What it is and how it works The Universal Asynchronous Receiver/Transmitter (UART) controller is the key component of the serial communications subsystem of a computer. The UART takes bytes of data and transmits the individual bits in a sequential fashion. At the destination, a second UART reassembles the bits into complete bytes. Serial transmission is commonly used with modems and for non-networked communication between computers, terminals and other devices. There are two primary forms of serial transmission: Synchronous and Asynchronous. Depending on the modes that are supported by the hardware, the name of the communication sub-system will usually include a A if it supports Asynchronous communications, and a S if it supports Synchronous communications. Both forms are described below.

3.4 Synchronous Serial Transmission Synchronous serial transmission requires that the sender and receiver share a clock with one another, or that the sender provide a strobe or other timing signal so that the receiver knows when to “read” the next bit of the data. In most forms of serial Synchronous communication, if there is no data available at a given instant to transmit, a fill character must be sent instead so that data is always being transmitted. Synchronous communication is usually more efficient because only data bits are transmitted between sender and receiver, and synchronous communication can be more costly if extra wiring and circuits are required to share a clock signal between the sender and receiver. A form of Synchronous transmission is used with printers and fixed disk devices in that the data is sent on one set of wires while a clock or strobe is sent on a different wire. Printers and fixed disk devices are not normally serial devices because most fixed disk interface standards send an entire word of data for each clock or strobe signal by using a separate wire for each bit of the word. In the PC industry, these are known as Parallel devices. The standard serial communications hardware in the PC does not support Synchronous operations. This mode is described here for comparison purposes only 3.5 Asynchronous Serial Transmission Asynchronous transmission allows data to be transmitted without the sender having to send a clock signal to the receiver. Instead, the sender and receiver must agree on timing parameters in advance and special bits are added to each word which are used to synchronize the sending and receiving units. When a word is given to the UART for Asynchronous transmissions, a bit called the "Start Bit" is added to the beginning of each word that is to be transmitted. The Start Bit is used to alert the receiver that a word of data is about to be sent, and to force the clock in the receiver into synchronization with the clock in the transmitter. These two clocks must be accurate enough to not have the frequency drift by more than 10% during the transmission of the remaining bits in the word. (This requirement was set in the days of mechanical teleprinters and is easily met by modern electronic equipment.)

After the Start Bit, the individual bits of the word of data are sent, with the Least Significant Bit (LSB) being sent first. Each bit in the transmission is transmitted for exactly the same amount of time as all of the other bits, and the receiver “looks” at the wire at approximately halfway through the period assigned to each bit to determine if the bit is a 1 or a 0. For example, if it takes two seconds to send each bit, the receiver will examine the signal to determine if it is a 1 or a 0 after one second has passed, then it will wait two seconds and then examine the value of the next bit, and so on. The sender does not know when the receiver has “looked” at the value of the bit. The sender only knows when the clock says to begin transmitting the next bit of the word.When the entire data word has been sent, the transmitter may add a Parity Bit that the transmitter generates. The Parity Bit may be used by the receiver to perform simple error checking. Then at least one Stop Bit is sent by the transmitter. When the receiver has received all of the bits in the data word, it may check for the Parity Bits (both sender and receiver must agree on whether a Parity Bit is to be used), and then the receiver looks for a Stop Bit. If the Stop Bit does not appear when it is supposed to, the UART considers the entire word to be garbled and will report a Framing Error to the host processor when the data word is read. The usual cause of a Framing Error is that the sender and receiver clocks were not running at the same speed, or that the signal was interrupted.Regardless of whether the data was received correctly or not, the UART automatically discards the Start, Parity and Stop bits. If the sender and receiver are configured identically, these bits are not passed to the host.If another word is ready for transmission, the Start Bit for the new word can be sent as soon as the Stop Bit for the previous word has been sent. Because asynchronous data is “self synchronizing”, if there is no data to transmit, the transmission line can be idle.

3.6 MICROCONTROLLER ATmega8

The ATmega8 is a low-power CMOS 8-bit microcontroller, based on the AVR enhanced RISC architecture. By executing powerful instructions in a single clock cycle, the ATmega8 achieves throughputs approaching 1 MIPS per MHz allowing the system designer to optimize power consumption versus processing speed.

Fig. 3.6.1 Pin Out of ATmega8 AVR core combines a rich instruction set with 32 general-purpose working registers. All the 32 registers are directly connected to the Arithmetic Logic Unit (ALU), allowing two independent registers to be accessed in one single instruction executed in one clock cycle. The resulting architecture is more code efficient while achieving throughputs up to ten times faster than conventional CISC microcontrollers.

3.6.1 The ATmega8 provides the following prominent features • High-performance, Low-power AVR® 8-bit Microcontroller • Advanced RISC Architecture   130 Powerful Instructions – Most Single-clock Cycle Execution 32 x 8 General Purpose Working Registers

  

Fully Static Operation Up to 16 MIPS Throughput at 16 MHz On-chip 2-cycle Multiplier

• High Endurance Non-volatile Memory segments          8K Bytes of In-System Self-programmable Flash program memory 512 Bytes EEPROM 1K Byte Internal SRAM Write/Erase Cycles: 10,000 Flash/100,000 EEPROM Data retention: 20 years at 85°C/100 years at 25°C Optional Boot Code Section with Independent Lock Bits In-System Programming by On-chip Boot Program True Read-While-Write Operation Programming Lock for Software Security

