Laser Based Voice and Data Communication

1 Laser Based Voice and Data Communication

CHAPTER 1

INTRODUCTION

1.1 Introduction to Project

Our final year project is based on the concept of laser (Light Amplification by

Stimulated Emission of Radiation) for transmitting analog as well as digital signals.

We have used phototransistor to receive the signal at receiver.

For voice transmission amplitude modulation of laser pulse was used to transmit the

voice signal. Condenser microphone converts the voice into electric pulse which was

then amplified and transmitted through laser. Photo detector at receiver detects the

laser light and voice was output through loud speaker.

Data transmission is based on pulse width modulation by the use of microcontroller.

Different width of laser pulse was used for different number and character. The

second microcontroller was used to decode the different characters and the received

data was displayed in LCD.

1.2 Team Members, Time Duration and Plan Followed

Our project comprises of team members Bijay Kumar Maharjan, Ghoshana Bista,

Ramila Shrestha and Toran Dura. The project was planned to be accomplished within

22 weeks. As for the plan followed, the majority of the time frame of the project was

spent in the concept formulation and design of suitable system to implement the

concept. The project was started with the development concept. The hardware was

connected in bread board according to the nature of concept and the corresponding

software was developed and then the circuit was tested for proper operation. Finally

the circuit was developed on matrix board.


1.3 Objectives of Project

1. To provide simple and cheap wireless communication for larger date rate with

less distortion.

2. To reduce the complexity for communication in the places where optical fiber

or any wired communication is very difficult and expensive.

1.4 Application of Project

1. Though this is just a small-scale demonstration, Free Space Optics (FSO) is a

very promising point-to-point communication technology.

2. These days the use of laser communication is widely done in satellite

application and communication between space crafts.

3. The light beam can be very narrow, which makes FSO hard to intercept,

improving security.

4. The project concept can be implemented for home automation data transfer.

5. Military application (Dedicated Base communication system)


CHAPTER 2

LITERATURE REVIEW

2.1 Literature Review

Laser based project has been attempted before but data were inputted through

computer. We have tried to simplify it by using 4x3 keypad which provides the

complete set of alphabetical letters. We have also tried to enhance it by implementing

voice communication as well. Laser communication is a modern technology in the

world of communication where bandwidth allocation, power requirement, and

dispersion parameter are becoming major hurdle due to rapid increase in number of

user. So considering these facts we put our interest in this project.

There were various methods for implementing this project but due to scarcity of

resources, components, we decided to use simple modulation and demodulation

techniques.

Hence we have designed communication system based on LASER that could be

implemented commercially facilitating the general people in terms of convenient

friendly system. Also it reduces the complexity for communication in some cases

where optical fiber or any wired communication is very difficult and expensive.


CHAPTER 3

BACKGROUND THEORY

3.1 The general principle of laser

Every atom has a certain energy levels, which may be high or low. Once excited by

heating, it goes to high energy level. After certain time in high energy level, it return

back to original energy level, consequently emitting energy in the form of light having

energy E=hf.

Incident photon with energy to E2-E1 interacts with an atom in conduction band,

causing it to return to low energy level with the emission of second photon. This

photon has same phase, frequency and polarization as first. This whole phenomenon

is known as stimulated emission, which gives the laser its spectral properties such as

narrow spectral width, highly directed beam and intense light.

Figure 3.1: Stimulated emission

Einstein demonstrated that for stimulated emission to dominate it was necessary that

the photon radiation density and population density (N2) of the upper energy level

must be increased relative to lower energy lever (N1). Thus when density of atom in

higher energy level is greater than lower energy level (i.e. N2>N1), this phenomenon


is known as population inversion and is fundamental condition for stimulated

emission. To achieve population inversion it is necessary to excite atoms in upper

energy level E2.This process is called pumping.[1]

Figure 3.2: Pumping process

Necessary requirement for lasing effects:

1 An active medium with in which a beam of electromagnetic wave is launched. It can be

solid, liquid or gas.

2 A resounding cavity, which contains the active media. If active media is liquid or gas,

this resounding cavity is limited by two spherical mirrors, one of this slightly transparent

in order to let that a beam of light escapes. Supposing the active media will be crystal,

two faces of crystal are polished so that they work as a mirror.

3 A source of external energy to excite the atoms of the active mean.

Properties of laser light:

1 It is narrow beam of coherent light i.e. all the waves are in same phase.

2 It is highly directed beam/intense light.

3 It has shorter spectral line width.

4 It has low lost coupling to fiber.

5 It is power efficient.

Active medium

Pumping energy

Stimulated

emission

Input photon


3.2 Optical detection principles

When device is reverse biased then the electric field developed across the p-n junction

sweeps mobile carriers (holes and electron) to their respective majority sides (p and n

type material). The depletion layer is therefore created on either side of the junction.

