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1. INTRODUCTION TO AN EMBEDDED SYSTEM

An Embedded System employs a combination of hardware & software (a computational engine) to

perform a specific function; is part of a larger system that may not be a computer works in a

reactive and time-constrained environment. Software is used for providing features and flexibility.

Hardware is used for performance & sometimes security.

An embedded system is a special purpose system in which the computer is completely encapsulated

by the device it controls. Unlike a general purpose computer, such as a PC, an embedded system

performs predefined tasks usually with very specific tasks design engineers can optimize it reducing

the size and cost of the product. Embedded systems are often mass produced, so the cost savings may

be multiplied by million of items.The core of any embedded system is formed by one or several

microprocessor or micro controller programmed to perform a small number of tasks.

Microcontroller are widely used in Embedded System products. An Embedded product uses the

microprocessor(or microcontroller) to do one task & one task only. A printer is an example of

Embedded system since the processor inside it perform one task only namely getting the data and

printing it. Contrast this with Pentium based PC. A PC can be used for any no. of applications such

as word processor, print server, bank teller terminal, video game player, network server or internet

terminal. Software for variety of applications can be loaded and run. Of course the reason a PC can

perform multiple task is that it has RAM memory and an operating system that loads the application

software into RAM & lets the CPU run it. In and Embedded system there is only one application

software that is typically burn into ROM.

One of the most critical needs of the embedded system is to decrease power consumptions and space.

This can be achieved by integrating more functions into the CPU chips. All the embedded processors

have low power consumptions in additions to some forms of I/O,Serial Com port & ROM all on a

single chip. In higher performance Embedded system the trend is to integrate more & more function

on the CPU chip & let the designer decide which feature he/she wants to use.

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2. LIST OF COMPONENTS USED

Printed Circuit Board

Microcontroller AT89S52

LCD

IN4007 diodes(4)

Step down transformer(12-0 V)

LEDs(2)

7805 Voltage Regulator

Crystal Oscillator - 11.0592MHz.

Capacitor - 33 uF (2), 10 uf (2), 1000 uf (1).

Resistor -470 ohm(6) ,10 kohm(2)

ON/OFF Switch.

Transistor- BC369 PNP.

Optocoupler 4N35(4)

RELAYS(4)

Ribbon Wire.

1738 TSOP

Remote

Soldering Material ( soldering iron,wire& stand)

Cutter

Cardboard

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3. TOOLS REQUIRED

KEIL MICROVISION3.0(COMPILER)

PCB WIZARD SOFTWARE

WINDOWS 7

SUPPLY

DIGITAL CAMERA

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4. PCB designing

A printed circuit board, or pcb is used to mechanically support and electrically connect electronics

componints using conductive paths ,tracks or signal traces etched from copper sheets laminated onto

a non-conductive substrate. It is also referred to as printed wiring board (PWB) or etched wiring

board. Printed circuit boards are used in virtually all but the simplest commercially produced

electronic devices.

A PCB populated with electronic components is called a printed circuit assembly (PCA), printed

circuit board assembly or PCB Assembly (PCBA).

Fig2.1.PCB Layout

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http://en.wikipedia.org/wiki/Laminatedhttp://en.wikipedia.org/wiki/Substrate_(electronics)http://en.wikipedia.org/wiki/Laminatedhttp://en.wikipedia.org/wiki/Substrate_(electronics)


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5REGULATED POWER SUPPLY

Power supplies are designed to convert high voltage AC mains to a suitable low voltage supply

for electronic circuits and other devices. A power supply can be broken down into a series of

blocks, each of which performs a particular function.

For example a 5V regulated supply:

Fig3.1.Block Diagram of Regulated Power Supply

Fig3.2.Power supply

Each of the blocks has its own function as described below

1. T ra ns f o r m er steps down high voltage AC mains to low voltage AC.

2. Rec ti f i er converts AC to DC, but the DC output is varying.

3. S m oo th i n g smoothes the DC from varying greatly to a small ripple.

4. Re g u la to r eliminates ripple by setting DC output to a fixed voltage.

5.1 TRANSFORMER

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Transformers convert AC electricity from one voltage to another with little loss of power.

Transformers work only with AC and this is one of the reasons why mains electricity is AC. The

two types of transformers

Step-up transformers increase voltage,

Step-down transformers reduce voltage.

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Most power supplies use a step-down transformer to reduce the dangerously high mainsvoltage to a safer low voltage. The input coil is called the primary and the output coil is

called the secondary. There is no electrical connection between the two coils,instead yjey are linked

by an alternaying magnetic field created in soft-iron core of the transformer. The two lines in the

middle of the circuit symbol represent the core.

Transformers waste very little power so the power out is (almost) equal to the power in. Note

that as voltage is stepped down current is stepped up. The ratio of the number of turns on each

coil, called the turn ratio, determines the ratio of the voltages. A step-down transformer haslarge

number of turns on its primary (input) coil which is connected to the high voltage mains

supply, and a small number of turns on its secondary (output) coil to give a low output voltage.

