Mobile Controlled Home Appliances

i

MOBILE CONTROLLED HOME APPLIANCES

A PROJECT REPORT

Submitted to




MAHARSHI DAYANAND UNIVERSITY, ROHTAK

I n partial fulfillment for the award of the degree

of

BACHELOR OF TECHNOLOGY

in

ELECTRONICS AND COMMUNICATION ENGINEERING

Submitted by

RAVINDER
FEJB10EC12
Under the supervision of

MR. MANOJ KUMAR
LECTURER
DEPARTMENT OF ECE


FACULTY OF ENGINEERING, J B KNOWLEDGE PARK,
VILLAGE-MANJHAWALI, FARIDABAD
JUNE 2014
i

DEPARMENT OF ELECTRONICS AND COMM ENGG
FACULTY OF ENGINEERING,
J B KNOWLEDGE PARK, VILLAGE-MANJHAWALI,
FARIDABAD


BONAFIDE CERTIFICATE


Certified that this project report “MOBILE CONTROLLED HOME
APPLIANCE” is the bonafide work of “RAVINDER FEJB10EC12” who
carried out the project work under my supervision.


Signature:
Name of Supervisor:
Designation:
Department:


Signature:
Name of Project Coordinator : Mr. Manoj kumar






Head of Department



DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. ii
ACKNOWLADGEMENT
I would like to take this opportunity to express my deepest sense of gratitude
and respect to my honorable teacher & project supervisor “ Mr.Manoj kumar “ ,
Lecturer , Department of Electronics & communication Engineering, J B
Knowledge park, who guided me in this project and help me in every stage where
any difficulty comes. And he has given me valuable suggestions on the
development of my project and suggested me to prepare this project paper.
I am also thankful to Professor A K Dubay, HOD, Department of
Electronic&Communication Engineering, MDU, for his kindness in providing me
access to the seminar library, internet connections and lab facilities. My heartiest
thank to all my respectable teachers, and my classmates for providing moral
support and encouragement to enhance my knowledge on Microcontroller.
Foremost and finally, it is generosity and guidance of Almighty God to
honor me with power and persistence to get this work done.



Date: 21/05/2014 The Author


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. iii
ABSTRACT
The project demonstrates a novel method which enables users to control
their home appliances and systems for remotely using a cell phone-based interface.
To access the control unit, the user should send an authentication code Dual Tone
Multi Frequency along with the required function to his/her home control system
via Global System for Mobile communication (GSM). Upon being properly
authenticated, the cell phone-based interface at home (control unit) would relay the
commands to a microcontroller that would perform the required function.
Four mobile keypad buttons generate four different DTMF signals at input
which is sent to the mobile at output section via GSM. Then the signal is
transferred to a Microcontroller. Microcontroller output is used to make active or
inactive two magnetic relays for four different state (i.e. keypad button 2 is used
make two relays idle. Keypad button 4 is used to make one relay one and so on.)


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. iv
Table of Contents
ACKNOWLADGEMENT ........................................................................................ ii
ABSTRACT ............................................................................................................. iii
Table of Contents ..................................................................................................... iv
LIST OF FIGURE .................................................................................................... vi
List of tables ........................................................................................................... vii
Chapter 1 INTRODUCTION ..................................................................................... 1
Chapter 2 DTMF BASICS ......................................................................................... 2
Chapter 3 DECODER M-8870 .................................................................................. 5
3.1 DECODER DESCRIPTION ............................................................................ 5
3.2 PIN DIAGRAM OF M- 8870 .......................................................................... 6
3.3 Pin discription .................................................................................................. 6
3.4 FEATURES ..................................................................................................... 8
Chapter 4 MICROCONTROLLER .........................................................................10
4.1 INTRODUCTION .........................................................................................10
4.2 PIN DIAGRAM .............................................................................................11
4.3 RC Oscillator..................................................................................................11
4.4 Internal Oscillator Block ................................................................................12
4.5 Special Features .............................................................................................12
4.6 HEXADECIMAL CODE OF PIC MICROCONTROLLER ........................14
Chapter 5 ULN 2003 IC ...........................................................................................16
5.1 INTRODUCTION .........................................................................................16
5.2 PIN DIAGRAM .............................................................................................17
5.3 Pin Description: .............................................................................................18
5.4 CIRCUIT DIAGRAM OF ULN2003 ............................................................19
Chapter 6 IN4007 DIODE .......................................................................................20
6.1 INTRODUCTION .........................................................................................20
6.2 FEATURES ...................................................................................................21
6.3 USED OF IN4007 DIODE ............................................................................21
Chapter 7 AUTOMATIVE RELAY IC 12VOLT ...................................................23