• Peripheral Features   Two 8-bit Timer/Counters with Separate Prescaler, one Compare Mode One 16-bit Timer/Counter with Separate Prescaler, Compare Mode, and Capture Mode            Real Time Counter with Separate Oscillator Three PWM Channels 8-channel ADC in TQFP and QFN/MLF package Eight Channels 10-bit Accuracy 6-channel ADC in PDIP package Six Channels 10-bit Accuracy Byte-oriented Two-wire Serial Interface Programmable Serial USART Master/Slave SPI Serial Interface Programmable Watchdog Timer with Separate On-chip Oscillator On-chip Analog Comparator

• Special Microcontroller Features     Power-on Reset and Programmable Brown-out Detection Internal Calibrated RC Oscillator External and Internal Interrupt Sources Five Sleep Modes: Idle, ADC Noise Reduction, Power-save, Power-down, and Standby

• I/O and Packages   23 Programmable I/O Lines 28-lead PDIP, 32-lead TQFP, and 32-pad QFN/MLF

• Operating Voltages 


2.7 - 5.5V (ATmega8L) 4.5 - 5.5V (ATmega8)

• Speed Grades   0 - 8 MHz (ATmega8L) 0 - 16 MHz (ATmega8)

• Power Consumption at 4 Mhz, 3V, 25°C    Active: 3.6 mA Idle Mode: 1.0 mA Power-down Mode: 0.5 μA

The Idle mode stops the CPU while allowing the USART, Two-wire interface, A/D Converter, SRAM, Timer/Counters, SPI port, and interrupt system to continue functioning. The Power-down mode saves the register contents but freezes the Oscillator, disabling all other chip functions until the next External Interrupt or Hardware Reset. In Power-save mode, the Asynchronous Timer continues to run, allowing the user to maintain a timer base while the rest of the device is sleeping. The ADC Noise Reduction mode stops the CPU and all I/O modules except Asynchronous Timer and ADC, to minimize switching noise during ADC conversions.

By combining an 8-bit RISC CPU with In-System Self-Programmable Flash on a monolithic chip, the Atmel ATmega8 is a powerful microcontroller that provides a highly flexible and cost-effective solution to many embedded control applications. 3.6.2 Pin Descriptions VCC- Digital supply voltage. GND- Ground. Port B (PB7..PB0) XTAL1/XTAL2/TOSC1/TOSC2 Port B is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port B output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port B pins that are externally pulled low will source current if the pull-up resistors are activated. The Port B pins are tri-stated when a reset condition becomes active, even if the clock is not running. Depending on the clock selection fuse settings, PB6 can be used as input to the inverting Oscillator amplifier and input to the internal clock operating circuit. Depending on the clock selection fuse settings, PB7 can be used as output from the inverting Oscillator amplifier. If the Internal Calibrated RC Oscillator is used as chip clock source, PB7..6 is used as TOSC2..1 input for the Asynchronous Timer/Counter2 if the AS2 bit in ASSR is set. Port C (PC5..PC0) Port C is an 7-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port C output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port C pins that are externally pulled low will source current if the pull-up resistors are activated. The Port C pins are tri-stated when a reset condition becomes active, even if the clock is not running. PC6/RESET If the RSTDISBL Fuse is programmed, PC6 is used as an I/O pin. Note that the electrical characteristics of PC6 differ from those of the other pins of Port C. If the RSTDISBL Fuse is unprogrammed, PC6 is used as a Reset input. A low level on this pin for longer than the minimum pulse length will generate a Reset, even if the clock is not running

Port D (PD7..PD0) Port D is an 8-bit bi-directional I/O port with internal pull-up resistors (selected for each bit). The Port D output buffers have symmetrical drive characteristics with both high sink and source capability. As inputs, Port D pins that are externally pulled low will source current if the pull-up resistors are activated. The Port D pins are tri-stated when a reset condition becomes active, even if the clock is not running. RESET Reset input. A low level on this pin for longer than the minimum pulse length will generate a reset, even if the clock is not running. Shorter pulses are not guaranteed to generate a reset. AVCC AVCC is the supply voltage pin for the A/D Converter, Port C (3..0), and ADC (7..6). It should be externally connected to VCC, even if the ADC is not used. If the ADC is used, it should be connected to VCC through a low-pass filter. Note that Port C (5..4) use digital supply voltage, VCC. AREF AREF is the analog reference pin for the A/D Converter. ADC7..6 (TQFP and QFN/MLF Package Only) In the TQFP and QFN/MLF package, ADC7..6 serve as analog inputs to the A/D converter. These pins are powered from the analog supply and serve as 10-bit ADC channels.

Fig 3.6.2.1 BLOCK DIAGRAM OF AT mega 8 The ATmega8 AVR is supported with a full suite of program and system development tools including: C compilers, macro assemblers, program debugger/simulators, in-circuit emulators, and evaluation kits.