Turns Ratio =

=

And Power Out = Power In

Where

Vs Is = Vp Ip

Vp = primary (input) voltage

Np = number of turns on primary coil

Ip = primary (input) current

Ns = number of turns on secondary coil

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Is = secondary (output) currenVs = secondary (output) voltage

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5.2 Bridge Rectifier

A bridge rectifier can be made using four individual diodes, but it is also available in special

packages containing the four diodes required.It is called a full-wave rectifier because it uses all

AC wave (both positive and negative sections). 1.4V is used up in the bridge rectifier because

each diode uses 0.7V when conducting and there are always two diodes conducting, as shown in

the diagram below. Bridge rectifiers are rated by the maximum current they can pass and the

maximum reverse voltage they can withstand (this must be at least three times the supply

RMS voltage so the rectifier can withstand the peak voltages). In this alternate pairs of diodes

conduct, changing over the connections so the alternating directions of AC are converted to the one

direction of DC.

Fig3.3.Output Full-wave varying DC

5.3 SMOOTHING

Smoothing is performed by a large value e le c tr ol y ti c c a p a c ito r connected across the DC supply

to act as a reservoir, supplying current to the output when the varying DC voltagevarying DC

voltage from the rectifier is falling. The diagram shows the unsmoothed varying DC

(dotted line) and the smoothed DC (solid line). The capacitor charges quickly near the peak of

the varying DC, and then discharges as it supplies current to the output.


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Fig3.4.Smoothing circuit and waveform

Note that smoothing significantly increases the average DC voltage to almost the peak value

(1.4 RMS value). For example 6V RMS AC is rectified to full wave DC of about 4.6V RMS

(1.4V is lost in the bridge rectifier), with smoothing this increases to almost the peak value

giving 1.4 4.6 = 6.4V smooth DC.

Smoothing is not perfect due to the capacitor voltage falling a little as it discharges, giving a small

ripple voltage.For many circuits a ripple which is 10% of the supply voltage is satisfactory and

the equation below gives the required value for the smoothing capacitor. A larger capacitor will

give fewer ripples. The capacitor value must be doubled when smoothing half-wave DC.

Smoothing capacitor for 10% ripple, C = 5 Io / Vs f

WhereC = smoothing capacitance in farads (F)

Io = output current from the supply in amps (A)

Vs = supply voltage in volts (V), this is the peak value of the unsmoothed DC

f = frequency of the AC supply in hertz (Hz)


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5.4 REGULATOR

Fig3.5.RegulatorVoltage regulator ICs are available with fixed (typically 5, 12 and 15V) or variable output

voltages. They are also rated by the maximum current they can pass. Negative voltage

regulators are available, mainly for use in dual supplies. Most regulators include some

automatic protection from excessive current (overload protection') and overheating (thermal

protection'). Many of the fixed voltage regulator ICs has 3 leads and look like power

transistors, such as the 7805 +5V 1A regulator shown on the right. They include a hole for

attaching a heat sink if necessary.


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5.5 Working of Power Supply

Transformer

Fig3.6.Transformer and its Waveform

The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable for

electronic circuits unless they include a rectifier and a smoothing capacitor.

Transformer + Rectifier

Fig3.7.Transformer with rectifier and their waveform

The varying DC output it is not suitable for electronic circuits unless they include a smoothing

capacitor.


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Transformer + Rectifier + Smoothing

Fig3.8.Transformer with rectifier and smoothing and their waveform

The smooth DC output has a small ripple. It is suitable for most electronic circuits.

Transformer + Rectifier + Smoothing + Regulator

Fig3.9.Transformer with rectifier ,smoothing and regulator circuit with their

waveform

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.


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6. MICROCONTROLLER 8051/52

In our day to day life the role of micro-controllers has been immense. They are used in a variety

of applications ranging from home appliances, FAX machines, Video games, Camera, Exerciseequipment, Cellular phones musical Instruments to Computers, engine control, aeronautics,

security systems and the list goes on.

6.1 Microcontroller versus Microprocessors

What is the difference between a microprocessor and microcontroller? The microprocessors

(such as 8086, 80286, 68000 etc.) contain no RAM, no ROM and no I/O ports on the chip itself.

For this reason they are referred as general- purpose microprocessors. A system designer using

general- purpose microprocessor must add external RAM, ROM, I/O ports and timers to make

them functional. Although the addition of external RAM, ROM, and I/O ports make the system

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

the not the case with microcontrollers. A microcontroller has a CPU (a microprocessor) in

addition to the fixed amount of RAM, ROM, I/O ports, and timers are all embedded together on

the chip: therefore, the designer cannot add any external memory, I/O, or timer to it. The fixed

amount of on chip RAM, ROM, and number of I/O ports in microcontrollers make them ideal for

many applications in which cost and space are critical. In many applications, for example a

TV remote control, there is no need for the computing power of a 486 or even a 8086

microprocessor. In many applications, the space it takes, the power it consumes, and the price

per unit are much more critical considerations than the computing power. These applications

most often require some I/O operations to read signals and turn on and off

certainbits. It is interesting to know that somemicrocontrollers manufactures have gone as far as integrating an ADC and other peripherals

into the microcontrollers.