This barrier has the effect of stopping the majority carriers crossing the junction in the

opposite direction to the field. However, the field accelerates minority carriers from

both sides to the opposite side of the junction forming the reverse leakage current of

the diode. Thus intrinsic conditions are created in the depletion region.

A photon incident in or near the depletion region of this device which has an energy

greater than or the equal to the band gap energy Eg (i.e. hf ≥ Eg) will excite an electron

from the valence band to the conduction band. This process leaves an empty hole in

the valence band and is known as the photo generation of an electron-hole (carrier)

pair. Carrier pairs so generated near the junction are separated and swept under the

influence of electric field to produce displacement by current in the external circuit in

excess of any reverse leakage current.

The depletion layer must be sufficiently thick to allow a large fraction of the incident

light to be absorbed in order to achieve maximum carrier-pair generation. [2]

3.3 Pulse Width Modulation

In pulse width modulation the average value of voltage (and current) is controlled by

turning the switch between supply and load on and off at a fast pace. The longer the

switch is on compared to the periods, the higher the power supplied to the load will

be. Duty cycle is expressed in percent, 100% being fully on. The advantage of using

the PWM is that power loss, the product of voltage and current, of the switching

device is close to zero. When it is in switch off condition then there is practically no

current and when it is on there will be almost no voltage drop across the switch.

Because of their duty cycle, on/off nature, they can use in digital controls too. [3]

3.4 Amplitude Modulation

Amplitude modulation (AM) is defined as a process in which the amplitude of the

carrier wave is varied linearly with the message signal [3]. It is a technique used in


electronic communication, most commonly for transmitting information via a radio

carrier wave.

The envelope of the amplitude modulated signal embeds the information bearing

signal. The total power of the transmitted signal varies with the modulating signal

whereas the carrier power remains constant.

The main defect of this modulation is that in an AM wave the signal is in the

amplitude variations of the carrier, practically all the natural and man noises consists

of electrical amplitude disturbances. As a receiver cannot distinguish between

amplitude that represents noise and that contain the desired signal so reception is

generally noisy.

3.5 Serial Communication

Serial communication uses a single data line instead of the 8-bit data line of parallel

communication. This helps to minimize the problem of transmission of data faced in

8-bit data communication. 8-bit data transmission works only if the cable is not too

long, since long cable diminishes and distorts the signal. Also an 8-bit data path is

expensive. Serial data communication uses either synchronous or asynchronous

method for the transmission of the data.

3.6 Asynchronous data communication

In this communication, transmitter and receiver are not synchronized. Each data

character has a bit which identifies its start and 1 or 2 bits, which identify its end

(framing). Since each character is individually identified, characters can be sent at

any time (asynchronously). When no data is being sent, the signal line is in a constant

high or masking state. Following the data bit is a parity bit, which is used to check for

errors in received data. Some system does not insert parity bit.

There are special IC chips for serial data communication, which are commonly known

as the UART (universal asynchronous receiver-transmitter) and USART (universal

synchronous-asynchronous receiver-transmitter). UART is basically the chips, a piece

of hardware that translates data between parallel and serial forms. UART is the


communication protocol to define the data formats need to be maintained to transmit

the data.

The UART usually does not directly generate or receive the external signals of

various connected equipment. Separate interface devices are used to convert the logic

level signals, such as RS-232 and RS-485. Some signaling schemes use modulation of

carrier signal (with or without wires like Bluetooth, Infra-red, optical fiber etc.)

communication may be “full duplex” or “half duplex” [4].

Figure 3.3: Asynchronous Data Format

3.7 Baud Rate

The baud rate of a data communications system is the number of symbol per second

transferred. A symbol may have more than two states, so it may represent more than

one binary bit (a binary bit always represents exactly two states). Therefore the baud

rate may not equal the bit rate, especially in the case of recent modems, which can

have (for example) up to nine bits per symbol. Microcontroller transfer and receive

data serially at many different baud rates.

Baud Rate TH1 (decimal) TH1 (Hex)

9600 -3 FD

4800 -6 FA

2400 -12 F4

1200 -24 E8

Note: XTAL= 11.0592 MHz

Table 3.1: Baud Rate

D0 D1 D2 D3 D4 D5 D6 D7

Receiver Transmitter

Start Stop


3.8 Component Used

3.8.1 Condenser Microphone

Microphone consists of a metal foil diaphragm which is attached to the needle. When

the vibration in the air vibrates the foil, it scratches the needle and the information is

carried on the foil. The condenser microphone is essentially a capacitor with one of

the plates moving with respect to pressure wave created by sound. Here, the

diaphragm acts as one plate of a capacitor, and the vibration produces changes in the

distance between plates.