A flexible interrupt module has its control registers in the I/O space with an additional global interrupt enable bit in the Status Register. All interrupts have a separate interrupt vector in the interrupt vector table. The interrupts have priority in accordance with their interrupt vector position. The lower the interrupt vector address, the higher the priority. The I/O Memory can be accessed directly, or as the Data Space locations following those of the Register File, 20h - 5Fh 3.6.3 I/O PORTS All AVR ports have true Read-Modify-Write functionality when used as general digital I/O ports. This means that the direction of one port pin can be changed without unintentionally changing the direction of any other pin with the SBI and CBI instructions. The same applies when changing drive value (if configured as output) or enabling/disabling of pull-up resistors (if configured as input). Each output buffer has symmetrical drive characteristics with both high sink and source capability. The pin driver is strong enough to drive LED displays directly. All port pins have individually selectable pull-up resistors with a supply-voltage invariant resistance. All I/O pins have protection diodes to both VCC and Ground. All registers and bit references in this section are written in general form. Three I/O memory address locations are allocated for each port, one each for the Data Register – PORTx, Data Direction Register – DDRx, and the Port Input Pins – PINx. The Port Input Pins I/O location is read only, while the Data Register and the Data Direction Register are read/write. In addition, the Pull-up Disable – PUD bit in SFIOR disables the pull-up function for all pins in all ports when set. Most port pins are multiplexed with alternate functions for the peripheral features on the device. Enabling the alternate function of some of the port pins does not affect the use of the other pins in the port as general digital I/O. 3.6.3.1 Ports as general purpose I/O: The ports are bi-directional I/O ports with optional internal pull-ups. Each port pin consists of three register bits: DDxn, PORTxn, and PINxn. The DDxn bits are accessed at the DDRx I/O address, the PORTxn bits at the PORTx I/O address, and the PINxn bits at the PINxI/O address. The DDxn bit in the DDRx Register selects the direction of this pin. If DDxn is written logic one, Pxn is configured as an output pin. If DDxn is written logic zero, Pxn is configured as an input pin. If

PORTxn is written logic one when the pin is configured as an input pin, the pull-up resistor is activated. To switch the pull-up resistor off, PORTxn has to be written logic zero or the pin has to be configured as an output pin. The port pins are tri-stated when a reset condition becomes active, even if no clocks are running. If PORTxn is written logic one when the pin is configured as an output pin, the port pin is driven high (one). If PORTxn is written logic zero when the pin is configured as an output pin, the port pin is driven low (zero). Normally, the pull-up enabled state is fully acceptable, as a high-impedance environment will not notice the difference between a strong high driver and a pull-up. If this is not the case, the PUD bit in the SFIOR Register can be set to disable all pull-ups in all ports.

Table 3.6.3.1 Selection Table

Fig 3.6.3.1 General

I/O Block diagram

Independent of the setting of Data Direction bit DDxn, the port pin can be read through the PINxn Register bit. The PINxn Register bit and the preceding latch constitute a synchronizer. This is needed to avoid meta stability if the physical pin changes value near the edge of the internal clock, but it also introduces a delay. The maximum and minimum propagation delays are denoted tpd, max and tpd, min respectively.

Fig 3.6.3.2 Timing Diagram while Reading an 1 Consider the clock period starting shortly after the first falling edge of the system clock. The latch is closed when the clock is low, and goes transparent when the clock is high, as indicated by the shaded region of the “SYNC LATCH” signal. The signal value is latched when the system clock goes low. It is clocked into the PINxn Register at the succeeding positive clock edge. As indicated by the two arrows tpd, max and tpd, min, a single signal transition on the pin will be delayed between ½ and 1½ system clock period depending upon the time of assertion.When reading back a software assigned pin value, a nop instruction must be inserted. The out instruction sets the “SYNC LATCH” signal at the positive edge of the clock. In this case, the delay tpd through the synchronizer is one system clock period.

Fig 3.6.3.3 Timing diagram when Reading an

3.7 8-bit Timer/Counter Register Description Timer/Counter Control Register – TCCR2

fig 3.7.1 TCCR 1 Bit 7 – FOC2: Force Output Compare The FOC2 bit is only active when the WGM bits specify a non-PWM mode. However, for ensuring compatibility with future devices, this bit must be set to zero when TCCR2 is written when operating in PWM mode. When writing a logical one to the FOC2 bit, an immediate Compare Match is forced on the waveform generation unit. The OC2 output is changed according to its COM21:0 bits setting. Note that the FOC2 bit is implemented as a strobe. Therefore it is the value present in the COM21:0 bits that determines the effect of the forced compare. A FOC2 strobe will not generate any interrupt, nor will it clear the timer in CTC mode using OCR2 as TOP. The FOC2 bit is always read as zero. • Bit 6,3 – WGM21:0: Waveform Generation Mode These bits control the counting sequence of the counter, the source for the maximum (TOP) counter value, and what type of waveform generation to be used. Modes of operation supported by the Timer/Counter unit are: Normal mode, Clear Timer on Compare Match (CTC) mode, and two types of Pulse Width Modulation (PWM) modes

table 3.7.1 TCCR modes • Bit 5:4 – COM21:0: Compare Match Output Mode These bits control the Output Compare Pin (OC2) behavior. If one or both of the COM21:0 bits are set, the OC2 output overrides the normal port functionality of the I/O pin it is connected to. However, note that the Data Direction Register (DDR) bit corresponding to OC2 pin must be set in order to enable the output driver. When OC2 is connected to the pin, the function of the COM21:0 bits depends on the WGM21:0 bit setting.

Timer/Counter Register – TCNT2

Fig 3.7.2 TCNT The Timer/Counter Register gives direct access, both for read and write operations, to the Timer/Counter unit 8-bit counter. Writing to the TCNT2 Register blocks (removes) the Compare Match on the following timer clock. Modifying the counter (TCNT2) while the counter is running, introduces a risk of missing a Compare Match between TCNT2 and the OCR2 Register.