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6.2 A brief history of 8051 Family

In 1981, Intel Corporation introduced an 8-bit microcontroller called the 8051. This

microcontroller had 128 bytes of RAM, 4K bytes of on-chip ROM, two timers, one serial port,

and four ports (8-bit) all on a single chip. The 8051 is an 8- bit processor, meaning the

the CPU can work on only 8 bit . The 8051 has a total of four I/O ports, each 8- bit wide.

Although 8051 can have a maximum of 64K bytes of on-chip ROM, many manufacturers put only

4K bytes on the chip.

The 8051 became widely popular after Intel allowed other manufacturers to make any flavor of

the 8051 they please with the condition that they remain code compatible with the 8051. This

has led to many versions of the 8051 with different speeds and amount of on-chip ROM

marketed by more than half a dozen manufacturers. It is important to know that although

there are different flavors of the 8051, they are all compatible with the original 8051 as far as

the instructions are concerned. This means that if you write your program for one, it will run on

any one of them regardless of the manufacturer. The major 8051 manufacturers are Intel, Atmel,

Dallas Semiconductors, Philips Corporation, Infineon.

6.3 8051 microcontroller

The 8051 is the original member of the 8051 family. Intel refers to it as MCS-51.

Other members of the 8051 family

There are two other members in the 8051 family of microcontrollers. They are the

8052 and the 8031.


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AT89C51 from ATMEL Corporation:

This popular 8051 chip has on-chip ROM in the form of flash memory. This is ideal for fast

development since flash memory can be erased in seconds compared to twenty minutes or more

needed for the earlier versions of the 8051. To use the AT89C51 to develop a microcontroller-

based system requires a ROM burner that supports flash memory: However, a ROM eraser is not

needed. Notice that in flash memory you must erase the entire contents of ROM in order to

program it again. The PROM burner does this erasing of flash itself and this is why a separate

burner is not needed. To eliminate the need for a PROM burner Atmel is working on a version

of the AT89C51 that can be programmed by the serial COM port of the PC.

Fig3.10. Atmel Microcontroller AT89C51

Hardware features

40 pin IC

4 Kbytes of Flash

128 Bytes of RAM

32 I/O lines

Two16-Bit Timer/Counters

Two-Level Interrupt Architecture

Full Duplex Serial Port

On Chip Oscillator and Clock Circuitry

Software features


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Bit Manipulations

Single Instruction Manipulation

Separate Program And Data Memory

4 Bank Of Temporary Registers

Direct, Indirect, Register and Relative Addressing.

In addition, the AT89C51 is designed with static logic for operation down to zero frequency and

supports two software selectable power saving modes. The Idle Mode stops the CPU while

allowing the RAM, timer/counters, serial port and interrupt system to continue functioning.

The Power down Mode saves the RAM contents but freezes the oscillator disabling all other

chip functions until the next hardware reset.

The Atmel Flash devices are ideal for developing, since they can be reprogrammed easy and fast.

If we need more code space for our application, particularly for developing 89Cxx projects

with C language. Atmel offers a broad range of microcontrollers based on the 8051

architecture, with on-chip Flash program memory.


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.

Fig3.11.Internal Architecture of AT89C51


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6.4 Pin description

The 89C51 have a total of 40 pins that are dedicated for various functions such as I/O, RD, WR,

address and interrupts. Out of 40 pins, a total of 32 pins are set aside for the four ports P0, P1, P2,

and P3, where each port takes 8 pins. The rest of the

pins are designated as Vcc, GND, XTAL1, XTAL, RST, EA, and PSEN. All these pins except

PSEN and ALE are used by all members of the 8051 and 8031 families. In other words,

they must be connected in order for the system to work, regardless of whether the

microcontroller is of the 8051 or the 8031 family. The other two pins, PSEN and ALE are used

mainly in 8031 based systems.

Fig3.12.MC 8051Vcc

Pin 40 provides supply voltage to the chip. The voltage source is +5V.


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GND

Pin 20 is the ground.

Oscillator Characteristics

XTAL1 and XTAL2 are the input and output, respectively, of an inverting amplifier

which can be configured for use as an on-chip oscillator, as shown in Figure. Either a quartz

crystal or ceramic resonator may be used. To drive the device from an external clock source,

XTAL2 should be left unconnected while XTAL1 is driven as shown in Figure.

Fig3.13.OscillatorConnections

It must be noted that there are various speeds of the 8051 family. Speed refers to the maximum

oscillator frequency connected to the XTAL. For example, a 12 sMHz chip must be

connected to a crystal with 12 MHz frequency or less. Likewise, a 20 MHz microcontroller

requires a crystal frequency of no more than 20 MHZ. When the 8051 is connected to a crystal

oscillator and is powered up, we can observe the frequency on the XTAL2 pin using oscilloscope.

