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.
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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.
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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
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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
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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
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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
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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
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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
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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
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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