Output Compare Register – OCR2

Fig 3.7.3 OCR2 The Output Compare Register contains an 8-bit value that is continuously compared with the counter value (TCNT2). A match can be used to generate an Output Compare interrupt, or togenerate a waveform output on the OC2 pin. 3.8 8-bit Timer/Counter0 Timer/Counter0 is a general purpose, single channel, 8-bit Timer/Counter module. The main features are: • Single Channel Counter • Frequency Generator • External Event Counter • 10-bit Clock Prescaler Overview A simplified block diagram of the 8-bit Timer/Counter is shown in Fig 3.8.1 . For the actual placement of I/O pinsCPU accessible I/O Registers, including I/O bits and I/O pins, are shown in bold. The device-specific I/O Register and bit locations are listed in the “8-bit Timer/Counter Register Description”.

Fig 3.8.1 8-bit Timer/Counter Registers The Timer/Counter (TCNT0) is an 8-bit register. Interrupt request (abbreviated to Int. Req. in the figure) signals are all visible in the Timer Interrupt Flag Register (TIFR). All interrupts are individually masked with the Timer Interrupt Mask Register (TIMSK). TIFR and TIMSK are not shown in the figure since these registers are shared by other timer units. The Timer/Counter can be clocked internally or via the prescaler, or by an external clock source on the T0 pin. The Clock Select logic block controls which clock source and edge the Timer/Counter uses to increment its value. The Timer/Counter is inactive when no clock source is selected. The output from the clock select logic is referred to as the timer clock (clkT0). Definitions Many register and bit references in this document are written in general form. A lower case “n” replaces the Timer/Counter number, in this case 0. However, when using the register or bit defines in a program, the precise form must be used i.e. TCNT0 for accessing Timer/Counter0 counter value and so on. Timer/Counter Clock Sources The Timer/Counter can be clocked by an internal or an external clock source. The clock source

is selected by the clock select logic which is controlled by the clock select (CS02:0) bits located in the Timer/Counter Control Register (TCCR0). Counter Unit .

Fig 3.8.2Counter Unit Block Diagram Operation The counting direction is always up (incrementing), and no counter clear is performed. The counter simply overruns when it passes its maximum 8-bit value (MAX = 0xFF) and then restarts from the bottom (0x00). In normal operation the Timer/Counter Overflow Flag (TOV0) will be set in the same timer clock cycle as the TCNT0 becomes zero. The TOV0 Flag in this case behaves like a ninth bit, except that it is only set, not cleared. However, combined with the timer overflow interrupt that automatically clears the TOV0 Flag, the timer resolution can be increased by software. A new counter value can be written anytime. Timing Diagrams The Timer/Counter is a synchronous design and the timer clock (clkT0) is therefore shown as a clock enable signal in the following figures. The figures include information on when Interrupt Flags are set. Figure contains timing data for basic Timer/Counter operation. The figure

shows the count sequence close to the MAX value.

Fig 3.8.3 timing diagrams 1

Fig 3.8.4 timing diagrams 2

4. SPECIFIED TECHNOLOGY

1 GSM MODEM
INTRODUCTION: What is GSM? GSM stands for Global System for Mobile Communication. It is a globally accepted standard for digital cellular communication. GSM is the name of a standardization group established in 1982 to create a common European mobile telephone standard that would formulate specifications for a pan-European mobile cellular radio system operating at 900 MHz. Functions – GSM Modes

Table 3.1.1: Functions – GSM Mode

The GSM modem basically consists of a • • • • SIM card holder to hold the activated SIM card for sending and receiving SMS. 5V AC power supply header to which the 5v ac adapter is connected. Power led which gives the indication of modem status that is on or off. 9 pin female to which the GSM antenna is connected.

Block Diagram of GSM:

Fig 3.1.1 : GSM Modem Block Diagram

GSM Network Setup:

TE

TA

ME

USER & APPLICATIONS

NETWORK

Fig 3.1.2 :GSM setup

This is the basic setup of a GSM. Here ME stands for Mobile Equipment, e.g. a GSM phone (equal to MS; Mobile Station). TE stands for Terminal Equipment,

e.g. a computer (equal to DTE; Data Terminal Equipment). TA stands for Terminal Adaptor, e.g. a GSM data card (equal to DCE; Data Circuit Terminating Equipment). Through the mobile equipment the network messages are sent and received. These messages are sent to the terminal adapter which is nothing but a GSM data card. Now if there is some data to be sent to the mobile equipment then the terminal equipment that is basically a computer or processor sends out AT COMMANDS to the terminal adapter which in turn sends the mobile equipment the required data as shown in fig 3.1.2 Connection Diagram: The GSM modem being a serial communication device is connected to the serial port or a serial device through a serial connector. The power input to the modem is given through a 9v ac adapter as shown in fig 2.2

Fig 3.1.3 : Connection Diagra

LED Status Indicator The LED will indicate different status of the modem: - OFF - ON Modem Switched off Modem is connecting to the network Modem is in idle mode

- Flashing Slowly

GSM SPECIFICATIONS: Before looking at the specification, it is important to understand the following basic terms: Bandwidth: The range of channels limits, the broader the bandwidth, the faster the data can be sent. Bits per second (BPS): A single on-off pulse of data. Eight bits are equivalent to one byte. Frequency: The number of cycles per unit of time. It is measured in hertz. Watt (W): A measure of power of transmitter. Frequency band: The frequency rage specified for GSM is 1850 to 1990 MHz. Duplex Distance: The duplex distance is 80 MHz. The duplex distance is the distance between uplink and downlink frequencies. A channel has two frequencies 80 MHz a part. Channel Separation: The separation between adjacent carrier frequencies is 200 KHz. Transmission rate: GSM is a digital system with the over the air bit rate of 270 Kbps. 9-PIN D-SUB Female Connector: PIN NAME DESIGNATION TYPE

1 2 3 4 5 6 7 8 9

X None Tx Rx DSR GND DTR CTS RTS X None

NC Transmit Data Receive Data Data Set Ready Ground Data Terminal Ready clear to send Request to send NC

NC Input Output Output Ground Input Output Input NC

Table 3.1.2 : PIN D-SUB Female Connector

GSM (Global System for Mobile Communication) : Open the Package directory from the CD and run the setup.exe file to start the installation. Once “Welcome to the Smart Modem Installation Program” appears on screen, click the “OK” button to continue. Click the “PC Software” Icon to install Smart modem software to the specified destination directory. Select “Program Group” or “Continue” button and wait till it finish the installation and check for the message “Smart Modem Setup was completed successfully”. This indicates that the installation is completed. Then click “OK” button. Observe the Smart Modem Package in program files.

