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MOUNT ZION COLLEGE OF ENGINEERING(Affiliated to M.G University&Approved by A.I.C.T.E)
Kadammanitta, Pathanamthitta Kerala-689649
Email: mountzion01@ sify.com Website: www.mountzioncollege.org
DEPARTMENT OF ELECTRONICS& COMMUNICATION
CERTIFICATE
Certified that this is a bonafide record of the mini project
work done by SAM SUNNY (56601) of SIXTH semester, Electronics
&Communication Engineering, under Mahatma Gandhi University during the
year 2010.
Project Guide HOD
Internal Examiner External Examiner
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MINI PROJECT 2010 LASER TORCH BASED VOICE TRANSMITTER AND RECEIVER
ACKNOWLEDGEMENT
To bring something into existence is truly the work of
ALMIGHTY. We thank GOD ALMIGHTY for making this venture a success.
We express our wholehearted thanks to the Management
of the college, Mr. Abraham Kalammannil, Chairman, for providing us an
opportunity to do our studies in this esteemed institution. We thank our Principal,
Prof.P.C.Mohandas for providing the facilities for our studies and constant
encouragement in all achievements.
At the outset we wish to place on record our sincere
thanks to quite a few people without whose help, this venture would not have been a
success.
We would like to express profound gratitude to our Head
of the department, Mr. Rangit Varghese, for his encouragement and for providing all
facilities for carrying out this project. We express our highest regard and sincere thanks
to our project Co-ordinators, Mr. Sreeji Krishnan, who provided the necessary
guidance and serious advice to carry out this project. We also express my gratitude to
our Project Guide, Mr. Sudheesh S.R, for her apt suggestions and support. Our sincere
thanks to all the staff members of the department of Electronics and Communication
who guided as throughout the entire course
1DEPT.OF E & C MZC, KADAMMANITTA
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PREFACE
Because of the differences in our college level studies and
industry level requirements, we are allotted a project to get knowledge about the on
goings at industries. I did the mini project that covered up a practical knowledge of
what I have studied so far in books. I did experienced an exposure to various electronics
devices and equipments which I would not have able to get easily anywhere else. I
learnt a lot about processes of communication like sampling, quantization, detection,
error correction, broadcasting and reception of signals. All the topics which were dealt
with in the project duration are mentioned in an easy manner here in the report which I
am submitting to our college for reference purpose
I am highly thankful to the college faculty and the management for the insertion of
such a training period in our curriculum.
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CONTENTS
1. INTRODUCTION
2. BLOCK DIAGRAM
3. BLOCK DIAGRAM EXPLANATION
3.1. CONDENSERMICROPHONE
3.2. TRANSMITTING SECTION
3.3. LASER TORCH
3.4. RECEIVING SECTION
3.5. LOUD SPEAKER
1. CIRCUIT DIAGRAM
4.1. TRANSMITTER
4.2. RECEIVER
2. COMPONENTSTUDY
5.1. OPERATIONAL AMPLIFIER
5.1.1. IC 741
5.1.1.1. NEGATIVE FEEDBACK CONTROL
5.2. VR (potentiometer/resistance variac/trimmer)
5.3. CAPACITOR
5.4. DIGITAL MULTIMETER (DMM)
5.5. BATTERY (9 VOLT)
5.6. LASER TORCH
5.7. MICROPHONE
5.7.1. CONDENSER MICROPHONE
5.8. INTEGRATED CIRCUIT (IC)
5.9. PHOTODIODES
5.9.1. PRINCIPLE OF OPERATION
5.9.2. APPLICATIONS
5.10. PHOTOTRANSISTORS
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3. CIRCUIT DESCRIPTION
6.1. TRANSMITTER
6.2. RECEIVER
6.2.1. CONDENSER MICROPHONE
1. WORKING
2. PCB DESIGN AND FABRICATION
8.1. PCB DESIGN
8.2. PCB FABRICATION
3. COMPONENTS LAYOUT
9.1. TRANSMITTER
9.2. RECEIVER
4. PRINTED CIRCUIT BOARD (PCB) LAYOUT
9.1. TRANSMITTER
9.2. RECEIVER
5. LIST OF TOOLS AND INSTRUMENTS REQUIRED
6. COMPONENTS REQUIRED
12.1. TRANSMITTER
12.2. RECEIVER
7. CONSTRUCTION AND TESTING
13.1. CONSTRUCTION
13.2. TESTING
8. SETTING UP LINK AND PRECAUTIONS
14.1 SETTING UP A
14.2. PRECAUTIONS
14.2.1. LASER SAFETY
9. ADVANTAGES
10.DISADVANTAGES
11.PROBLEM FACED
12.APPLICATIONS
13.CONCLUSION
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REFERENCES
APPENDIX
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LIST OF FIGURES
1. TRANSMITTER……………………………………………..… xii2. RECEIVER…………………………………………………… xiii3. SYMBOL OF OP AMP…………………………………...…….xiv4. SYMBOL OF IC 741……………………………………...…… xv5. NEGATIVE FEED BACK CONTROL CIRCUIT………….….xvi6. CERAMIC CAPACITOR………………………………….… xvii7. 9V BATTERY………………………………………...……….xviii8. LASER TORCH………………………………………………...xix9. REVERSE BIASED CIRCUIT…………………………...…….xxi10.PHOTODIODE SYMBOL……………………………………..xxii11.PHOTODIODE CIRCUIT………………………………… …xxii12. PHOTOTRANSISTOR SYMBOL……………………….... …xxv13.PHOTOTRANSISTOR USED AS A PHOTODIODE………..xxvi14.COMPONENTS LAYOUT…………………………………..xxxiv15. PCB LAYOUT………………………………………………xxxiv
LIST OF TABLES
1. COMPONENTS REQUIRED FOR TRANSMITTER…xxxvii2. COMPONENTS REQUIRED FOR RECEIVER……….xxxviii
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1. INTRODUCTION
Laser as a communication medium can provide a good substitute for the
present day communication systems as the problem of interference faced in
case of electromagnetic waves is not there and high deal of secrecy is
available. Laser communications offers a viable alternative to RF
communications for inter satellite links and other applications where high-
performance links are a necessity. High data rate, small antenna size, narrow
beam divergence, and a narrow field of view are characteristics of laser
communications that offer a number of potential advantages for system
design. The present paper involves the study of wireless, open channel
communication system using laser a carrier for voice signals. Using this
circuit we can communicate with your own neighbours wirelessly. Instead of
RF signals, light from a laser torch is used as the carrier in the circuit. The
laser torch can transmit light up to a distance of about 500 meters. The
phototransistor of the receiver must be accurately oriented towards the laser
beam from the torch. If there is any obstruction in the path of laser beam, no
sounds will be heard from the receiver.