Figure 3.4: Condenser Mic

3.8.2 Loud speaker

The basic working of speaker is just opposite to that of microphone. When the needle

scratches the foil, the patterns in the foil move the diaphragm and recreate the sound.

The modern speaker does the same thing but electronically.

Figure 3.5: Speaker

3.8.3 Transistor

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

and electrical power .It is composed of semiconductor material what 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.[5] Because the controlled (output) power can be higher than the controlling

(input) power, a transistor can amplify a signal. Today, some transistors are packaged

individually, but many more are found embedded in integrated circuits. The transistor

used is BC 548, BC547 and BD139.

Figure 3.6: Transistor symbol

3.8.4 Laser Torch

A laser is a device that emits light (electromagnetic radiation) through a process

of optical amplification based on the stimulated of photons. The term "laser"

originated as an acronym for Light Amplification by Stimulated Emission of

Radiation. The emitted laser light is notable for its high degree of spatial and

temporal coherence.

3.8.5 OPERATIONAL AMPLIFIER

An operational amplifier (“op-amp”) is a DC coupled high-gain electronic voltage amplifier

with a different input and, usually, a single-ended output. An op-amp produces and output

voltage that is typically hundreds of thousands of times larger than the voltage difference

between its input terminals. The operational amplifier can be used in two configurations:

1. Inverting Configuration

2. Non inverting Configuration

The circuit symbol for an op-amp is shown to the right, where:

V+: non-inverting input


V−: inverting input

Vout: output

VS+: positive power supply

VS−: negative power supply

Figure 3.7: Symbol of IC741

The amplifier's differential inputs consist of a V+ input and a V− input, and ideally the op-amp

amplifies only the difference in voltage between the two, which is called the differential input

voltage. The output voltage of the op-amp is given by the equation:

Vout = AOL (V+

- V-)

Where V+ is the voltage at the non-inverting terminal, V− is the voltage at the inverting

terminal and AOL is the open-loop gain of the amplifier (the term "open-loop" refers to the

absence of a feedback loop from the output to the input).

Figure3.8: Negative Feedback

When the circuit is operated as a non-inverting linear amplifier, Vin will appear at the

(+) and (−) pins and create a current i through Rg equal to Vin/Rg. Since Kirchhoff’s


current law states that the same current must leave a node as enter it, and since the

impedance into the (−) pin is near infinity, we can assume the overwhelming majority

of the same current i travels through Rf, creating an output voltage equal

to Vin + i × Rf. By combining terms, we can easily determine the gain of this

particular type of circuit.

I=Vin/Rg

Vout = Vin +i*RF=Vin+ (Vin/Rg*RF) =Vin +Vin*RF/Rg = Vin (1+RF/Rg)

G=Vout/Vin

G=1+RF/Rg

Figure 3.9: Pin Configuration of IC741

3.8.6 LM 386 IC

General Description:

The LM 386 is a power amplifier designed for use in low voltage consumer

applications. The gain is internally set to 20 to keep external part count low, but the

addition of an external resistor and capacitor between pins 1 and 8 will increase the

gain to any value from 20 to 200. The inputs are ground referenced while the output

automatically biases to one-half the supply voltage. The quiescent power drain is only

24 mill watts when operating from a 6 volt supply, making LM 386 ideal for battery

operation.


Figure 3.10: General figure of LM386

Features:

1. Battery operation

2. Minimum external parts

3. Wide supply voltage range: 4V-12V or 5V-18V

4. Low quiescent current drain: 4mA

5. Voltage gains from 20 to 200

6. Ground referenced input

7. Self-centering output quiescent voltage

8. Low distortion: 0.2% (Av =20, Vs = 6V, R = 8ohm, Po = 125mW, f = 1kHz)

3.8.7 Phototransistor L14F1

Phototransistor is a semiconductor device with electrical characteristics that are light

sensitive. Phototransistor differs from photodiodes in that the primary photoelectric

current is multiplied internally in the device, thus increasing the sensitivity to light.

The application of a small amount of light causes the device to switch from a low

current to a high current condition. Phototransistor combines a photodiode and

transistor together to generate more output current than a photodiode by itself. The

L14F1 is silicon photo Darlington’s mounted in a narrow angle.

Features:

1. Hermetically sealed package

2. Narrow reception angle


Figure 3.11:L14F1 phototransistor

3.8.8 AT89C51

AT89C51 is an 8-bit microcontroller and belongs to Atmel's 8051 family. ATMEL

89C51 has 4KB of Flash programmable and erasable read only memory (PEROM)

and 128 bytes of RAM. It can be erased and program to a maximum of 1000 times.