3.2 GSM AT Commands :
1. line settings A serial link handler is set with the following default values (factory settings): autobaud, 8 bits data, 1 stop bit, no parity, RTS/CTS flow control. Please use the +IPR, +IFC and +ICF commands to change these settings. 2. Command line Commands always start with AT (which means ATtention) and finish with a <CR> character. 3. Information responses and result codes Responses start and end with <CR><LF>, except for the ATV0 DCE response format) and the ATQ1 (result code suppression) commands. If command syntax is incorrect, an ERROR string is returned. If command syntax is correct but with some incorrect parameters, the +CME ERROR: <Err> or +CMS ERROR: <SmsErr> strings are returned with different error codes. If the command line has been performed successfully, an OK string is returned.

In some cases, such as “AT+CPIN?” or (unsolicited) incoming events, the product does not return the OK string as a response. In the following examples <CR> and <CR><LF> are intentionally omitted.

General behaviors :

1. SIM Insertion, SIM Removal SIM card Insertion and Removal procedures are supported. There are software functions relying on positive reading of the hardware SIM detect pin. This pin state (open/closed) is permanently monitored. When the SIM detect pin indicates that a card is present in the SIM connector, the product tries to set up a logical SIM session. The logical SIM session will be set up or not depending on whether the detected card is a SIM Card or not. The AT+CPIN? command delivers the following responses:

If the SIM detect pin indicates “absent”, the response to AT+CPIN? Is “+CME ERROR 10” (SIM not inserted). If the SIM detect pin indicates “present”, and the inserted Card is a SIM Card, the response to AT+CPIN? is “+CPIN: xxx” depending on SIM PIN state. If the SIM detect pin indicates “present”, and the inserted Card is not a SIM Card, the response to AT+CPIN? is CME ERROR 10. These last two states are not given immediately due to background initialization. Between the hardware SIM detect pin indicating “present” and the previous results the AT+CPIN? sends “+CME ERROR: 515” (Please wait, init in progress). When the SIM detect pin indicates card absence, and if a SIM Card was previously inserted, an IMSI detach procedure is performed, all user data is removed from the product (Phonebooks, SMS etc.). The product then switches to emergency mode. 2. Background initialization After entering the PIN (Personal Identification Number), some SIM user data files are loaded into the product (Phonebooks, SMS status, etc.). Please be aware that it might take some time to read a large phonebook.

The AT+CPIN? command response comes just after the PIN is checked. After this response user data is loaded (in background). This means that some data may not be available just after PIN entry is confirmed by ’OK’.

The reading of phonebooks will then be refused by “+CME ERROR: 515” or “+CMS ERROR: 515” meaning, “Please wait, service is not available, init in progress”. This type of answer may be sent by the product at several points:

when trying to execute another AT command before the previous one is completed (before response), when switching from ADN to FDN (or FDN to ADN) and trying to read the relevant phonebook immediately, when asking for +CPIN? status immediately after SIM insertion and before the product has determined if the inserted card is a valid SIM Card.

Short Messages commands 1. Parameters definition <da> <dcs> <dt> Destination Address, coded like GSM 03.40 TP-DA Data Coding Scheme, coded like in document [5]. Discharge Time in string format :

“yy/MM/dd,hh :mm :ss z”(Year [00-99], Month [01-12], Day [01-31], Hour, z Minute, Second and Time Zone [quarters of an hour] ) <fo> value is First Octet, coded like SMS-SUBMIT first octet in document [4], default

17 for SMS-SUBMIT <index> <length> length of Place of storage in memory. Text mode (+CMGF=1): number of characters PDU mode (+CMGF=0):

the TP data unit in octets

<mem1> +CMGD). <mem2> <mid> <mr> <oa> <pid> <pdu> hexadecimal

Memory used to list, read and delete messages (+CMGL, +CMGR and Memory used to write and send messages (+CMGW, +CMSS). CBM Message Identifier. Message Reference. Originator Address. Protocol Identifier. For SMS : GSM 04.11 SC address followed by GSM 03.40 TPDU in

format, coded as specified in doc [4] For CBS : GSM 03.41 TPDU in hexadecimal format <ra> <sca> <scts> Recipient Address. Service Center Address Service Center Time Stamp in string format :

“yy/MM/dd,hh :mm :ss ± zz” (Year/Month/Day, Hour: Min: Seconds ± Time Zone) <sn> <st> <stat> <tooa> <tora> <tosca> <total1> <total2> <used1> <used2> <vp> CBM Serial Number Status of a SMS-STATUS-REPORT Status of message in memory. Type-of-Address of <oa>. Type-of-Address of <ra>. Type-of-Address of <sca>. Number of message locations in <mem1>. Number of messages locations in <mem2. Total number of messages locations in <mem1>. Total number of messages locations in <mem2. Validity Period of the short message, default value is 167

2. Select message service +CSMS 2.1 Description : The supported services are originated (SMS-MO) and terminated short message (SMS-MT) + Cell Broadcast Message (SMS-CB) services. 2.2 Syntax : 2.3 Defined values : <service> 0: 1: SMS AT commands are compatible with GSM 07.05 Phase 2 version 4.7.0. SMS AT commands are compatible with GSM 07.05 Phase 2 + version

COMMAND AT+CSMS=0 Note: SMS AT command phase 2 version 4.7.0 AT+CSMS=1 Note: SMS AT command phase 2 + AT+CSMS? Note: Current values ?