RST

Pin 9 is the reset pin. It is an input and is active high (normally low). Upon applying a high


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pulse to this pin, the microcontroller will reset and terminate all activities. This is often

referred to as a power on reset. Activating a power-on reset will cause all values in the

registers to be lost. Notice that the value of Program Counter is 0000 upon reset, forcing the

CPU to fetch the first code from ROM memory location 0000. This means that we must place the

first line of source code in ROM location 0000 that is where the CPU wakes up and expects to

find the first instruction. In order to RESET input to be effective, it must have a minimum

duration of 2 machine cycles. In other words, the high pulse must be high for a minimum of 2

machine cycles before it is allowed to go low.

EA

All the 8051 family members come with on-chip ROM to store programs. In such cases, the EA

pin is connected to the Vcc. For family members such as 8031 and

8032 in which there is no on-chip ROM, code is stored on an external ROM and is

fetched by the 8031/32. Therefore for the 8031 the EA pin must be connected to ground to

indicate that the code is stored externally. EA, which stands for external access, is pin number

31 in the DIP packages. It is input pin and must be connected to either Vcc or GND. In

other words, it cannot be left unconnected.

PSEN

This is an output pin. PSEN stands for program store enable. It is the read strobe to external

program memory. When the microcontroller is executing from external memory, PSEN is

activated twice each machine cycle.

ALE

ALE (Address latch enable) is an output pin and is active high. When connecting a microcontroller

to external memory, port 0 provides both address and data. In other words the microcontroller

multiplexes address and data through port 0 to save pins. The ALE pin is used for de-multiplexing


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the address and data by connecting to the G pin of the 74LS373 chip.

I/O port pins and their functions

The four ports P0, P1, P2, and P3 each use 8 pins, making them 8-bit ports. All the ports upon

RESET are configured as output, ready to be used as output ports. To use any of these as input

port, it must be programmed.

Port 0

Port 0 occupies a total of 8 pins (pins 32 to 39). It can be used for input or output. To use the

pins of port 0 as both input and output ports, each pin must be connected externally to a

10K-ohm pull-up resistor. This is due to fact that port 0 is an open drain, unlike P1, P2 and P3.

With external pull-up resistors connected upon reset, port 0 is configured as output port. In

order to make port 0 an input port, the port must be programmed by writing 1 to all the bits of

it. Port 0 is also designated as AD0-AD7, allowing it to be used for both data and address.

When connecting a microcontroller to an external memory, port 0 provides both address and

data. The microcontroller multiplexes address and data through port 0 to save pins. ALE

indicates if P0 has address or data. When ALE=0, it provides data D0- D7, but when ALE=1 ithas address A0-A7. Therefore, ALE is used for de- multiplexing address and data with the

help of latch 74LS373.

Port 1

Port 1 occupies a total of 8 pins (pins 1 to 8). It can be used as input or output. In contrast to

port 0, this port does not require pull-up resistors since it has already pull-up resistors

internally. Upon reset, port 1 is configures as an output port. Similar to port 0, port 1 can be

used as an input port by writing 1 to all its bits.

Port 2

Port 2 occupies a total of 8 pins (pins 21 to 28). It can be used as input or output. Just like P1,


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6.5 Memory Space Allocation

Internal ROM

The 89C51 has 4K bytes of on-chip ROM. This 4K bytes ROM memory has memory

addresses of 0000 to 0FFFh. Program addresses higher than 0FFFh, which exceed the internal

ROM capacity, will cause the microcontroller to automatically fetch code bytes from external

memory. Code bytes can also be fetched exclusively from an external memory, addresses 0000h

to FFFFh, by connecting the external access pin to ground. The program counter doesnt care

where the code is: the circuit designer decides whether the code is found totally in internal

ROM, totally in external ROM or in a combination of internal and external ROM.

Internal RAM

The 128 bytes of RAM inside the 8051 are assigned addresses 00 to 7Fh. These

128 bytes can be divided into three different groups as follows:

1. A total of 32 bytes from locations 00 to 1Fh are set aside for register banks and the

stack.2. A total of 16 bytes from locations 20h to 2Fh are set aside for bit addressable

read/write memory and instructions.

A total of 80 bytes from locations 30h to 7Fh are used for read and write storage, or what is

normally called a scratch pad. These 80 locations of RAM are widely used for the purpose of

storing data and parameters by 8051 programmers.


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7. LIQUID CRYSTAL DISPLAY

Liquid crystal displays (LCD) are widely used in recent years as compares to LEDs. This is

due to the declining prices of LCD, the ability to display numbers, characters and graphics,

incorporation of a refreshing controller into the LCD, their by relieving the CPU of the task of

refreshing the LCD and also the ease of programming for characters and graphics. HD

44780 based LCDs are most commonly used.The LCD, which is used as a display in the system, is LMB162A. The main features of this

LCD are: 16 X 2 display, intelligent LCD, used for alphanumeric characters & based on ASCII

codes. This LCD contains 16 pins, in which 8 pins are used as 8-bit data I/O, which are

extended ASCII. Three pins are used as control lines these are Read/Write pin, Enable pin and

Register select pin. Two pins are used for Backlight and LCD voltage, another two pins are for

Backlight & LCD ground and one pin is used for contrast change.