In 40 pin AT89C51, there are four ports designated as P1, P2, P3 and P0. All these

ports are 8-bit bi-directional ports, i.e., they can be used as both input and output

ports. Except P0 which needs external pull-ups, rest of the ports have internal pull-ups.

When 1s are written to these port pins, they are pulled high by the internal pull-ups

and can be used as inputs. These ports are also bit addressable and so their bits can

also be accessed individually.

Port P0 and P2 are also used to provide low byte and high byte addresses, respectively,

when connected to an external memory. Port 3 has multiplexed pins for special

functions like serial communication, hardware interrupts, timer inputs and read/write

operation from external memory. AT89C51 has an inbuilt UART for serial

communication. It can be programmed to operate at different baud rates. Including

two timers & hardware interrupts, it has a total of six interrupts.

http://www.engineersgarage.com/microcontroller/8051projects/interface-serialport-RS232-AT89C51


Pin Diagram:

Figure 3.12: Pin diagram of AT89C51.

Pin Description of Microcontroller AT89C51:

Pin

No

Function Name

1

8 bit input/output port (P1) pins

P1.0

2 P1.1

3 P1.2

4 P1.3

5 P1.4

6 P1.5

7 P1.6

8 P1.7

9 Reset pin; Active high Reset

10 Input (receiver) for serial

communication RxD

8 bit input/output port

(P3) pins P3.0


11 Output (transmitter) for

serial communication TxD P3.1

12 External interrupt 1 Int0 P3.2

13 External interrupt 2 Int1 P3.3

14 Timer1 external input T0 P3.4

15 Timer2 external input T1 P3.5

16 Write to external data

memory Write P3.6

17 Read from external data

memory Read P3.7

18 Quartz crystal oscillator (up to 24 MHz)

Crystal 2

19 Crystal 1

20 Ground (0V) Ground

21

8 bit input/output port (P2) pins

/

High-order address bits when interfacing with external

memory

P2.0/ A8

22 P2.1/ A9

23 P2.2/ A10

24 P2.3/ A11

25 P2.4/ A12

26 P2.5/ A13

27 P2.6/ A14

28 P2.7/ A15

29 Program store enable; Read from external program memory PSEN

30 Address Latch Enable ALE

Program pulse input during Flash programming Prog

31

External Access Enable; Vcc for internal program executions EA

Programming enable voltage; 12V (during Flash

programming) Vpp

32 8 bit input/output port (P0) pins

Low-order address bits when interfacing with external

memory

P0.7/ AD7

33 P0.6/ AD6

34 P0.5/ AD5

35 P0.4/ AD4

36 P0.3/ AD3


Table 3.2: Pin Description of Microcontroller AT89C51

Features:

1. Sixteen bits program counter and data pointer.

2. Eight bits CPU with resistors A and B.

3. Eight bits programs status word.

4. Eight bits stack program.

5. Internal ROM and EEPORM.

6. Internal RAM of 128 bytes.

7. Four resistors banks each contain eight registers.

8. Sixteen bytes, which may be addressed.

9. Eight bytes of general-purpose data memory.

10. Thirty-two input/output pins are arranged as four 8 bit ports.

11. Two 16-bit timers/counters T0 and T1.

12. Full duplex serial data receiver/transmitter.

13. Two external and three internal interrupt sources.

14. Oscillator and clock circuits.

3.8.9 Keypad

A 4x4 keypad has 3 columns and 4 rows. The rows and columns are connected with

micro-controller port. There are 12 buttons in a 4x3 matrix keypad. The clear

advantage of using matrix keypad is that only 8 micro controller pins are connected

for 12 buttons. Circuit layout for 4x3 matrix keypad is shown in figure below.

37 P0.2/ AD2

38 P0.1/ AD1

39 P0.0/ AD0

40 Supply voltage; 5V (up to 6.6V) Vcc


Figure 3.13: 4x3 Matrixes

Rows are used as input and columns are used as output. All eight pins are set high at

first. Row 1 is the set low. All four columns are checked if anyone is low. A low

would appear if first column were pressed. If no column was low, row 1 is set high

and row 2 is set low. Again the columns are checked for low. If no key was pressed

row 2 was set high and row 3 is set low and process continued. To get the value of the

key pressed, as soon as a column is low for a low row is obtained, 8-bit binary value

of the key was returned. For example if first column key were pressed the binary

value would be 11101110.