POSSIBLE RESPONSE +CSMS: 1,1,1 OK Note: SMS.MO, SMS-MT and SMS-CB supported +CSMS: 1,1,1 OK Note: SMS.MO, SMS-MT and SMS-CB supported +CSMS: 1,1,1 OK Note: GSM 03.40 and 03.41(SMS AT command phase 2 version 4.7.0 +CSMS: (0,1) OK

AT+CSMS=? Note :possible services

3. New Message Acknowledgement +CNMA 3.1 Description : This command allows reception of a new message routed directly to the TE to be acknowledged. In TEXT mode, only positive acknowledgement to the network (RP-ACK) is possible. In PDU mode, either positive (RP-ACK) or negative (RP-ERROR) acknowledgement to the network is possible.

Acknowledge with +CNMA is possible only if the +CSMS parameter is set to 1 (+CSMS=1) when a +CMT or +CDS indication is shown (see +CNMI command). If no

acknowledgement is given within the network timeout, an RP-ERROR is sent to the network, the <mt> and <ds> parameters of the +CNMI command are then reset to zero (do not show new message indication).

3.2 Syntax : Command syntax in text mode : AT+CNMA Command syntax in PDU mode : AT+CNMA [ = <n> [ , <length> [ <CR> PDU is entered Note: PDU is entered using <ackpdu> format instead of <pdu> format (e.g.. SMSC address field is not present). Example of acknowledgement of a new message in TEXT mode <ctrl-Z / ESC> ] ] ]

Command AT+CMGF=1 Note : Set TEXT mode AT+CNMI=2,2,0,0,0 Note : <mt>=2

Possible responses OK Note : TEXT mode valid OK +CMT :”123456”,”98/10/01,12:30 00=00”,129,4. 32,240,”15379”,129,5<CR><LF> Received message Note : message received OK Note : send positive acknowledgement to the network +CMS ERROR : 340 Note : no+CNMA acknowledgement expected

AT+CNMA Note : acknowledge the message received AT+CNMA Note : try to acknowledge again

Example of acknowledgement of a new message in PDU mode:
COMMAND AT+CMGF=0 Note : Set PDU mode POSSIBLE RESPONSE OK Note : Set PDU mode +CMT:,29 07913366003000F1240B913366920547 F30000003003419404800B506215D42E CFE7E17319 Note: message received

AT+CNMA=2<length><CR> …Pdu message…<Ctrl-Z/ESC> Note: negative acknowledgement for the message

OK Note: negative acknowledgement to the network (RP-ERROR) with PDU Message(<ackpdu> format)

3.3 Defined values : <n>: Type of acknowledgement in PDU mode 0: 1: 2: send RP-ACK without PDU (same as TEXT mode) send RP-ACK with optional PDU message send RP-ERROR with optional PDU message

<length> length>: Length of the PDU message

4. Preferred Message Storage +CPMS

4.1 Description : This command allows the message storage area to be selected (for reading, writing, etc).

4.2 Syntax :

4.3 Defined values : <mem1>: Memory used to list, read and delete messages. It can be: - “SM” - “BM” : SMS message storage in SIM (default) : CBM message storage (in volatile memory).

- “SR” : Status Report message storage (in SIM if the EF-SMR file exists, otherwise in the M E non volatile memory) Note : “SR” ME non volatile memory is cleared when another SIM card is inserted. It is kept, even after a reset, while the same SIM card is used.

<mem2> - “SM”

: Memory used to write and send messages : SMS message storage in SIM (default).

If the command is correct, the following message indication is sent: +CPMS: <used1>,<total1>,<used2>,<total2>

When <mem1> is selected, all following +CMGL, +CMGR and +CMGD commands are related to the type of SMS stored in this memory.

5. Preferred Message Format +CMGF 5.1 Description : The message formats supported are text mode and PDU mode. In PDU mode, a complete SMS Message including all header information is given as a binary string (in hexadecimal format). Therefore, only the following set of characters is allowed: {‘0’,’1’,’2’,’3’,’4’,’5’,’6’,’7’,’8’,’9’, ‘A’, ‘B’,’C’,’D’,’E’,’F’}. Each pair or characters is converted to a byte (e.g.: ‘41’ is converted to the ASCII character ‘A’, whose ASCII code is 0x41 or 65). In Text mode, all commands and responses are in ASCII characters. The format selected is stored in EEPROM by the +CSAS command.

HOW TO RUN THE APPLICATION: Select Start --> Programs --> Smart Modem --> Smart Modem, then the following form is appeared as shown below Login

At the application loading time, checks the modem working conditions.

If the modem is not responding, then it displays, Modem not responding message. Click OK to close the application.

Then check The modem is Switched Off. The modem is connected to PC-Comport.

If the PC-Comport is already in open mode, then it displays, Port Already Open message. Click OK to close the application. Then Close the Comport.

If the Simcard is not inserted into Modem, then it displays, Check The Simcard message. Click OK to close the application. Insert the Simcard.