Table3.3.LCDpin

description

Pin Symbol I/O Description

1 VSS - Ground

2 VCC - +5V power supply

3 VEE - Power supply to control contrast

4 RS I RS=0 to select command register, RS=1 to select data

5 R/W I R/W=0 for write, R/W=1 for read

6 E I/O Enable

7 DB0 I/O The 8 bit data bus


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1








The LCD discuss in this section has the most common connector used for the Hitachi 44780

based LCD is 14 pins in a row and modes of operation and how to program and interface with

microcontroller is describes in this section.

VCC, VSS, VEE

The voltage VCC and VSS provided by +5V and ground respectively while VEE is used for

controlling LCD contrast. Variable voltage between Ground and Vcc is used

to specify the contrast (or "darkness") of the characters on the LCD screen.

RS (register select)

There are two important registers inside the LCD. The RS pin is used for their selection as

follows. If RS=0, the instruction command code register is selected, then allowing to user to

send a command such as clear display, cursor at home etc.. If RS=1, the data register is selected,

allowing the user to send data to be displayed on the LCD.

R/W (read/write)


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The R/W (read/write) input allowing the user to write information from it. R/W=1, when it read and

R/W=0, when it writing.

EN (enable)

The enable pin is used by the LCD to latch information presented to its data pins. When data is

supplied to data pins, a high power, a high-to-low pulse must be applied to this pin in order to

for the LCD to latch in the data presented at the data pins.

D0-D7 (data lines)

The 8-bit data pins, D0-D7, are used to send information to the LCD or read the contents of

the LCDs internal registers. To displays the letters and numbers, we send ASCII codes for the

letters A-Z, a-z, and numbers 0-9 to these pins while making RS =1. There are also command

codes that can be sent to clear the display or force the cursor to the home position or blink the

cursor.

We also use RS =0 to check the busy flag bit to see if the LCD is ready to receive the

information. The busy flag is D7 and can be read when R/W =1 and RS =0, as follows: if R/W

=1 and RS =0, when D7 =1(busy flag =1), the LCD is busy taking care of internal operation and

will not accept any information, when D7=0 the LCD is ready to receive new information


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7.2 Interfacing of micro controller with LCD display

In most applications, the "R/W" line is grounded. This simplifies the application because when

data is read back, the microcontroller I/O pins have to be alternated between input and output

modes.

In this case, "R/W" to ground and just wait the maximum amount of time for each instruction

(4.1ms for clearing the display or moving the cursor/display to the "home position", 160s for

all other commands) and also the application software is simpler, it also frees up a

microcontroller pin for other uses. Different LCD execute instructions at different rates and to

avoid problems later on (such as if the LCD is changed to a slower unit). Before sending

commands or data to the LCD module, the Module must be initialized. Once the initialization is

complete, the LCD can be written to with data or instructions as required. Each character to

display is written like the control bytes, except that the "RS" line is set. During initialization, by

setting the "S/C" bit during the "Move Cursor/Shift Display" command, after each character is

sent to the LCD, the cursor built into the LCD will increment to the next position (either right

or left). Normally, the "S/C" bit is set (equal to "1")

Table 3.4.LCD Command Code

Commands and Instruction set


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Only the instruction register (IR) and the data register (DR) of the LCD can be controlled by the

MCU. Before starting the internal operation of the LCD, control information is temporarily

stored into these registers to allow interfacing with various MCUs, which operate at different

speeds, or various peripheral control devices. The internal operation of the LCD is determined

by signals sent from the MCU.

Sending Commands to LCD

To send commands we simply need to select the command register. Everything is same as we

have done in the initialization routine. But we will summarize the common steps and put them in

a single subroutine.

Following are the steps:

1. Move data to LCD port

2. Select command register

3. Select write operation

4. Send enable signal5. Wait for LCD to process the command

Fig 3.14.LCD INTERFACING WITH MICROCONTROLLER


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Fig3.15.ircuit diagram of LCD interfacing


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8. Relay Driver using Optocoupler (Auto Electro Switching)

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 most have double throw (changeover) switch contacts as

shown in the diagram.

Fig3.16.Relay Driver

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

COM = Common, always connect to 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.

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.

Fig3.17.Relay Devices

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

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this current and a transistor is usually used to amplify the small IC current to the larger value required

for the relay coil.

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8.1 Pole and Throw

Since relays are switches, the terminology applied to switches is also applied to relays. A relay will

switch one or more poles, each of whose contacts can be thrown by energizing the coil in one of

three ways:

Normally-open (NO) contacts connect the circuit when the relay is activated; the circuit isdisconnected when the relay is inactive. It is also called a Form A contact or make contact.