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. v
7.1 INTRODUCTION .........................................................................................23
7.2 Relay Design ..................................................................................................24
7.3 Relay Construction .........................................................................................25
7.4 WORKING OF RELAY ................................................................................26
7.5 Relay Basics ...................................................................................................27
7.6 relay operation................................................................................................28
Chapter 8 POWER SUPPLY ...................................................................................31
8.1 BLOCK DIAGRAM ......................................................................................31
8.2 TRANSFORMER ..........................................................................................31
8.3 Bridge rectifier ...............................................................................................31
8.4 7805 ................................................................................................................31



DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. vi
LIST OF FIGURE

Fig. 2-1 Row and Column Frequency Correspondence ............................................. 3
Fig. 3-1pin digram of m-8870 .................................................................................... 6
Fig. 4-1 Pin digram of pic 18f4520 ..........................................................................11
Fig. 4-2 HEXADECIMAL CODE OF PIC MICROCONTROLLER ....................14
Fig. 5-1 pin digram of ULN2003 .............................................................................17
Fig. 5-2 CIRCUIT DIAGRAM OF ULN2003 ........................................................19
Fig. 6-1 diode symbol and figure .............................................................................20
Fig. 6-2 use of IN4007 .............................................................................................22
Fig. 7-1 relays ..........................................................................................................23
Fig. 7-2 relay internal structure ................................................................................25
Fig. 7-3 Relay ...........................................................................................................26
Fig. 7-4 Energized Relay (ON) ................................................................................29
Fig. 7-5 De – Energized Relay .................................................................................30
Fig. 8-1 circuit digram of power supply ..................................................................31



DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. vii
List of tables

Table 2-1 Frequencies generated on Key presses ...................................................... 2
Table 2-2 Row and Column Frequency Correspondence ......................................... 3
Table 3-1 Values of Decoder output for various frequencies .................................... 5
Table 5-1 pin description of ULN2003 ....................................................................18

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 1
Chapter 1
INTRODUCTION
The remote control technologies have been used in the fields like factory
automation, space exploration, in places where human access is difficult. As this
has been achieved in the domestic systems partially, many corporations and
laboratories are researching the methods which enable human to control and
monitor efficiently and easily in the house or outdoor. Controlling the domestic
system regardless of time and space is an important challenge. As the mobile
phone enables us to connect with the outside devices via mobile communication
network regardless of time and space, the mobile phone is a suitable device to
control domestic systems.
This project proposes a method to control a domestic system using a mobile
phone, irrespective of the phone model and mobile phone carrier. The system
suggested consists of the mobile phone normally registered in communication
service and a mobile phone that can receive a call from another phone. Existing
methods for control and monitoring, using mobile phones have usage problems
because the cost and need for continuous control. One of the disadvantage, being
the lack of feedback during the process. This paper proposes to solve the problems
of existing methods of control that use simple voice call. Method proposed uses the
DTMF (Dual Tone Multi Frequency) generated when a keypad button of the
mobile phone is pressed by the user. The mobile phone user controls the system by
sending the DTMF tone to the access point. Mobile communication network
coverage is larger than that of LANs, thus user can take advantage of mobile
phones to control the system



DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 2
Chapter 2
DTMF BASICS
DTMF is a generic communication term for touch tone . The tones produced
when dialing on the keypad on the phone could be used to represent the digits, and
a separate tone is used for each digit. However, there is always a chance that a
random sound will be on the same frequency which will trip up the system. It was
suggested that if two tones were used to represent a digit, the likelihood of a false
signal occurring is ruled out. This is the basis of using dual tone in DTMF
communication.
DTMF dialing uses a keypad with 12/16 buttons. Each key pressed on the
phone generates two tones with specific frequencies, so a voice or a random signal
cannot imitate the tones. One tone is generated from a high frequency group of
tones and the other from low frequency group.
The frequencies generated on pressing different phone keys are shown in the
Table 1.
Table 2-1 Frequencies generated on Key presses
Button

Low
Frequency(Hz)
High
Frequency(Hz)
1 697 1209
2 697 1336
3 697 1477
4 770 1209
5 770 1336
6 770 1477
7 852 1209
8 852 1336

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 3





Each row and column of the keypad corresponds to a certain tone and
creates a specific frequency. Each button lies at the intersection of the two tones as
shown in Table 2.

Fig. 2-1 Row and Column Frequency Correspondence


Table 2-2 Row and Column Frequency Correspondence
1 2 3 697
4 5 6 770
7 8 9 852
9 852 1477
0 941 1209
* 941 1336
# 941 1477

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 4
* 0 # 941
1209 1336 1477 Frequency(Hz)

When a button is pressed, both the row and column tones are generated by
the telephone instrument. These two tones will be unique and different from tones
of other keys. So, whenever we say that there is a low and high frequency
associated with a button, it is actually the sumof two waves is transmitted. This
fundamental principle can be extended to various applications. DTMF signals can
be transmitted over a radio to switch on or switch off home appliances, flash lights,
motors, cameras, warning systems, irrigation systems and so on. These encoded
data can be stored in a microcontroller and can be transmitted serially to another
system for processing. Block diagram for the proposed method is shown in Figure


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 5
Chapter 3
DECODER M-8870
3.1 DECODER DESCRIPTION

The decoder used is M-8870. For operating functions. M-8870 includes a
band split filter that separates the high and low tones of the received pair, and a
digital decoder that verifies both the frequency and duration of the received tones
before parsing the resulting 4-bitcode to the output bus. The M-8870 decoder uses
a digital counting technique to determine the frequencies of the limited tones and
to verify that they correspond to standard DTMF frequencies. Table 3 shows
values of Decoder output for various frequencies.

Table 3-1 Values of Decoder output for various frequencies
Button Low
Frequency(Hz)
High
Frequency(Hz)
Q1 Q2 Q3 Q4
1 697 1209 0 0 0 1
2 697 1336 0 0 1 0
3 697 1477 0 0 1 1
4 770 1209 0 1 0 0
5 770 1336 0 1 0 1
6 770 1477 0 1 1 0
7 852 1209 0 1 1 1
8 852 1336 0 0 0 0
9 852 1477 1 0 0 1
0 941 1209 1 0 1 0
* 941 1336 1 0 1 1
# 941 1477 1 1 0 0

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 6


3.2 PIN DIAGRAM OF M- 8870

Fig. 3-1pin digram of m-8870
3.3 Pin discription

1. IN+ Non-Inverting Op-Amp (Input).
2. IN- Inverting Op-Amp (Input).

3. GS Gain Select. Gives access to output of front end differential
amplifier for connection of feedback resistor.

4. V-Ref Reference Voltage (Output). Nominally VDD/2 is used to bias
inputs at mid-rail .


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 7
5. INH Inhibit (Input). Logic high inhibits the detection of tones
representing characters A, B, C and D. This pin input is internally pulled
down.

6. PWDN Power Down (Input). Active high. Powers down the device and
inhibits the oscillator. This pin input is internally pulled down.

7. OSC1 Clock (Input).

8. OSC2 Clock (Output). A 3.579545 MHz crystal connected between
pins OSC1 and OSC2 completes the internal oscillator circuit.