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2. BLOCK DIAGRAM
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CO
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3. BLOCK DIAGRAM EXPLANATION
3.1 CONDENSER MICROPHONE
It is also called a capacitor or electrostatic microphone. Condenser means
capacitor, which stores energy in the form of an electric field. Condenser microphones
require power from a battery or external source. Condenser also tends to be more
sensitive and responsive than dynamic, making them well suited to capturing subtle
nuances in a sound.
The diaphragm vibrates when struck by sound waves, changing the distance between
the two plates and therefore changing the capacitance. Specifically when the plates are
closer together capacitance increases and a charge current occurs and this current will
be used to trigger the transmitting section.
3.2 TRANSMITTING SECTION
The transmitter section comprises condenser microphone, transistor
amplifier BC548 followed by an op-amp stage built around IC1. The gain of
the op-amp can be controlled with the help of 1-mega ohm pot meter VR1.
The AF output from IC1 is coupled to the base of transistor Bd139, which in
turn, modulates the laser beam. The transmitter uses 9V power supply.
however, the 3-volt laser torch ( after the removal of its battery) can be
directly connected to the circuit--with the body of the torch connected to the
emitter of BD139 and the spring-loaded lead protruding from inside the
torch to circuit ground.
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3.3 LASER TORCH
Here we use the light rays coming from laser torch as the medium for transmission.
Laser had potential for the transfer of data at extremely high rates, specific
advancements were needed in component performance and systems engineering,
particularly for space-qualified hardware. Free space laser communications systems are
wireless connections through the atmosphere. They work similar to fibre optic cable
systems except the beam is transmitted through open space. The laser systems operate in
the near infrared region of the spectrum. The laser light across the link is at a
wavelength of between 780 - 920 nm. Two parallel beams are used, one for
transmission and one for reception.
3.4 RECEIVING SECTION
The receiver circuit uses an NPN phototransistor as the light sensor that is followed by
a two stage transistor preamplifier and LM386-based audio power amplifier. The
receiver doesn't need any complicated alignment. Just keep the phototransistor oriented
towards the remote transmitter's laser point and adjust the volume control for a clear
sound.
3.5 LOUD SPEAKER
A loudspeaker (or "speaker") is an electro acoustic transducer that converts an
electrical signal into sound. The speaker moves in accordance with the variations of an
electrical signal and causes sound waves to propagate through a medium such as air or
water.
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4. CIRCUIT DIAGRAM
4.1 TRANSMITTER
Fig 4.1. Transmitter
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4.2 RECEIVER
Fig 4.2. Receiver
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5. COMPONENT STUDY
5.1 OPERATIONAL AMPLIFIER
An op amp is a high-gain, direct-coupled differential linear amplifier whose response
characteristics are externally controlled by negative feedback from the output to the
input. OP amps, widely used in computers, can perform mathematical operations such
as summing, integration, and differentiation. OP amps are also used as video and audio
amplifiers, oscillators, etc. in the communication electronics. Because of their versatility
op amps are widely used in all branches of electronics both in digital and linear circuits.
OP amps lend themselves readily to IC manufacturing techniques. Improved IC
manufacturing techniques, the op amp's adaptability, and extensive use in the design of
new equipment have brought the price of IC ops amps from very high to very
reasonable levels. These facts ensure a very substantial role for the IC op amp in
electronics.
Fig shows the symbol for an op amp. Note that the operational amplifier has two inputs
marked (-) and (+). The minus input is the inverting input. A signal applied to the minus
terminal will be shifted in phase 180° at the output. The plus input is the non-inverting
input. A signal applied to the plus terminal will appear in the same phase at the output
as at the input. Because of the complexity of the internal circuitry of an op amp, the op
amp symbol is used exclusively in circuit diagrams.