3.8.10 LCD

LCD (Liquid Crystal Display) screen is an electronic display module and find a wide

range of applications. A 16x2 LCD display is very basic module and is very

commonly used in various devices and circuits. These modules are preferred over

seven segments and other multi segment LED. The reasons being: LCDs are

economical; easily programmable; have no limitation of displaying special &

even custom characters (unlike in seven segments), animations.

http://www.engineersgarage.com/content/seven-segment-display

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http://www.engineersgarage.com/microcontroller/8051projects/create-custom-characters-LCD-AT89C51

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A 16x2 LCD means it can display 16 characters per line and there are 2 such lines. In

this LCD each character is displayed in 5x7 pixel matrix. This LCD has two registers,

namely, Command and Data.

The command register stores the command instructions given to the LCD. A

command is an instruction given to LCD to do a predefined task like initializing it,

clearing its screen, setting the cursor position, controlling display etc. The data

register stores the data to be displayed on the LCD. The data is the ASCII value of the

character to be displayed on the LCD.[6]

Figure3.14: Pin configuration of LCD.

Pin Description:

Pin No Function Name

1 Ground (0V) Ground

2 Supply voltage; 5V (4.7V – 5.3V) Vcc

3 Contrast adjustment; through a variable resistor

VEE

4 Selects command register when low; and data

register when high

Register Select

5 Low to write to the register; High to read from

the register

Read/write

6 Sends data to data pins when a high to low pulse

is given

Enable


7

8-bit data pins

DB0

8 DB1

9 DB2

10 DB3

11 DB4

12 DB5

13 DB6

14 DB7

15 Backlight VCC (5V) Led+

16 Backlight Ground (0V) Led-

Table 3.3: Pin Description of LCD

3.9 Programming Language (C)

The C language is a high level language for computers microprocessor and other

programmable device. It consists of a set of syntactic and semantic rules used to

define computer programs. This syntax is much closer to human language. Therefore,

C language is easier to read, write, understand and programming. It enables

programmer to write programs that are more or less independent of a particular type

of computer. It also enables a programmer to precisely specify what data a computer

is to act upon, how these data are to be stored or transmitted and what actions are to

be taken under various circumstances to achieve the desire results. [7]


CHAPTER 4

METHODOLOGY

4.1 Algorithm

Algorithm for Voice Communication:

1. First the input voice is taken through condenser microphone.

2. The voice signal is amplified through preamplifier phase.

3. Then, the signal is transmitted through laser light.

4. The phototransistor at receiving side converts the signal into electrical signal.

5. The electrical signal is passed through two transistor amplifier phases.

6. Then LM386 audio amplifier amplifies the signal and drive speaker to generate

voice output.

Algorithm for Data Communication:

1. First the data are inputted to microcontroller through keypad.

2. Microcontroller displays the data in LCD and gives the corresponding pulse

duration to laser via one of the pins

3. The phototransistor detects the laser light incident on it and converts it into

electrical signal.

4. These electrical signals are fed to second microcontroller which sends data to

LCD.

5. Then, LCD displays the received data.


4.2 System Diagram

4.2.1 Block Diagrams

Figure 4.1: Block diagram for voice communication

Figure 4.2: Block diagram for data communication

Loudspeaker

Condenser

Microphone

Pre amplifier

Phase Laser Torch

Phototransistor

Audio

Amplifier

Keypad

Micro Controller

LCD Laser Source Phototransistor

Micro Controller

LCD


4.2.2 Flow Charts

Transmitter

Receiver

Figure 4.3: Flow chat for voice transmission

Start

Get input from Condenser

Mic

Amplifies the voice

signal

Modulate voice signal using laser light

And transmit the voice

Detect the voice using photo

transistor

Demodulates the voice signal

Pass the voice to the

loudspeaker

Stop


Transmitter

Receiver

Figure 4.4: Flow chart for data transmission

Display result on

LCD

Detect the data from

photo transistor

Stop

Send data to the

Microcontroller

Start

Get data form Keypad

Modulate data and transmit

through laser beam

Display on LCD

Send data to the

Microcontroller


4.3 Software Implementation

4.3.1 Data Communication

In our project, the information is carried in the pulse duration of the laser beam. The

information is quantized, i.e. the pulse duration within a certain period denotes one

character, whereas the pulse duration within another period denotes other character.

This is achieved by associating delays along with the output of the port to which the

laser is connected (P3.1).

The program basically takes the input from the keypad, and compares it with each of

26 alphabets and the other three symbols. After identifying the character, the output

port was set high (SETB P3.1), and the associated delay was run before setting it low

(CLR P3.1). In this way, the particular information was transmitted.

The laser beam would strike the photo detector and the signal would be sufficiently

amplified and fed to one of the ports of the microcontroller at the receiving side

(P3.0). The port is set as an input port.