If the modem is working properly Error indicator indicates the Green Signal, other wise it indicates the Red Signal.

At the time of checking modem conditions, if any command Buttons or Menu options are selected from application, then it displays, Please wait Checking Modem Connections… message. Wait for the selection of Command Buttons or Menu options until the Error indicates the Green Signal. How to work with Modem:

The application contains the following facilities: SMS Voice Call Data Call Internet

The Menu Contains File Configuration Help Exit

Configuration:

Select the Configuration option from Main Menu then open the configuration form Set the Comport, Baud Rate and Number of Rings, then Click Config Command Button

At Modem Configuration time, the Config, and Cancel buttons are shown in Disable Mode and status will be shown in the Status Bar. After the Completion of Modem Configuration Config, and Cancel Command Buttons are shown in Enable Mode.

Select the Cancel button to Exit from the Configuration Form. SMS (Short Message Services):

Select the SMS option from File Menu, and then open the SMS Form. Type the Message in Text Box (Data Must be below 150 Characters). Enter the phone number to send the Message.

After sending the Message, a message is displayed Message is Send. Select the Cancel button to Exit from the SMS Form. Voice Call:

Select the Voice Call option from File Menu, and then open the Voice Call Form. Enter the phone number to be call.

After the establishment of connection, if the other end is also received, then message is displayed, Modem is Connected…, then Click the OK Button. If the Disconnect is selected then, Modem disconnects the connection. Select the Cancel button to Exit from the Voice Form. Data Call:

Select the Data Call option from File Menu, open the Data Call Form. Enter the phone number to get connected.

After the establishment of connection, Send/Receive, Disconnect options are in Enable mode.

If Send/Receive is selected then, open the Send/Receive form. We can Send or Receive the data simultaneously.

Select the Cancel button to Exit from the Send/Receive form.

If Disconnect is selected then, it gets disconnected.

Select the Cancel button to Exit from the Data Form. Internet:

Select the Internet option from File Menu, and then open the Internet Form. Internet form contains Initialize, APN, Save, Cancel options.

First select Initialize option then, Modem gets initialized. After the modem initialization message is displayed Modem Initialized. If the modem is not initialized then message is displayed Modem not responding. Again select the Initialize option.

If modem is initialized, select APN option then, it prompts for Simcard Name (like “airtelgprs.com”, ”hutchgprs.com”). If APN address is correct then it connects to Network, message is displayed Connected to Network. If APN address is wrong message is displayed Invalid Simcard Name. Again select the APN option. Select the Save option to save the initialized commands.

After saving the commands Double Click dialing icon, then it establish the Internet connection. Select the Cancel button to Exit from the Internet form.

Relays A relay is an electrical switch that opens and closes under the control of another electrical circuit. In the original form, the switch is operated by an electromagnet to open or close one or many sets of contacts. It was invented by Joseph Henry in 1835. Because a relay is able to control an output circuit of higher power than the input circuit, it can be considered to be, in a broad sense, a form of an electrical amplifier Operation When a current flows through the coil, the resulting magnetic field attracts an armature that is mechanically linked to a moving contact. The movement either makes or breaks a connection with a fixed contact. When the current to the coil is switched off, the armature is returned by a force approximately half as strong as the magnetic force to its relaxed position. Usually this is a spring,

but gravity is also used commonly in industrial motor starters. Most relays are manufactured to operate quickly. In a low voltage application, this is to reduce noise. In a high voltage or high current application, this is to reduce arcing. If the coil is energized with DC, a diode is frequently installed across the coil, to dissipate the energy from the collapsing magnetic field at deactivation, which would otherwise generate a spike of voltage and might cause damage to circuit components. Some automotive relays already include that diode inside the relay case. Alternatively a contact protection network, consisting of a capacitor and resistor in series, may absorb the surge. If the coil is designed to be energized with AC, a small copper ring can be crimped to the end of the solenoid. This "shading ring" creates a small out-of-phase current, which increases the minimum pull on the armature during the AC cycle.[1]

The following is a 5 pin relay: _______________________________ | 1 | | | 5

---------|---+

o------------|---------------| | |

| |-----------/---- s | 3 | / / s s

---------|-----------o/ coil s | | | 2 |

|

4

o---s-------|---------------| | | | | | |

----------|---------------------+

|_____________________________|

By analogy with the functions of the original electromagnetic device, a solid-state relay is made with a thyristor or other solid-state switching device. To achieve electrical isolation an optocoupler can be used which is a light-emitting diode (LED) coupled with a photo transistor. In the above diagram pin 3 is connected to pin 5, by default. By sending +12V between pin 1 and pin 2, you will will turn on a switch. Pin 1 and pin 2 will disconnect, and pin 5 and pin 4 will connect.

Relays…more Info

Relay showing coil and switch contacts A relay is an electrically operated switch. Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches. Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits, the link is magnetic and mechanical. The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil. The maximum output current for the popular 555 timer IC is 200mA so these devices can supply relay coils directly without amplification.

Relays are usuallly SPDT or DPDT but they can have many more sets of switch contacts, for example relays with 4 sets of changeover contacts are readily available. For further information about switch contacts and the terms used to describe them please see the page on switches. Most relays are designed for PCB mounting but you can solder wires directly to the pins providing you take care to avoid melting the plastic case of the relay. The supplier's catalogue should show you the relay's connections. The coil will be obvious and it may be connected either way round. Relay coils produce brief high voltage 'spikes' when they are switched off and this can destroy transistors and ICs in the circuit. To prevent damage you must connect a protection diode across the relay coil. The animated picture shows a working relay with its coil and switch contacts. You can see a lever on the left being attracted by magnetism when the coil is switched on. This lever moves the switch contacts. There is one set of contacts (SPDT) in the foreground and another behind them, making the relay DPDT.