Fig 5.1 symbol of op-amp
5.1.1 IC-741
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An operational amplifier often referred to as op Amp, is a very high gain high
performance amplifier designed to amplify ac and dc signal voltages. Modern integrated
circuit technology and large-scale production techniques have brought down the prices
of such amplifiers within reach of all amateurs, experimenters and hobbyists. The Op
Amp is now used as a basic gain element, like an elegant transistor, in electronic
circuits.
The availability of two input terminals simplifies feedback circuitry and makes the
operational amplifier a highly versatile device. If a feedback is applied from the output
to the inverting input terminal, the result is a negative feedback, which gives a stable
amplifier with precisely controlled gain characteristics. On the other hand, if the
feedback is applied to the non-inverting input, the result is positive feedback, which
gives oscillators and multivibrator. Special effects are obtained by combination of both
types of feedback.
Fig 5.1.1 symbol of IC741
5.1.1.1 NEGATIVE FEEDBACK CONTROL
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Fig 5.1.1.1. Negative feedback control circuit
The above figure shows the basic circuit, including the negative feedback loop of an op
amp. The output is fed back to the inverting input terminal in order to provide negative
feedback for the amplifier. The input signal is applied to the inverting input. As a result,
the output will be inverted. It is possible to operate the op amp as a non-inverting
amplifier by applying the signal to the plus input. In this circuit the feedback network is
still connected to the inverting input.
5.2 VR (potentiometer/resistance variac/trimmer):
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fig 6.2 symbol
The potentiometer is a resistor of variable resistance. It has three terminals; a fixed
resistance is found between two of the terminals and the third terminal slides along the
fixed resistor. Often, it is used to control the volume in an audio amplifier.
5.3 CAPACITOR:
The capacitor plays a crucial role in electronics -- it stores electrons for when they're
needed most. Capacitors consist of two conducting plates placed near each other. Inside
the capacitor, the terminals connect to two metal plates separated by a dielectric. The
dielectric can be air, paper, plastic or anything else that does not conduct electricity and
keeps the plates from touching each other..
fig 5.3. Ceramic capacitor
They can store electric charge for later discharge. Direct current through a capacitor
will charge the capacitor for a short time, and then stop flowing. Alternating current,
because of the changing electric fields it generates, can “flow” across a capacitor.
5.4 DIGITAL MULTIMETER (DMM)
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The DMM is an instrument that is able to measure voltage, current, and resistance in a
circuit, or across circuit components and displays its measurements on a digital display.
5.5 BATTERY (9 VOLT)
If you look at any battery, you'll notice that it has two terminals. One terminal is marked
(+), or positive, while the other is marked (-), or negative. In an normal flashlight
batteries, the ends of the battery are the terminals. In a large car battery, there are two
heavy lead posts that act as the terminals.
Electrons collect on the negative terminal of the battery. If you connect a wire between
the negative and positive terminals, the electrons will flow from the negative to the
positive terminal as fast as they can (and wear out the battery very quickly -- this also
tends to be dangerous, especially with large batteries, so it is not something you want to
be doing). Normally, you
connect some type of load
to the battery using the
wire.
Fig 5.5: 9V Battery
Inside the battery itself, a chemical reaction produces the electrons. The speed of
electron production by this chemical reaction (the battery's internal resistance) controls
how many electrons can flow between the terminals. Electrons flow from the battery
into a wire, and must travel from the negative to the positive terminal for the chemical
reaction to take place. That is why a battery can sit on a shelf for a year and still have
plenty of power unless electrons are flowing from the negative to the positive terminal,
the chemical reaction does not take place. Once you connect a wire, the reaction starts.
5.6 LASER TORCH
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For this project we have removed the laser assembly from a small laser pointer. The
power supply circuit is the green board attached to the brass laser head. We carry
similar laser pointers in our catalog that are easily disassembled for this project. The
power supply circuit came conveniently marked with a plus and a minus next to two
holes in the board. We solder the black negative lead from the battery clip to the hole
marked minus. We solder one of the coil leads to the hole marked plus. We solder the
red positive lead of the battery clip to the other lead from the coil.
Fig 3.7. Laser torch
5.7 MICROPHONE
Sound is an amazing thing. All of the different sounds that we hear are caused by
minute pressure differences in the air around us. What's amazing about it is that the air
transmits those pressure changes so well, and so accurately, over relatively long
distances. It was a metal diaphragm attached to a needle, and this needle scratched a
pattern onto a piece of metal foil. The pressure differences in the air that occurred when
you spoke toward the diaphragm moved the diaphragm, which moved the needle, which
was recorded on the foil. When you later ran the needle back over the foil, the vibrations
scratched on the foil would then move the diaphragm and recreate the sound. The fact
that this purely mechanical system works shows how much energy the vibrations in the
air can have! All modern microphones are trying to accomplish the same thing as the
original, but do it electronically rather than mechanically. A microphone wants to take
varying pressure waves in the air and convert them into varying electrical signals. There
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are five different technologies commonly used to accomplish this conversion. We use
condenser mic in our project.
5.7.1 CONDENSER MICROPHONES - A condenser microphone is
essentially a capacitor, with one plate of the capacitor moving in response to sound
waves.