9. VSS Ground (Input). 0 V typical.

10. TOE Three State Output Enable (Input). Logic high enables the
outputs Q1-Q4. This pin is pulled up internally.

11-14. Q1-Q4 Three State Data (Output). When enabled by TOE, provide
the code corresponding to the last valid tone-pair received (see Table 1).
When TOE is logic low, the data outputs are high impedance.

15. StD Delayed Steering (Output).Presents a logic high when a received
tone-pair has been registered and the output latch updated; returns to logic
low when the voltage on St/GT falls below VTSt.


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 8
16. ESt Early Steering (Output). Presents a logic high once the digital
algorithm has detected a valid tone pair (signal condition). Any momentary
loss of signal condition will cause ESt to return to a logic low.

17. St/GT Steering Input/Guard time (Output) Bidirectional. A voltage
greater than VTSt detected at St causes the device to register the detected
tone pair and update the output latch. A voltage less than VTSt frees the
device to accept a new tone pair. The GT output acts to reset the external
steering time-constant; its state is a function of ESt and the voltage on St.

18. VDD Positive power supply (Input). +5 V typical.
3.4 FEATURES
• Complete DTMF Receiver
• Low power consumption
• Internal gain setting amplifier
• Adjustable guard time
• Central office quality
• Power-down mode
• Inhibit mode
• Backward compatible with MT8870C/MT8870C-1
3.4 APPLICATION

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 9
• Paging systems
• Repeater systems/mobile radio
• Credit card systems
• Remote control
• Personal computers
• Telephone answering machine



DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 10
Chapter 4
MICROCONTROLLER
4.1 INTRODUCTION
Microconroller pic18f4520 is used for this purpose. It is a 40 pin miro
controller.10bit A/D converter is used. Here 16pin is used for i/o purpose.
Microcontroller pic18f4520 is divided into 5port.
PortA -8pin
PortB -8pin
PortC -8pin
PortD -8pin
PortE -4pin
In microcontroller pic18f4520 PortC and PortD is used for i/o purpose.The o/p of
decoder is apply to the i/p of pic micro controller.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 11
4.2 PIN DIAGRAM

Fig. 4-1 Pin digram of pic 18f4520
4.3 RC Oscillator
For timing insensitive applications, the “RC” and “RCIO” device options
offer additional cost savings. The actual oscillator frequency is a function of
several factors:
• supply voltage
• values of the external resistor (REXT) and capacitor (CEXT)
• operating temperature
Given the same device, operating voltage and temperature and component
values, there will also be unit-to-unit frequency variations. These are due to factors
such as:
• normal manufacturing variation

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 12
• difference in lead frame capacitance between package types (especially for
low CEXT values)
• variations within the tolerance of limits of REXT and CEXT
In the RC Oscillator mode, the oscillator frequency divided by 4 is available on the
OSC2 pin. This signal may be used for test purposes or to synchronize other logic.
4.4 Internal Oscillator Block
The PIC18F4520 devices include an internal oscillator block which
generates two different clock signals; either can be used as the microcontroller’s
clock source. This may eliminate the need for external oscillator circuits on the
OSC1 and/or OSC2 pins. The main output (INTOSC) is an 8 MHz clock source,
which can be used to directly drive the device clock. It also drives a post scaler,
which can provide a range of clock frequencies from 31 kHz to 4 MHz. The
INTOSC Output is enabled when a clock frequency from 125 kHz to 8 MHz is
selected.
The other clock source is the internal RC oscillator (INTRC), which
provides a nominal 31 kHz output. INTRC is enabled if it is selected as the device
clock
source; it is also enabled automatically when any of the following are enabled:
• direct or INTOSC postscaler) is selected by configuring the IRCF bits of
the OSCCON register Power-up Timer
• Fail-Safe Clock Monitor
• Watchdog Timer
• Two-Speed Start-up
4.5 Special Features
• Memory Endurance: The Enhanced Flash cells for both program memory
and data EEPROM are rated to last for many thousands of erase/write

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 13
cycles – up to 100,000 for program memory and 1,000,000 for EEPROM.
Data retention without refresh is conservatively estimated to be greater
than 40 years.