The relay's switch connections are usually labelled COM, NC and NO:
• • • •

COM = Common, always connect to this, it is the moving part of the switch. NC = Normally Closed, COM is connected to this when the relay coil is off. NO = Normally Open, COM is connected to this when the relay coil is on. Connect to COM and NO if you want the switched circuit to be on when the relay coil is on. Connect to COM and NC if you want the switched circuit to be on when the relay coil is off.



Choosing a relay You need to consider several features when choosing a relay:
1. Physical

size

and

pin

arrangement

If you are choosing a relay for an existing PCB you will need to ensure that its dimensions

and pin arrangement are suitable. You should find this information in the supplier's catalogue.
2. Coil

voltage

The relay's coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.
3. Coil

resistance

The circuit must be able to supply the current required by the relay coil. You can use Ohm's law to calculate the current:

supply voltage

coil resistance

Relay coil current =
4. For example: A 12V supply relay with a coil resistance of 400

passes a current of 30mA.

This is OK for a 555 timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current.
5. Switch ratings (voltage and current)

)

the relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".
6. Switch contact arrangement (SPDT, DPDT etc)

)

Most relays are SPDT or DPDT which are often described as "single pole changeover"

(SPCO) or "double pole changeover" (DPCO). For further information please see the page on switches. Protection diodes for relays Transistors and ICs must be protected from the brief high voltage produced when a relay coil is switched off. The diagram shows how a signal diode (eg 1N4148) is connected 'backwards' across the relay coil to provide this protection. Current flowing through a relay coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the relay coil which is very likely to damage transistors and ICs. The protection diode allows the induced voltage to drive a brief current through the coil (and diode) so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs.

Reed relays Reed relays consist of a coil surrounding a reed switch. Reed switches are normally operated with a magnet, but in a reed relay current flows through the coil to create a magnetic field and close the reed switch. Reed relays generally have higher coil resistances than standard relays (1000 Reed Relay Photograph © Rapid Electronics

for example) and a wide range of supply voltages (9-20V for example). They are

capable of switching much more rapidly than standard relays, up to several hundred times per second; but they can only switch low currents (500mA maximum for example). The reed relay shown in the photograph will plug into a standard 14-pin DIL socket ('IC holder'). For further information about reed switches please see the page on switches.

Relays and transistors compared Like relays, transistors can be used as an electrically operated switch. For switching small DC currents (< 1A) at low voltage they are usually a better choice than a relay. However transistors cannot switch AC or high voltages (such as mains electricity) and they are not usually a good choice for switching large currents (> 5A). In these cases a relay will be needed, but note that a low power transistor may still be needed to switch the current for the relay's coil! The main advantages and disadvantages of relays are listed below: Advantages of relays:
• • • •

Relays can switch AC and DC, transistors can only switch DC. Relays can switch high voltages, transistors cannot. Relays are a better choice for switching large currents (> 5A). Relays can switch many contacts at once.

Disadvantages of relays:
• •

Relays are bulkier than transistors for switching small currents. Relays cannot switch rapidly (except reed relays), transistors can switch many times per second. Relays use more power due to the current flowing through their coil. Relays require more current than many ICs can provide, so a low power transistor may be needed to switch the current for the relay's coil.

• •

5. POWER SUPPLY

A variable regulated power supply, also called a variable bench power supply, is one where you can continuously adjust the output voltage to your requirements. Varying

the output of the power supply is the recommended way to test a project after having double checked parts placement against circuit drawings and the parts placement guide. This type of regulation is ideal for having a simple variable bench power supply. Actually this is quite important because one of the first projects a hobbyist should undertake is the construction of a variable regulated power supply. While a dedicated supply is quite handy e.g. 5V or 12V, it's much handier to have a variable supply on hand, especially for testing. Most digital logic circuits and processors need a 5 volt power supply. To use these parts we need to build a regulated 5 volt source. Usually you start with an unregulated power To make a 5 volt power supply, we use a LM7805 voltage regulator IC (Integrated Circuit). The IC is shown below.

Fig 8.1 LM7805 voltage regulator The LM7805 is simple to use. You simply connect the positive lead of your unregulated DC power supply (anything from 9VDC to 24VDC) to the Input pin, connect the negative lead to the 8.1 CIRCUIT FEATURES • Brief description of operation: Gives out well regulated +5V output, output current capability of 100 mA



Circuit protection: Built-in overheating protection shuts down output when regulator IC gets too hot

• • • •

Circuit complexity: Very simple and easy to build Circuit performance: Very stable +5V output voltage, reliable operation Availability of components: Easy to get, uses only very common basic components Design testing: Based on datasheet example circuit, I have used this circuit successfully as part of many electronics projects

• • • •

Applications: Part of electronics devices, small laboratory power supply Power supply voltage: Unregulated DC 8-18V power supply Power supply current: Needed output current + 5 mA Component costs: Few dollars for the electronics components + the input transformer cost

8.2 BLOCK DIAGRAM

Fig 8.2.1 BLOCK DIAGRAM of Power supply

8.3EXAMPLE CIRCUIT DIAGRAM

Fig 8.3.1 Circuit DIAGRAM of Power supply

6. CIRCUIT DIAGRAM

7.RESULT

8. CONCLUSION

9. REFERENCE

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