5.8 INTEGRATED CIRCUIT (IC)
An integrated circuit is a pre-made circuit shrunk down to small size and put on a
chip. IC’s save circuit makers time by serving common purposes like amplifying a
signal which would otherwise have to be done by a new circuit built from scratch every
time.
5.9 PHOTODIODES
If a conventional silicon diode is connected in the reverse-biased circuit , negligible
current will flow through the diode and zero voltage will develop across R1. If the diode
casing is now carefully removed so that the diode's semiconductor junction is revealed,
and the junction is them exposed to visible light in the same circuit, the diode current
will rise, possibly to as
Fig. 5.9.1 Reverse-biased diode circuit
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high as 1 mA, producing a significant output across R1. Further investigation will show
that the diode current (and thus the output voltage) is directly proportional to light
intensity, and that the diode is therefore photosensitive.
In practice, all silicon junctions are photosensitive, and a photodiode can be regarded as
a conventional diode housed in a case that lets external light reach its photosensitive
semiconductor junction. Fig.5.9.2 shows the standard photodiode symbol.
In use, the photodiode is reverse biased and the output voltage is taken from across a
series-connected load resistor. This resistor may be connected between the diode and
ground, as in fig. 1, or between the diode and the positive supply line, as in fig.5.9.3
Fig 5.9.2 Photodiode symbol
The human eye is sensitive to a range of light radiation, as shown in fig. 5.9.4. It has a
peak spectral response to the color green, which has a wave length of about 550 nm, but
has a relatively low sensitivity to the color violet (400 nm) at one end of the spectrum
and to dark red (700 nm) at the other. Photodiodes also have spectral response
characteristics, and these are determined by the chemistry used in the semiconductor
junction material. Fig.5.9.4 shows typical response curves of a general-purpose
photodiode, and infrared (IR) photodiode.
Photodiodes have a far lower light-sensitivity than cadmium-sulphide LDRs, but give a
far quicker response to changes in light level. Generally, LDRs are ideal for use in slow-
acting direct-coupled light-level sensing applications, while photodiodes are ideal for
use in fast-acting AC-coupled signaling applications. Typical photodiode
applications include IR remote-control circuits.
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Fig 5.9.3 Photodiode circuit
A photodiode is a semiconductor diode that functions as a photo detector. Photodiodes
are packaged with either a window or optical fibre connection, in order to let in the light
to the sensitive part of the device. They may also be used without a window to detect
vacuum UV or X-rays.
A phototransistor is in essence nothing more than a bipolar transistor that is encased in a
transparent case so that light can reach the base-collector junction. The phototransistor
works like a photodiode, but with a much higher sensitivity for light, because the
electrons that are generated by photons in base-collector junction are injected into the
base, this current is then amplified by the transistor operation. A phototransistor has a
slower response time than a photodiode however.
5.9.1 PRINCIPLE OF OPERATION
A photodiode is a p-n junction or p-i-n structure. When light with sufficient photon
energy strikes a semiconductor, photons can be absorbed, resulting in generation of a
mobile electron and electron hole. If the absorption occurs in the junction's depletion
region, these carriers are swept from the junction by the built-in field of the depletion
region, producing a photocurrent.
Photodiodes can be used in either zero bias or reverse bias. In zero bias, light falling on
the diode causes a voltage to develop across the device, leading to a current in the
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forward bias direction. This is called the photovoltaic effect, and is the basis for solar
cells — in fact; a solar cell is just a large number of big, cheap photodiodes.
Diodes usually have extremely high resistance when reverse biased. This resistance is
reduced when light of an appropriate frequency shines on the junction. Hence, a reverse
biased diode can be used as a detector by monitoring the current running through it.
Circuits based on this effect are more sensitive to light than ones based on the
photovoltaic effect.
Avalanche photodiodes have a similar structure; however they are operated with much
higher reverse bias. This allows each photo-generated carrier to be multiplied by
avalanche breakdown, resulting in internal gain within the photodiode, which increases
the effective responsivity of the device.
Because of their greater band gap, silicon-based photodiodes generate less noise than
germanium-based photodiodes, but germanium photodiodes must be used for
wavelengths longer than approximately 1 µm.
5.9.2 APPLICATIONS
P-N photodiodes are used in similar applications to other photodetectors, such as
photoconductors, charge-coupled devices, and photomultiplier tubes.
Photodiodes are used in consumer electronics devices such as compact disc players
smoke detectors, and the receivers for remote controls in VCRs and televisions.
In other consumer items such as camera light meters, clock radios (the ones that dim the
display when its dark) and street lights, photoconductors are often used rather than
photodiodes, although in principle either could be used.
Photodiodes are often used for accurate measurement of light intensity in science and
industry. They generally have a better, more linear response than photoconductors.
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5.10 PHOTOTRANSISTORS
The standard symbol of a phototransistor, which can be regarded as a conventional
transistor housed in a case that enables its semiconductor junctions to be exposed to
external light. The device is normally used with its base open circuit, in either of the
configurations shown in fig. 5.10.2, and functions as follows.
Fig. 5.10.1Phototransistor symbol.