The microcontroller checks the status of this port continuously to check for any

transmitted data. When the port is high, it initiates a counter which is incremented on

each 1s delay. When the port goes low, the counter is stopped. The microcontroller

then read the character serially from SBUF register and displays it in the LCD.

4.3.2 LCD Interfacing

For interfacing LCD with microcontroller we have used Port 0 of the microcontroller

as the output port which in turn is connected to 8 data pins of the LCD .Additionally

three pins of Port 2 of the microcontroller are connected to three pins namely RS, RW

and E for driving the displaying the necessary information.

The LCD used was a 16 pin 16*2 LCD. The pin configuration and other information

are described in hardware aspect. The software aspect of LCD is described below.

RS, Register select:


There are two register inside the LCD. The RS pin allows for the selection of the

registers. If the RS pin is low, it selects the instruction command code register,

allowing us to send control command such as clear screen, cursor position setting etc.

When RS pin is high, it selects data register, allowing us to send data to the LCD for

display.

R/W, Read/Write:

It allows for the user to read information from LCD or write information to it. If R/W

=1 it allows for reading of data from LCD whereas if R/W=0, it allows writing data

into LCD.

E, Enable:

This is used to latch information presented to its data pins. When data is sent to the

data pins, high to low pulse of at minimum 450ns width needs to be applied to this pin

to latch the data.

Data Pins, D0-D7:

The data are from the microcontroller are sent in parallel format to the LCD to display

through these pins. These pins are also used to send control or command codes to

LCD for various functions. The command codes are briefly listed below:

1H: Clear Display Screen

2H: Return home

4H: Shift cursor to left

6H: Shift cursor to right

5H: Shift display right

7H: Shift display left

8H: Display off, cursor off

AH: Display off, cursor on

CH: Display on, cursor off

EH: Display on, cursor blinking

FH: Display off, cursor blinking

10 H: Shift cursor position to left


14H: Shift cursor position to right

18H: Shift the entire display to the left

1CH: Shift the entire display to the right

80H: Force cursor to beginning of the first line

C0H: Force cursor to beginning of the second line

In our configuration, the data pins were connected to port 0. Similarly the RS, R/W

and E pins were connected to P2.0, P2.1 and P2.2 respectively. Upon initialization,

the LCD would call command subroutine, which would be enable RS= 0 for

command and R/W=0 for write. Then the commands for cursor on, and cursor

position would be transmitted to LCD. After that, the data would be moved on to P0

and latched with a high to a low pulse on E. further, the data write subroutine would

be called, which would move data onto P0 and then put RS=1 denoting data to be

displayed on the LCD. The LCD was used in byte mode. Therefore, the Enable pin

“E” was strobe only once.

4.4 Working principle

4.4.1 Data Communication

For the transmission of data, the information is carried in the pulse duration of the

laser. When a key is pressed, corresponding 8 bit binary value is given to micro-

controller. Depending upon the value of row and column, micro-controller displays

one of the 26 alphabets. Then it stores these data in SBUF register to transmit it

serially. The two transistors connected as a Darlington pair at the transmission port

provide sufficient current to drive the laser. Then pulse duration of data is given to

laser. The length of the pulse duration carries the information of the data.

Photo transistor at receiving side detects the incidence laser light which causes it to

switch from low current condition to the high current condition. The output voltage of

photo transistor is then amplified and fed to the receiving port of second micro-

controller. When micro-controller detects the logic 1, data are serially read from the

SBUF register. The received data are then displayed on LCD.


4.4.2 Voice Communication

The circuit is based upon the principle of LIGHT MODULATION where instead of,

Radio frequency signals; light from a laser torch is used as the carrier in the circuit.

Here, the transmitter uses 9V power supply. Audio signal or voice is taken as input

from the condenser mic, which is, followed transistor amplifier BC548 along with op-

amp stage built around UA741.The gain of the op-amp can be controlled with the help

of 1 mega ohms pot meter.

The AF output from op-amp UA741 is coupled to the base of the power transistor

BD139, which in turn, modulates the laser. However, the three volts laser torch can

be directly connected to the emitter of BD139 and the spring loaded lead protruding

from inside the torch to the ground. In the transmitter circuit, audio signal of the non-

sinusoidal waveform and having a few mV of amplitude is taken as input from

condenser mic.Condenser mic is directly followed by the transistor amplifier stage

consist of BC548.Transistor BC548 is connected in common emitter configuration.

Resistor R1 is the source resistor, which is directly connected to the power-supply.R2,

R3 and capacitor C1 are acting as self-biasing circuits, which is used for the biasing

transistor.