Normally-closed (NC) contacts disconnect the circuit when the relay is activated; the circuit is

connected when the relay is inactive. It is also called a Form B contact or break contact.

Change-over(CO), or double-throw (DT), contacts control two circuits: one normally-open contact

and one normally-closed contact with a common terminal. It is also called a Form C contact or

transfer contact (break before make). If this type of contact utilizes make before break

functionality, then it is called a Form D contact.

The following designations are commonly encountered:

SPST Single Pole Single Throw. These have two terminals which can be connected or

disconnected. Including two for the coil, such a relay has four terminals in total. It is ambiguous

whether the pole is normally open or normally closed. The terminology SPNO and SPNC is

sometimes used to resolve the ambiguity.

SPDT Single Pole Double Throw. A common terminal connects to either of two others. Including

two for the coil, such a relay has five terminals in total.

Fig3.18.Designations

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DPST Double Pole Single Throw. These have two pairs of terminals. Equivalent to two SPST

switches or relays actuated by a single coil. Including two for the coil, such a relay has six terminals

in total. The poles may be Form A or Form B (or one of each).

DPDT Double Pole Double Throw. These have two rows of change-over terminals. Equivalent to

two SPDT switches or relays actuated by a single coil. Such a relay has eight terminals, including the

coil.

The "S" or "D" may be replaced with a number, indicating multiple switches connected to a single

actuator. For example 4PDT indicates a four pole double throw relay (with 14 terminals)

8.2 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:

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RELAY COIL CURRENT= SUPPLY VOLTAGE / COIL RESISTANCE

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.

4. 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".

5. Switch contact arrangement (SPDT, DPDT etc)

Most relays are SPDT or DPDT which are often described as "single pole changeover" (SPCO) or "doubl

pole changeover" (DPCO).

Protection diodes for relays

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.

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

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.

Fig3.19.Protection Diodes

8.3 Relay and transistor comparison as switch

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 (such as mains electricity) and in simple circuits 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 higher voltages than standard transistors.

Relays are often 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.

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8.4 Relay Driver

For its interfacing with microcontrollers or other low current digital ICs, a power or current amplifier

circuit is required, known as relay driver circuits. Mainly two such driver circuits i.e. ULN2003 and

transistor/optocouplers are often used. An Auto Electro Switching/Relay Driver circuit comprises of

an optocoupler which will isolate the controller from the outer spikes or fluctuations or from theexternal hardware and at the same time it drives a power transistor i.e. make it on when a signal from

the controller pin is applied to it. Optocoupler actually comprises of a diode and a phototransistor. It

comes in a DIP IC package. Thus signal from the MCU is given to the LED part or the driving part.

When LED begins to glow then the phototransistor acts as on switch or short circuit. This output is

given to power transistor, which will amplify the current of the signal and then can be used to drive a

relay. The I/P signal is connected to the Relays common terminal and the O/P can be taken from

the relays NO terminal. When relay is ON then NO is connected to the common terminal of the

relay.

Fig3.20.Block diagram of Relay driver

Optocoupler

It has one IR LED and a phototransistor. One pin of the LED is connected to the MCU to get a signal

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(0 or 1) and the pin is given ground. When the signal from the MCU is 0, then LED emits light. This

light will turn on the NPN transistor. Emitter of the transistor is grounded. Collector is connected to

the PNP transistor whose emitter is connected to Vcc and collector to the relay.

The purpose of using the optocouplers is to pass the supply from the PC/MCU to the appliances & is

for isolation of the port of the PC/MCU from an external hardware. The voltage signal from the

PC/MCU is being converted into light by the LED and then further converted into voltage by thephototransistor. This ensures that there is no physical connection between the PC and the appliances.

The signal from the PC/MCU is coupled only through light so that if in any case

the external hardware ( in this case :appliances) produces an error voltage it will not be passed over to

the port of the PC/MCU and will not damage the internal circuitry of the PC/MCU.

Fig3.21.Optocoupler (4N35) Pin Diagram

Table3.5Pin description

Pin no.

Function

1 Anode

2 Cathode

3 NC

4 Emitter5 Collector

6 Base

The 4N35 optoisolators consist of a gallium arsenide infrared emitting diode driving a silico

phototransistor in a 6-pin dual in-line package. There is no electrical connection between the two, just

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beam of light. The light emitter is nearly always an LED. The light sensitive device may be a photodiode

phototransistor, or more esoteric devices such as thyristors, triacs etc. To carry a signal across the isolatio

barrier, optocouplers are operated in linear mode.