• Self-programmability: These devices can write to their own program
memory spaces under internal software control. By using a bootloader
routine located in the protected Boot Block at the top of
program memory, it becomes possible to create an application that can
update itself in the field.

• Extended Instruction Set: The PIC18F2420/2520/4420/4520 family
introduces an optional extension to the PIC18 instruction set, which adds
8 new instructions and an Indexed Addressing mode. This extension,
enabled as a device configuration option, has been specifically designed
to optimize re-entrant application code originally developed in high-level
languages, such as C.

• Enhanced CCP module: In PWM mode, this module provides 1, 2 or 4
modulated outputs for controlling half-bridge and full-bridge drivers. Other
features include Auto-Shutdown, for disabling PWM outputs on interrupt or
other select conditions and Auto-Restart, to reactivate outputs once the
condition has cleared.

• Enhanced Addressable USART: This serial communication module is
capable of standard RS-232 operation and provides support for the LIN
bus protocol. Other enhancements include automatic baud rate detection and
a 16-bit Baud Rate Generator for improved resolution. When the

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 14
microcontroller is using the internal oscillator block, the USART provides
stable operation for applications that talk to the outside world without using
an external crystal (or its accompanying power requirement).

• 10-bit A/D Converter: This module incorporates programmable
acquisition time, allowing for a channel to be selected and a conversion to be
initiated without waiting for a sampling period and thus, reduce code
overhead.

• Extended Watchdog Timer (WDT): This enhanced version incorporates
a 16-bit prescaler allowing an extended time-out range that is stable across
operating voltage and temperature.
4.6 HEXADECIMAL CODE OF PIC MICROCONTROLLER

Fig. 4-2 HEXADECIMAL CODE OF PIC MICROCONTROLLER

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 15


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 16
Chapter 5
ULN 2003 IC
5.1 INTRODUCTION
Ideally suited for interfacing between low-level logic circuitry and multiple
peripheral power loads, the Series ULN2003IC high-voltage, high-current
Darlington arrays feature continuous load current ratings to 500 mA for each of the
seven drivers. At an appropriate duty cycle depending on ambient temperature and
number of drivers turned ON simultaneously, typical power loads totaling over 230
W (350 mA x 7, 95 V) can be controlled. Typical loads include relays, solenoids,
stepping motors, magnetic print hammers, multiplexed LED and incandescent
displays, and heaters. All devices feature open-collector outputs with integral
clamp diodes
• 600mA peak)
• Output voltage 50V.
• Integrated suppression Seven Darlingtons per package output current
500mA. Diodes for inductive loads outputs can be paralleled for high
current.
• TTL/CMOS/PMOS/DTL compatible input pinned opposite outputs to
simplify layout.
The ULN2003 is a high voltage, high current Darlington array containing
seven open collector
Darlington pairs with common emitters. Each channel rated at 500mA and
can withstand peak currents of 600mA. Suppression diodes are included for
inductive load driving and the inputs are pinned opposite the outputs to simplify
board layout.


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 17
This versatile device is useful for driving a wide range of loads including
solenoids, relays DC motors,
LED displays filament lamps, thermal print heads and high power buffers. The
ULN2003A is supplied in 16 pin plastic DIP packages with a copper lead frame to
reduce thermal resistance.
5.2 PIN DIAGRAM

Fig. 5-0-1 pin digram of ULN2003

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 18
5.3 Pin Description:
Table 0-1 pin description of ULN2003
Pin
No
Function Name
1 Input for 1
st
channel Input 1
2 Input for 2
nd
channel Input 2
3 Input for 3
rd
channel Input 3
4 Input for 4
th
channel Input 4
5 Input for 5
th
channel Input 5
6 Input for 6
th
channel Input 6
7 Input for 7
th
channel Input 7
8 Ground (0V) Ground
9 Common free wheeling diodes Common
10 Output for 7
th
channel Output 7
11 Output for 6
th
channel Output 6
12 Output for 5
th
channel Output 5
13 Output for 4
th
channel Output 4
14 Output for 3
rd
channel Output 3
15 Output for 2
nd
channel Output 2
16 Output for 1
st
channel Output 1