.In practice, the collector and emitter current of the transistor are virtually identical and,
since the base is open circuit, the device is not subjected to significant negative
feedback. Consequently, the alternative fig. 5.10.2(b) circuit, in which R1 is connected
to Q1 emitter, gives a virtually identical performance to that of fig. The sensitivity of a
phototransistor is typically one hundred times greater than that of a photodiode, but is
useful maximum operating frequency (a few hundred kilohertz) is proportionally lower
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than that of a photodiode by using only its base and collector terminals and ignoring the
emitter, as shown in fig.
Phototransistors are solid-state light detectors with internal gain that are used to provide
analog or digital signals. They detect visible, ultraviolet and near-infrared light from a
variety of sources and are more sensitive than photodiodes, semiconductor devices that
require a pre-amplifier. Phototransistors feed a photocurrent output into the base of a
small signal transistor. For each illumination level, the area of the exposed collector-
base junction and the DC current gain of the transistor define the output.
Fig. 5.10.3 Phototransistor used as a photodiode
The base current from the incident photons is amplified by the gain of the transistor,
resulting in current gains that range from hundreds to several thousands. Response time
is a function of the capacitance of the collector-base junction and the value of the load
resistance. Photodarlingtons, a common type of phototransistor, have two stages of gain
and can provide net gains greater than 100,000. Because of their ease of use, low cost
and compatibility with transistor-transistor logic (TTL), phototransistors are often used
in applications where more than several hundred nanowatts (nW) of optical power are
available. Selecting phototransistors requires an analysis of performance specifications.
Collector current is the total amount of current that flows into the collector terminal.
Collector dark current is the amount of collector current for which there is no optical
input. Typically, both collector current and collector dark current are measured in
milliamps (mA). Peak wavelength, the wavelength at which phototransistors are most
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responsive, is measured in nanometers (nm). Rise time, the time that elapses when a
pulse waveform increases from 10% to 90% of its maximum value, is expressed in
nanoseconds (ns). Collector-emitter breakdown voltage is the voltage at which
phototransistors conduct a specified (nondestructive) current when biased in the normal
direction without optical or electrical inputs to the base. Power dissipation, a measure of
total power consumption, is measured in milliwatts (mW).
6. CIRCUIT DESCRIPTION
There are two sections: the transmitter board and the receiver board, both powered
by a separate 9V battery or a fixed voltage power supply, depending on your needs. The
transmitter board has an electret microphone module at one end, and the laser diode at
the other end. The electronics modulates the intensity of the laser beam according to the
output of the microphone. The laser diode has an inbuilt collimating lens, and is simply
a module that connects to the transmitter board. The receiver uses a photodiode as the
receiving element, and the onboard amplifier powers a small 4-36 ohm speaker. This
board is therefore a high gain amplifier with a basic audio output stage.
But what about results - are they better? Sure. Because this design uses a higher
power (and visible) laser beam, the range is improved, and alignment is easier and not
all that critical, especially over a few hundred meters. The quality of sound transmitted
by the link is quite surprising. Clearly, this project is ideal for setting up a speech
channel between two areas, say adjacent houses or offices on opposite sides of the
street. Or you could use it as a link between the work shop and the house. For duplex
(two way) communication, you'll obviously need two laser 'channels’. An important
feature of transmission by laser beam is privacy. Because a laser beam is intentionally
narrow, it's virtually impossible for someone to tap into the link without you knowing.
If someone intercepts the beam, the link is broken, signaling the interception. Fibre-
optic cables also have high security, as it's very difficult to splice into the cable without
breaking the link. However it's theoretically possible; so for the highest security, you
probably can't beat a line-of-sight laser beam.
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Where the transmission distance is no more than meter of so, a LED (or two for
increased power) can be substituted for the laser diode. For instance, where the link is
being used for educational purposes, such as demonstrating fibre-optic coupling, or the
concept of communication over a light beam. Obviously the security of the transmission
is much lower as LEDs transmit light in all directions. While this laser link can be
adapted for use as a perimeter protector. Now to a description of how it all works, it's
really very simple. We'll start with the transmitter.
6.1TRANSMITTER
A laser diode needs a certain value of current, called the threshold current, before it
emits laser light. A further increase in this current produces a greater light output. The
relationship between output power and current in a laser diode is very linear, once the
current is above the threshold, giving a low distortion when the beam is amplitude
modulated. For example, the 65Onm 5mW laser diode used in this project has a typical
threshold current of 3OmA and produces its full output when the current is raised by
approximately 1OmA above the threshold to 4OmA.
Further increasing the current will greatly reduce the life of the laser diode, and
exceeding the absolute maximum of 8OmA will destroy it instantly. Laser diodes are
very fragile and will not survive electrostatic discharges and momentary surges!
However, if used within specifications, the typical life of one of these lasers is around
20,000 hours. In the transmitter circuit (Fig.1) the laser diode is supplied via an
adjustable constant-current source. Note that the metal housing for the laser diode and
the lens also acts as a heat sink. The laser diode should not be powered without the
metal housing in place. The increasing the voltage at VR1 reduces the laser current. The
setting of VR1 determines the quiescent brightness of the laser beam, and therefore the
overall sensitivity of the system. The electric microphone is powered through R1 and is
coupled to the non inverting input of 1C1 a via capacitor. This input is held at a fixed
DC voltage to give a DC output to bias.