These circuit arrangements provide or establish a stable operating point. Voice

Transmission through LASER. The biasing voltage is obtaining by R2 and R3

resistors network. Self-bias is used for obtaining entire audio signal as input.

Capacitor C1 is the coupling capacitor, since audio input signal is having a non-

sinusoidal waveform of different amplitude and frequency, coupling capacitor is used

to reject some of the dc noise/line as well as level from audio input signal. The self-

biased circuit is connected with the BC548 in CE configuration.

It is transistor amplifier stage, where the low amplitude audio signal is amplified to

the desired voltage. The output is taken from the collector terminal; so inverted audio

input signal is obtained. Transistor pre-amplifier stage is coupled with op-amp stage

built by ua741. C2 is the blocking capacitor while R4 is the op-amp stage resistor.

Op-amp ua741 is easily available general-purpose operational amplifier. Here pin no.

1 and 5 are not connected in order to nullify input-offset voltage. Pin no. 7 and 4 are

VCC as well as –VEE supply voltage. Pin no. 3 is non-inverting input while pin no. 2

is inverting input. Between pin no. 2 and 6, 1 mega-ohm pot meter is connected as

voltage series negative feedback, which controls the infinite gain of the op-amp.


Resistors R5 and R6 of its value acts as a voltage-divider network, thus it gives a

fixed voltage at the non-inverting pin. Input inverted audio signal is applied to the

inverting pin. Op-amp works on the differences into the applied two input voltage and

provide an output at pin no. 6. Since, input is applied to the inverting pin the output is

also an inverting one. Thus, again we get in phase high power and high amplitude

level audio signal. Capacitors C3, C4 and resistor R7 are acting as diffusion

capacitors and feedback resistor respectively. These diffusion capacitors stored the

carriers like holes and electron sin the base and thus provide self-biasing of the

transistor.

Power dissipation rate of UA741 is very high, which is not practical for driving other

electronics devices, so heat sink power transistor BD139 is used. Power transistor

BD139 absorbs most of the power and supplies the suitable power to drive the laser

torch. This in turns modulates the laser beam, since laser torch acts like a balanced

modulator, where two signals – one is message signal (audio signal) and carrier laser

signal, superimposed. So, laser beam modulates and transmits the signals to large

distances.

RECEIVER:

The receiver circuit uses an NPN phototransistor (L14F1) as the light sensor. Here,

the phototransistor receives the audio signal of low power and low amplitude

that is followed by a two-stage transistor pre-amplifier. In the pre-amplifier stage R8

is a source resistor, which is directly connected to the power supply. The pre amplifier

stage is RC coupled amplifier in CE configuration.C5, C6 are the junction

capacitances, which are taken in to the account when we consider high frequency

response, which is limited by their presence. Resistors R9 and R12 are used to

establish the biasing of the transistor BC548. R11 is self-bias resistor, which is used to

avoid degeneration.C7 is a bypass capacitor, which acts as to prevent loss of

amplification due to negative feedback arrangement.

Transistors BC548 are the amplifier transistors, which amplifies the signal because

the signal obtained by the phototransistor is of few mV. C8 is the blocking capacitor,

which is connected to the variable resistor VR2, which in turn followed by audio

power amplifier IC LM386.


Pin no. 1 and 10 is followed by C10, which is an external capacitor, used to

compensate internal error amplifier and thus avoid instability. Volume control can be

adjusted from variable resistor VR2 of 10 kilo- ohms.LM386 provides suitable power

output useful for drive the loudspeaker of 0.5W. From the pin no. 5, the high power as

well as suitable amplitude received audio signal is taken as output.R14 and C13 are

bypass arrangement used to prevent loss of amplification. C12 capacitor is used for

preventing the noise as well as the hum produced. From the loudspeaker, the audio

output is heard.

4.5 Laser Interfacing

In our circuit, we have salvaged a laser diode off a laser pen torch. The batteries were

discarded and the electrodes were connected to corresponding power source.

Although any color of laser could be chosen, the red laser diode was chosen because

of its ease of availability and due to higher wavelength of red light, relatively longer

distances could be covered.

For data communication laser is interfaced with a microcontroller at port 3.1. The

program present in the micro controller drives the laser. For voice communication

laser is driven by the output of the Op amp.


CHAPTER 5

DISCUSSION AND CONCLUSION

Discussion

At present there are various techniques which are being successfully used for

transmission of data. The data transmission techniques employ RF, FM signals for

transmission of data. Here we have look into data transmission by using laser. The

user can see the information on LCD when we type on keypad, by using laser for

transmission; higher data rates are available with lower error rate which is

advantageous over RF signal. The data is transmitted in the form of infrared rays. The

photo transistor in the receiver unit converts the received data into electrical data and

displayed on the LCD. Since it is LOS, due to environment changes the disturbances

can occurs in the message signal by which the quality of the output decreases. The

main problem with lasers is the beam dispersion can occurs due to external factors. In

order to overcome these problems most advanced powerful lasers are to be employed.

