Laser Based Voice and Data Communication
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Transcript of 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.
2 Laser Based Voice and Data Communication
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)
3 Laser Based Voice and Data Communication
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.
4 Laser Based Voice and Data Communication
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
5 Laser Based Voice and Data Communication
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
6 Laser Based Voice and Data Communication
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
7 Laser Based Voice and Data Communication
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
8 Laser Based Voice and Data Communication
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
9 Laser Based Voice and Data Communication
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
10 Laser Based Voice and Data Communication
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
11 Laser Based Voice and Data Communication
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
12 Laser Based Voice and Data Communication
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.
13 Laser Based Voice and Data Communication
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
14 Laser Based Voice and Data Communication
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.
15 Laser Based Voice and Data Communication
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
16 Laser Based Voice and Data Communication
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
17 Laser Based Voice and Data Communication
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
18 Laser Based Voice and Data Communication
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.
19 Laser Based Voice and Data Communication
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
20 Laser Based Voice and Data Communication
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]
21 Laser Based Voice and Data Communication
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.
22 Laser Based Voice and Data Communication
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
23 Laser Based Voice and Data Communication
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
24 Laser Based Voice and Data Communication
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
25 Laser Based Voice and Data Communication
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:
26 Laser Based Voice and Data Communication
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
27 Laser Based Voice and Data Communication
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.
28 Laser Based Voice and Data Communication
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.
29 Laser Based Voice and Data Communication
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.
30 Laser Based Voice and Data Communication
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.
31 Laser Based Voice and Data Communication
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.
32 Laser Based Voice and Data Communication
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.
33 Laser Based Voice and Data Communication
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.
34 Laser Based Voice and Data Communication
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.
35 Laser Based Voice and Data Communication
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.
36 Laser Based Voice and Data Communication
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
37 Laser Based Voice and Data Communication
APPENDECES
38 Laser Based Voice and Data Communication
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
39 Laser Based Voice and Data Communication
CIRCUIT DIAGRAM
Data transmitter
40 Laser Based Voice and Data Communication
Data Receiver
41 Laser Based Voice and Data Communication
Circuit diagram for voice communication
Figure: Analog transmitter
Figure: Analog receiver