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 19
5.4 CIRCUIT DIAGRAM OF ULN2003

Fig. 5-0-2 CIRCUIT DIAGRAM OF ULN2003
ULN2003 is a high voltage and high current Darlington array IC. It contains
seven open collector darlington pairs with common emitters. A darlington pair is
an arrangement of two bipolar transistors.
ULN2003 belongs to the family of ULN200X series of ICs. Different
versions of this family interface to different logic families. ULN2003 is for 5V
TTL, CMOS logic devices. These ICs are used when driving a wide range of loads
and are used as relay drivers, display drivers, line drivers etc. ULN2003 is also
commonly used while driving Stepper Motors. Refer Stepper Motor interfacing
using ULN2003.
Each channel or darlington pair in ULN2003 is rated at 500mA and can
withstand peak current of 600mA. The inputs and outputs are provided opposite to
each other in the pin layout. Each driver also contains a suppression diode to
dissipate voltage spikes while driving inductive loads. The schematic for each
driver is given above.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 20
Chapter 6
IN4007 DIODE
6.1 INTRODUCTION
In normal biasing state of diode anode terminal is connected to positive of
the power source and cathode is connected to ground. When anode voltage crosses
inner barrier potential of diode the device goes into conduction state where
there is increase in output current with constant Vak (voltage across anode and
cathode) voltage. But in this paper we have observed the behavior of diode at
different biasing conditions.




In the case of semiconductor diodes, the temperature range over which the
temperature dependence of the forward voltage is linear, can be increased by
lowering the operating current along with the increase of the
sensitivity (dV
f
/dT) which is found to vary logarithmically with I. The temperature
and current dependence of forward voltage V
f
can be explained by using the theory
of the p-n junction. The capacitance-voltage (C-V) measurements of p-n junctions
are carried out at different temperatures and are discussed in light of the theory of
the p-n junction. The band gap E
g
, estimated from V
f
-Tmeasurement, is found to
be ∼1.17 eV, whereas it is found to be 1.189 eV from C-V measurement.
on the basis of above observations we can say that on practical analysis
diode is much more than a switch or for the matter rectyifier . we have seen the
sinusoid variations of current and resistances, which is independent of each other.
We have seen diode behaving as amplifier with albiet low gain.we are carrying out
Fig. 6-0-1 diode symbol and figure

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 21
further studies to put the above behaviour interms of mathmetical/solid state
devices equation. This may leads to complete new out look toward a conventional
diode and new applicaions will come in picture

6.2 FEATURES

• Diffused Junction
• High Current Capability and Low Forward Voltage Drop
• Surge Overload Rating to 30A Peak
• Low Reverse Leakage Current
• Lead Free Finish, RoHS Compliant

6.3 USED OF IN4007 DIODE
It is used for reverse Voltage protection.The IN4007 is staple for many
powers, DC to DC setup and broadband projects.This is a simple, common
rectified Diode.
IN4007 rated for upto 1A/1000V


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 22

Fig. 6-0-2 use of IN4007


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 23
Chapter 7
AUTOMATIVE RELAY IC 12VOLT
7.1 INTRODUCTION
Relays are used where it is necessary to control a circuit by a low-
power signal. Relay are used when you need to switch a higher current
then a switch can handle. A relay is an electrically operated switch

Fig. 7-0-1 relays
We know that most of the high end industrial application devices have
relays for their effective working. Relays are simple switches which are
operated both electrically and mechanically. Relays consist of a n
electromagnet and also a set of contacts. The switching mechanism is
carried out with the help of the electromagnet. There are also other
operating principles for its working. But they differ according to their
applications. Most of the devices have the application of relays.
The main operation of a relay comes in places where only a low-power
signal can be used to control a circuit. It is also used in places where
only one signal can be used to control a lot of circuits. The application of