We also transmitted the voice signal with help of laser, first with help of condenser

microphone voice is input and preamplifier amplifies voices signal and with help of

laser voice is transmitted and phototransistor detect voice and converts into electrical

signal and pass to the loudspeaker, due to environment factors we hear distorted voice

in the loudspeaker.


Conclusion

The final year project “Laser based Voice and Data Communication” was completed

successfully. The project completely based on wireless communication system.

Although, wireless communication predominantly means the use of radio frequency

for communication, we explored the use of light based carriers for transfer of

information.

The emphasis of the project was to study various wireless technologies that are used

for data communication between two microcontroller controlled devices. Besides this

we have gained practical knowledge of microcontroller interfacing and the project

software development.

Although the optical data communication technology is prevailing from last decade as

optical fiber communication devices available in the market, the project was carried

out to get all ideas that are behind such wireless system. And by now, we think we are

successful in the respect. Also we hope our effort will be worthwhile if the project

work will be hopeful for those who seek to carry out any project related to optical data

communication using laser technology.


Limitation

1. The factors such as beam dispersion, background light, shadowing, rain, fog,

snow, pollution, smog causes an attenuated received signal and lead to higher

bit error rates.

2. Limited up to the range of 500m.

3. For longer distance communication high power laser is required which is

costly.


Future Enhancement

1. Multiple voice, data, picture, video can be multiplied simultaneously to

perform communication using Multiplexer.

2. Half duplex or even full duplex communication can be achieved by

software implementation

3. A more power laser can be used to increase the range of communication.

4. Laser can be replaced by IR laser that can’t be visible by bare eye.


REFERENCES

Books

1. Gerd Keiser, “Optical Fiber Communication”, Second Edition.

2. John M. Senior, “Optical Fiber Communications”, Second Edition, 2004.

3. Sharma D.K., (1999), “Communication System-I; Course Manual”, Institute

of Engineering, Tribhuvan University, Kathmandu.

4. Douglas V. Hall, “Microprocessor and Interfacing”, Tata McGraw-Hill.1999

5. Sedra, Adel.S and Smith, Kenneth C, “Microelectronic Circuits”, Oxford

University Press, 1998.

6. Mazidi Muhammad Ali, Mazzidi Janice Gillispie, “The 8051 Microntroller

and Embedded Systems”, Pearson Prentice Hall.

7. Krishna Kandel, “Learning by C”, First edition, 2007.

Websites

http://www.engineersgarage.com/electronic-components/16x2-lcd-module-datasheet

http://www.engineersgarage.com/insight/AT89C51.

http://www.electronicsforu.com/electronicsforu/microcontrollers.

http://www.ti.com/lsds/ti/analog/amplifiersandlinears/amplifiersandlinears.page.

http://www.physicsforums.com/showthread.php?t=368913.

http://www.engineersgarage.com/insight/AT89C51

http://www.electronicsforu.com/electronicsforu/microcontrollers

http://www.ti.com/lsds/ti/analog/amplifiersandlinears/amplifiersandlinears.page

http://www.physicsforums.com/showthread.php?t=368913


BIBLIOGRAPHY

i. Excerpt from Mazidi Muhammad Ali, Mazzidi Janice Gillispie, “The 8051

Microntroller and Embedded Systems”, Pearson Prentice Hall

ii. Excerpt from Datasheets of LM741 published by National Semiconductor


APPENDECES


COST ESTIMATION

S.N. Components Quantity Cost per

piece (Rs)

Total cost (Rs)

1 AT89C51 2 200 400

2 LCD 2 400 800

3 Laser Touch 2 500 1000

4 Resistor and Capacitor Few 200

5 Transistor(BC548,BC549,BD139) Few 300 300

6 LM386 1 100 100

7 741 IC Op-Amp 1 50 50

8 Photo Transistor(LI4F1) 2 400 800

9 Condenser Micro-Phone 1 30 30

10 Speaker 1 30 30

11 Keypad 1 200 200

12 Matrix Board 4 100 400

13 Crystal Oscillator 2 50 100

14 Potentiometer 2 10 20

15 Wires Few 50 50

Total Cost Rs.4500


CIRCUIT DIAGRAM

Data transmitter


Data Receiver


Circuit diagram for voice communication

Figure: Analog transmitter

Figure: Analog receiver

Documents

Laser Based Voice and Data Communication