6.2 RECEIVER
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The transmitted signal is picked up by the photo detector diode in the receiver (shown in
Fig.2). The output voltage of this diode is amplified by the common emitter amplifier
around T4. This amplifier has a gain of 20 or so, and connects via VR2 to IC2, an
LM386 basic power amplifier IC with a gain internally set to 20.This IC can drive a
speaker with a resistance as low as four ohms, and 35OmW when the circuit is powered
from a 9V supply. Increasing the supply voltage will increase the output power
marginally. Incidentally, the photodiode used for this project has a special clear
package, so it responds to visible light, and not just infrared.
6.3 MICROPHONE
Sound is an amazing thing. All of the different sounds that we hear are caused by
minute pressure differences in the air around us.
What's amazing about it is that the air transmits those pressure changes so well, and
so accurately, over relatively long distances. It was a metal diaphragm attached to a
needle, and this needle scratched a pattern onto a piece of metal foil. The pressure
differences in the air that occurred when you spoke toward the diaphragm moved the
diaphragm, which moved the needle, which was recorded on the foil. When you later
ran the needle back over the foil, the vibrations scratched on the foil would then move
the diaphragm and recreate the sound. The fact that this purely mechanical system
works shows how much energy the vibrations in the air can have! All modern
microphones are trying to accomplish the same thing as the original, but do it
electronically rather than mechanically. A microphone wants to take varying pressure
waves in the air and convert them into varying electrical signals. There are five different
technologies commonly used to accomplish this conversion:
6.3.1 Condenser microphones - A condenser microphone is essentially a capacitor,
with one plate of the capacitor moving in response to sound waves. The movement
changes the capacitance of the capacitor, and these changes are amplified to create a
measurable signal. Condenser microphones usually need a small battery to provide a
voltage across the capacitor.
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7. WORKING
In all of the laser communicators on this page, the laser light is amplitude
modulated. This simply means that the amount of light the laser emits varies over time.
To understand what is going on, it helps to consider how a loudspeaker makes sound. A
loudspeaker is a paper cone attached to a coil of wire that sits in a magnetic field from a
strong permanent magnet. When an electric current flows in the loudspeaker coil, the
coil becomes an electromagnet, and it moves toward or away from the permanent
magnet. As it moves, the paper cone pushes on the air around it, compressing the air in
front of it, and expanding the air behind it. Waves of compressed and expanded air
travel to your ear, and cause your eardrum to move in time to the movements of the
paper cone. The laser communicator adds two components to the loudspeaker concept.
We take the electrical signal that goes to the loudspeaker, and connect it instead to the
laser, so the laser gets brighter and dimmer as the electric current varies. The second
component is the receiver, which converts the light back into an electric current. This
current varies in time with the first current, because the amount of light that it receives
is varying in time. This second electric current is used to move the paper cone of a
loudspeaker, just as before. However, now the loudspeaker can be quite a distance away
from the original electric current, without any wires connecting the two.
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8. PCB DESIGN AND FABRICATION
8.1 PCB DESIGN
Designing of PCB is a major step in the production of PCB is a major. It forms a
distinct factor in electronic performance and reliability. The productivity of a PCB,
its assembly and service ability also depends on the design.
The designing of a PCB consists of designing of the layout followed by the
preparation of the artwork. The layout should include all the relevant aspects in
details of the PCB design while the art work preparation brings it to the form
required for the production process. The layout can be designed with the help of any
one of the standard layout edition softwares such as Eagle, Orcad or Edwin XP.
Hence a concept, clearly defining all the details of the circuits and partly of the
equipment, is a prerequisite and the actual layout can start. Depending on the
accuracy required, the artwork might be produced a 1:1 or 2:1 even 4:1 scale. It is
best prepared on a 1:1 scale.
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8.2 PCB FABRICATION
PCB fabrication involves the following steps.
1) First the layout of the PCB is generated using the software ORCAD.
First step involves drawing the circuit CIS which is a section of ORCAD.
Then the layout is obtained using layout plus. This layout is printed on a
paper.
2) This printed layout is transferred to a Mylar sheet and touched with black
ink.
3) The solder side of the Myler sheet is placed on the shining side of the
copper board and is placed in a frame. It is than exposed to sunlight,
with the Mylar sheet facing the sunlight.
4) The exposed copper board is put in hydrogen peroxide solution. It is
then put in hot water; shook till unexposed region becomes transparent.
5) This is put in cold water and then the rough side is struck in to the skill
screen. This is then pressed and dried well.
6) The plastic sheet of the five - star is removed leaving the pattern on the
screen.
7) A copper clad sheet is cut to the size and cleaned. This is then placed
under the screen.
8) Acid resist ink is spread on the screen, So that the pattern of the tracks
and pad is obtained on the copper clad sheet. It is dried.
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9) The dried sheet is then etched using ferric chloride solution till all the
unwanted copper is etched away.
10) The unwanted resist ink is removed using sodium hydroxide solution,
holes are then drilled.