Pin Description of 4N35

The IC package may also be called an IC or a chip. It is important to note that each type ofoptocoupler may use different pin assignments. For carrying a linear signal across isolation barrier

there are two types of optocouplers. Both types use an infrared light emitting diode (LED) to generate

and send a light signal across an isolation barrier. The difference is in the detection method. Some

optocouplers use a phototransistor detector while others use a photodiode detector which drives the

base of a transistor. The phototransistor detector uses the transistors collector base junction to detect

the light signal. This necessitates that the base area be relatively large compared to a standard

transistor. The result is a large collector to base capacitance which slows the collector rise time and

limits the effective frequency response of the device. In addition the amplified photocurrent flows in

the collector base junction and modulates the response of the transistor to the photons. This causes the

transistor to behave in a non-linear manner. Typical phototransistor gains range from 100 to 1000. The

photodiode/transistor detector combination on the other hand uses a diode to detect the photons and

convert them to a current to drive the transistor base. The transistor no longer has a large base area.

The response of this pair is not affected by amplified photocurrent and the photodiode capacitance

does not impair speed.

Optocoupler Operation

Optocouplers are good devices for conveying analog information across a power supply isolation

barrier, they operate over a wide temperature range and are often safety agency approved they do,

however, have many unique operating considerations.

Optocouplers are current input and current output devices. The input LED is excited by changes in

drive current and maintains a relatively constant forward voltage. The output is a current which is

proportional to the input current. The output current can easily be converted to a voltage through a

pull-up or load resistor.

Applications

AC mains detection

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Reed relay driving

Switch mode power supply feedback

Telephone ring detection

Logic ground isolation

Logic coupling with high frequency noise rejection.

Features

Interfaces with common logic families

Input-output coupling capacitance < 0.5 pF

Industry Standard Dual-in line 6-pin package

5300 VRMS isolation test voltage

Lead-free component

Transistor

A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is

composed of a semiconductor material with at least three terminals for connection to an external

circuit. A voltage or current applied to one pair of the transistor's terminals changes the current

flowing through another pair of terminals. Because the controlled (output) power can be higher than

the controlling (input) power, a transistor can amplify a signal.

Transistor as a Current Amplifier

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A PNP transistor can be used as a current amplifier for driving the relay. Since approx 500-1000mA

current can be passed from emitter to collector in PNP transistor with a little base current (approx 1-

2mA).

Fig3.22.Transistor as a current amplifier

Here a circuit shows interfacing of relay with PNP transistor. Here Vin signal is given to transistor from

microcontroller or other low current digital devices. By using this driver circuit a relay can be derived from

microcontroller, but has a drawback i.e. whenever the relay gets off a back spike is generated in the base o

transistor which can harm the controller or other digital devices. This problem can be solved by using an

optocoupler between controller and transistor's base as shown:

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Fig3.23.Use of Optocoupler in current amplifier

In this circuit the MCU is connected to transistor through optocoupler, means there is an optical

connected between microcontroller and transistor not any physical connection, so the spike generated

by the transistor wouldn't be reach controller, so by using this circuit we've isolated the controller

from Power transistor. The PNP Power transistor used here is BC369.

Power Transistor (BC 369)

High current gain

High collector current

Low collector-emitter saturation voltage

Complementary type: BC 368 (NPN)

Fig3.24.Power Transistor BC369

Terminal 1: Emitter

Terminal 2: Collector

Terminal3: Base

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9. TSOP 1738

The TSOP 17XX series are miniaturized receivers for infrared remote control systems. PIN diode

and preamplifier are assembled on lead frame, the epoxy package is designed as IR filter. Thedemodulated output signal can directly be decoded by a microprocessor. TSOP 17XX is the standard

IR remote control receiver series, supporting all major transmission codes. Here XX refers to the

frequency of the infrared carrier signal on which the code is modulated, which is 38 KHz in our case.

It has three pins .GND and Vcc are connected to the power supply with VCC as 5V and Vout which

becomes 0V, or GND when the demodulated bit received is high i.e. 5V and vice versa.

Fig3.25.TSOP 1738

Features

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_ Photo detector and preamplifier in one package

_ Internal filter for PCM frequency

_ TTL and CMOS compatibility

_ Output active low

_ Low power consumption

_ High immunity against ambient light

_ Continuous data transmission possible (up to 2400 bps)

_ Suitable burst length .10 cycles/burst

_Available for carrier frequencies of 27 kHz up to 62 kHz

_ No external components necessary

_Output microcomputer-compatible

_High sensitivity for large transmitting range (120 ft/ 35 m)

_Maximum interference safety against optical and electrical disturbances

_High quality level ISO 9001

_Automated large-volume production

Typical applications

TV sets

Video recorders

Sat receivers

DVD (Digital Versatile Disk)

Slide projectors

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Hi-fi components

Fig3.26.RC5 Code transmission

12 CIRCUIT DIAGRAMS

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Fig4.4.Microcontroller with relay driver

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R ?

+T ?

T R A N S F O R M E R

1 5

4 8

R ?

D 1

L E D

V o l t a g e R e g u l a t o r 7 8 0 5

Q ?

P N P C B E

R ?

P

C ?

U ?

1 3V I N V O U T

K ?

R E L A Y S P D T

35

412

T s o p 1 7 3 8

Q ?