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 24
relays started during the invention of telephones. They played an
important role in switching calls in telephone exchanges. They were also
used in long distance telegraphy. They were used to switch the signal
coming from one source to another destination. After the invention of
computers they were also used to perform Boolean and other logical
operations. The high end applications of relays require high power to be
driven by electric motors and so on. Such relays are called contactors.
7.2 Relay Design
There are only four main parts in a relay. They are
 Electromagnet
 Movable Armature
 Switch point contacts
 Spring
The figures given below show the actual design of a simple relay.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 25

Fig. 7-0-2 relay internal structure
7.3 Relay Construction
It is an electro-magnetic relay with a wire coil, surrounded by an iron
core. A path of very low reluctance for the magnetic flux is provided for
the movable armature and also the switch point contacts. The movable
armature is connected to the yoke which is mechanically connected to
the switch point contacts. These parts are safely held with the help of a
spring. The spring is used so as to produce an air gap in the circuit when
the relay becomes de-energized.


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 26
7.4 WORKING OF RELAY
The working of a relay can be better understood by explaining the
following diagram given below.

Fig. 7-0-3 Relay

The diagram shows an inner section diagram of a relay. An iron
core is surrounded by a control coil. As shown, the power source is
given to the electromagnet through a control switch and through contacts
to the load. When current starts flowing through the control coil, the
electromagnet starts energizing and thus intensifies the magnetic field.
Thus the upper contact arm starts to be attracted to the lower fixed arm
and thus closes the contacts causing a short circuit for the power to the
load. On the other hand, if the relay was already de-energized when the

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 27
contacts were closed, then the contact move oppositely and make an
open circuit.
As soon as the coil current is off, the movable armature will be
returned by a force back to its initial position. This force will be almost
equal to half the strength of the magnetic force. This force is mainly
provided by two factors. They are the spring and also gravity.
Relays are mainly made for two basic operations. One is low voltage
application and the other is high voltage. For low voltage applications,
more preference will be given to reduce the noise of the whole circuit.
For high voltage applications, they are mainly designed to reduce a
phenomenon called arcing.
7.5 Relay Basics
The basics for all the relays are the same. Take a look at a 4 – pin
relay shown below. There are two colours shown. The green colour
represents the control circuit and the red colour represents the load
circuit. A small control coil is connected onto the control circuit. A
switch is connected to the load. This switch is controlled by the coil in

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 28
the control circuit. Now let us take the different steps that occour in a
relay.
7.6 relay operation
Energized Relay (ON)
As shown in the circuit, the current flowing through the coils
represented by pins 1 and 3 causes a magnetic field to be aroused. This
magnetic field causes the closing of the pins 2 and 4. Thus the switch
plays an important role in the relay working. As it is a part of the load
circuit, it is used to control an electrical circuit that is connected to it.
Thus, when the relay in energized the current flow will be through the
pins 2 and 4.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 29

Fig. 7-0-4 Energized Relay (ON)

De – Energized Relay (OFF)
As soon as the current flow stops through pins 1 and 3, the switch
opens and thus the open circuit prevents the current flow through pins 2
and 4. Thus the relay becomes de-energized and thus in off position.

DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 30

Fig. 7-0-5 De – Energized Relay
De-Energized Relay (OFF)


DEPARTMENT OF ELECTRONICS AND COMMUNICATION ENGG. 31
Chapter 8
POWER SUPPLY
8.1 BLOCK DIAGRAM
v

Fig. 8-1 circuit digram of power supply
8.2 TRANSFORMER
An step down transformer is used for bringing 230 v AC down to
9v AC.
8.3 Bridge rectifier
Convert AC to DC.
8.4 7805
It is a 5 volt regulator. It take 7 to 35 v as input and produce an
output of 5 v.