11) The components are soldered neatly on the board without dry soldering
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9. COMPONENTS LAYOUT
9.1 TRANSMITTER 9.2 RECEIVER
Fig.14
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10. PRINTED CIRCUIT BOARD (PCB) LAYOUT
10.1 TRANSMITTER
10.2 RECEIVER
Fig 15
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11. LIST OF TOOLS AND INTRUMENTS REQUIRED
Following tools and instruments are used for preparing the project
1 Soldering iron
2 Desoldering pump
3 Drill Machine
4 Multimeter
5 Filer
6 Tweezers
7 Screw driver
8 Dual power supply
9 Flux
10 Desoldering wick
11 Petrol
12 Brush
13 Soldering Wire
12. COMPONENTS REQUIRED
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12.1 TRANSMITTER:
Sl.No. NAME OF THE COMPONENT QUANTITY
1. Resistance (8.2 K) 2
2 Resistance (1.8 M) 1
3. Resistance (10 K) 1
4. Resistance (15 K) 2
5. Resistance (82 ohm) 1
6. Variable Resistance (1 M) 1
7. Capacitor (1 mf) 1
8. Capacitor (0.1 mf) 1
9. Capacitor (470 mf) 1
10. Capacitor (1000 mf) 111. Semiconductor T1 BC548 112. Semiconductor T2 BD139 113. Condenser MIC 114. IC UA741 115. PCB 1
Table 1
12.2 RECEIVER:
Sl.No. NAME OF THE COMPONENT QUANTITY
1. Resistor (6.8 K) 1
2 Resistor (4.7M) 1
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3. Resistor (470 K) 1
4. Resistor (2.2 K) 2
5. Resistor (1 K) 1
6. Resistor (10 K) 1
7. Variable resistor (50 K) 1
8. Capacitor (0.01 mf) 1
9. Capacitor(47 pf) 1
10. Capacitor (0.1 mf) 211. Capacitor (1 mf) 112. Capacitor (100 mf) 213. Capacitor(10mf) 114. Capacitor(470 mf) 115. Semiconductor 2N5777 116. Semiconductor BC549 217. LM 386 118. P.C.B 119. 8 ohm Speaker 1
Table 2
13. CONSTRUCTION AND TESTING
13.1CONSTRUCTION
As the photos show, both the transmitter and the receiver are built on silk- screened
PCBS. As usual fit the resistors, pots and capacitors first, taking care with the polarity
of the electrolytic. IC sockets are not essential, although servicing is obviously made
easier if they are used. In which case, fit these next, followed by the transistors and
photo transistors
The photo diode/ transistors, is mounted directly on the receiver PCB. When first
mounted, the active side of the diode (Black Square inside the package) will face
towards the centre of the board. You then bend the diode over by almost 180' so the
active surface now faces outwards. The polarized microphone element solders directly
to the transmitter PCB. The negative lead is marked with a minus sign and is the lead
that connects to the metal case. The laser diode is also polarized, and has three leads. Of
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these, only two are used, shown on the circuit. Take care when soldering the laser in
place, as too much heat can destroy it. The diode can be mounted on the board, or
connected with leads to it. Connect a clip lead to the inside of the laser pointer where
the battery touched. Usually there is a small spring to which you can attach the clip lead.
The other end of the battery usually connects to the case of the laser. Since there are
many different styles of laser pointer, you may have to experiment with clip lead
placement to get the laser to work with the new external battery pack. You may also
have to hold down the laser's push button switch by wrapping a rubber band or some
wire around it. Finally, connect the speaker and 9V battery clips, then check over the
boards for any soldering errors or incorrectly installed components
13.2 TESTING
First of all, it's most important that you don't look directly into the laser beam. If you
do, it could cause permanent eye damage. Also, you are responsible for the safety of
others near the laser, which means you must stop others from also looking into the
beam, and take all necessary safety steps. This is covered by legislation.
Both the receiver and the transmitter can be powered by separate 9V batteries or
suitable DC supplies. Before applying power to the transmitter PCB, set VRI to its
halfway position, to make sure the laser current is not excessive. To be totally sure, you
could set VRI fully anticlockwise, as this setting will reduce the laser current to zero.
Then apply power to the board. If the laser doesn't produce light, slowly adjust VRI
clockwise. The laser diode should emit a beam with an intensity adjustable with VRI.
At this stage, keep the beam intensity low, but high enough to clearly see. If you are not
getting an output, check the circuit. You won't see the laser beam intensity change with
the modulating signal.
To check that the system is working, place the two PCBs on the workbench, spaced a
meter or go apart. You might need to put a sheet of paper about 2Omm in front of the
photodiode to reduce the intensity of light from the laser beam. Set the volume control
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of the speaker to about halfway. If the volume control setting is too high you'll get
acoustic feedback.
Move the laser diode assembly so the beam points at the receiver's photodiode. It's
useful to adjust the beam so it's out of focus at the photodiode, to make alignment even
easier. You should now be able to hear the speaker reproducing any audio signal picked
up by the microphone.
.