P N P C B E

U ?

4 N 3 5

1 6

2

5

4

R ?

2 0

D e v i c e

K ?

R E L A Y S P D T

35

412

2 2 0 V A

Y ?

C R Y S T A L

U ?

4 N 3 5

1 6

2

5

4

4 0

K ?

R E L A Y S P D T

35

412

- +

D I O D E B R I D G E

1

2

3

4

U ?

4 N 3 5

1 6

2

5

4

0 - 1 2 V

V C C

D e v i c e 1

Q ?

P N P C B E

N

2

2

0

V

A

C

C ?C A P N P

D e v i c e

U ?

A T 8 9 C 5 2

91 81 9 2 9

3 0

3 1

12345678

2 12 22 32 42 52 62 72 8

1 01 11 21 31 41 51 61 7

3 93 83 73 63 53 43 33 2

R S TX T A L 2X T A L 1 P S E N

A L E / P R O G

E A / V P P

P 1 . 0 / T 2P 1 . 1 / T 2 - E XP 1 . 2P 1 . 3P 1 . 4P 1 . 5P 1 . 6P 1 . 7

P 2 . 0 / A 8P 2 . 1 / A 9

P 2 . 2 / A 1 0P 2 . 3 / A 1 1P 2 . 4 / A 1 2P 2 . 5 / A 1 3P 2 . 6 / A 1 4P 2 . 7 / A 1 5

P 3 . 0 / R X DP 3 . 1 / T X D

P 3 . 2 / I N T OP 3 . 3 / I N T 1

P 3 . 4 / T OP 3 . 5 / T 1

P 3 . 6 / W RP 3 . 7 / R D

P 0 . 0 / A D 0P 0 . 1 / A D 1P 0 . 2 / A D 2P 0 . 3 / A D 3P 0 . 4 / A D 4P 0 . 5 / A D 5P 0 . 6 / A D 6P 0 . 7 / A D 7

-

K ?

R E L A Y S P D T

35

412

R e c . S e c t i

D e v i c e

O u t p u t 5 V D C

Q ?

P N P C B E

U ?

4 N 3 5

1 6

2

5

4

R 1R

Fig4.5. Microcontroller with TSOP and relay driver

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13. C CODE FOR PROJECT

#define f40k 1

#define DATA P1

#define RS P35

#define RW P36

#define E P37

#include

#define IRDet P32 //IR serial input stream

sbit IRLED = P3^3;//LED will blink on valid IR code

#define CH1 P20

#define CH2 P21

#define CH3 P22

#define CH4 P23

#define CH5 P24

#define CH6 P25

#define CH7 P26

#define CH8 P27

#define TRUE 1

#define FALSE 0

unsigned char RC5RxAddress, RC5RxCommand; // Last command received on RC5bit RC5Avail=FALSE; // And if the data are new...

unsigned char Repeating;

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TR0=TRUE; // Start timer 1

IRLED = 0;

IE0=0; // Clear any pending int's

EX0=TRUE; // Prepare for another startbit...

void main(void)

{

lcd_initialize();

P2=0xff;

ACC=0x83;

lcd_cmd();

lcd_display("IR REMOTE ",12);

ACC=0xC4;

lcd_cmd();

lcd_display("Control ",12);

secdelay(3);

ACC=0x80;

lcd_cmd();

lcd_display("RECEIVING...... ",12);

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ACC=0xC0;

lcd_cmd();

lcd_display("DEVICE CODE ",12);

EX0 = 1; // Enable EX0 Interrupt

EA = 1; // Enable Global Interrupt Flag

CH1 = 0; // Initial state, this output is low

P2=0xff;

while (1)

{

if(RC5Avail) {

RC5Avail = FALSE; // reset the variable for next check

switch(RC5RxCommand) {

case 0x01:

{

CH1 = ~CH1; // toggle the output

ACC='1';

lcd_datawrite();

}

break;

case 0x02:

{

CH2 = ~CH2;

ACC='2';

lcd_datawrite();

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}

break;

case 0x03:

{

CH3 = ~CH3;

ACC='3';lcd_datawrite();

}

break;

case 0x04:

{

CH4 = ~CH4;

ACC='4';

lcd_datawrite();

}

break;

case 0x05:

{

CH5 = ~CH5;

ACC='5';

lcd_datawrite();

}

break;

case 0x06:

{

CH6 = ~CH6;

ACC='6';

lcd_datawrite();

}

break;

case 0x07:

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{

CH7 = ~CH7;

ACC='7';

lcd_datawrite();

}

break;case 0x08:

{

CH8 = ~CH8;

ACC='8';

lcd_datawrite();

}

break;

}

}

}

}

}

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14. PROJECT PHOTOGRAPH

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. REFERENCES

GOOGLE

www.alldatasheets.com

www.datasheets.com

www.electronicsforyou.com

www.projectguidance.com

8051 microcontroller and embedded systems by Ali Mazidi\

WIKIPEDIA

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