14. SETTING UP LINK AND PRECAUTIONS
14.1SETTING UP A LINK
Once you've tested the link, you'll probably be keen to put it to use. For a short link
of say 100 meters, all you need do is position the receiver so the laser beam falls on the
photodiode. Once the link is established, adjust VRI higher the laser current, the shorter
will be its life. If you have an ammeter, connect it to measure the current taken by the
transmitter board. Most of the current is taken by the laser, so adjust VRI to give a total
current consumption of no more than 45Ma. Also, focus the laser so all of the beam is
striking the photodiode. At close range, there's probably no need to focus the beam. In
fact, because of the high output power (5mW) of the laser diode, excellent results will
be obtained over reasonably short distances (20 meters or so) with rough focusing and
quiescent current adjustments. But the longer the distance between the transmitter and
the receiver, the more critical the adjustments. For example, for distances over 20
meters, you might have to put a piece of tube over the front of the photodiode to limit
the ambient light falling on it. This diode is responsive to visible light, so a high
ambient light could cause it to saturate. For very long distances, say half a kilometer,
you'll probably need a parabolic reflector for the laser beam, to focus it directly onto the
photodiode.
For short ranges (a meter or so), or for educational or testing purposes, you can use a
conventional red LED. Adjust the quiescent current with VR1. The light output of a
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LED is not focused, and simply spreads everywhere, so a reflector might help the
sensitivity. Warnings The laser diode in this project is a class 3B laser and you should
attach a warning label to the transmitter.. Remember that, as for any hazardous device,
the owner of a laser is responsible for its proper use.
14.2 PRECAUTIONS
14.2.1. LASER SAFETY:
Safety instructions for lasers: Laser beams may damage the eyes severely or may cause
blindness if they radiate into the eyes directly or indirectly. Therefore the laser
electronics must be installed in such a manner that radiation into the eyes will be
impossible neither directly nor indirectly via marrow’s in the room.
When using lasers with an output power higher than 1 mW, you should check about the
legal regulations for prevention of accidents and be very careful. Normal laser pointers
sold in shops have typically output power of 1..5 mW (power depends on laser pointer
model and what country regulations say on maximum power). This power level is
normally not very hazardous, but can cause permanent dotages your eye if you stare at
the beam.
We should be very careful with higher power lasers and lasers on that power range that
emit invisible radiation, because they can cause immediate eye damage (and very high
power lasers can cause skin burns or fire).With any high power laser make sure that you
have safe operating environment, necessary regulations/permissions and somebody that
takes care that these legal regulations are observed. Lasers use coherent light which has
very different properties to a standard lighting effect. This is what makes lasers one of
the most beautiful forms of light, but also one of the most dangerous light sources if not
used with proper cautions
2. In the transmitter schematic, no ballast resistor is shown because most small
LASER power supplies already have one built in. Yours may differ, and a resistor may
be needed.
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3. The receiver should be kept away from bright lights. You may put a piece of wax
paper in front of photo transistor to keep the LASER from swamping it.
15. ADVANTAGES
1. Less costly
2. Circuit can be easily constructed
3. High data rate
4. No communication licenses required.
5. The laser transmission is very secure because it has a narrow beam.
6. There are no recurring line costs.
7. Compatibility with copper or fiber interfaces and no bridge or router
requirements.
8. Lasers can also transmit through glass, however the physical properties of the
glass have to be considered.
9. Narrow beam divergence
10.Laser transmitter and receiver units ensure easy, straightforward
systems alignment and long-term stable, service free operation,
especially in inaccessible environments, optical wireless systems
offer ideal, economical alternative to expensive leased lines for
buildings.
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16. DISADVANTAGES
1. To avoid 50Hz hum noise in the speaker, keep the phototransistor away from
AC light sources such as bulbs. The reflected sunlight, however, does not cause
any problem. But the sensor should not directly face the sun.
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17. PROBLEMS FACED
Although this project was successfully completed, however a few hurdles that came
during the construction of the circuit were the breaking of the thin electrical wires after
it had been soldered and the breaking of the photodiode receiver’s leg leading to an
error in reception of data.
Moreover the connections with the OP-AMP chip have to be dealt with very carefully
because one wrong connection may damage the whole chip. If the supply to laser is
greater than it will not glow.
All these things are to be taken care of, for the efficient working of the project.
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18. APPLICATIONS
1. Using this circuit we can communicate with our neighbors wirelessly
2. It can be used in inaccessible areas.
3. In future it can be commissioned in satellites for communication.
4. It can be used in conference halls.
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19. CONCLUSION
After the successful working of the project, it can be concluded that this project is
suitable for easily communication. There can be further up gradations in the project
which could lead to a much better system for communication. Some of the possible
ways are as follows:-
Instead of the short range laser, high range lasers can be used which range a few
hundred meters.
Provisions have to be made for cases when there is no heavy traffic.
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REFERENCES
1. [Online] // wikipedia. - www.wikipedia.com.2. [Online] // circuitstoday. - www.circuitstoday.com.3. [Online] // electronics schematics. - www.electroschematic.com.4. [Online] // electronics for you. - www.efy.com.5. D.ROY CHOUDHARY SHALIN B. JAIN LINEAR INTEGRATED CIRCUITS [Book]. - DELHI : NEW AGE INTERNATIONL PUBLISHERS, THIRD EDITION 2009.6. ELECTRONICS FOR YOU MAGAZINE [Book].7. GUPTA J.B. ELECTRONICS DEVICE & CIRCUITS [Book]. - INDIA : S.K. KATARIA & SONS, FIRST EDITION DEC 2000. - Vol. 1.8. KUMAR N. SURESH ELECTRONICS DEVICE & CIRCUITS [Book]. - 2008.9. MEHTA V.K. PRINCIPLES OF ELECTRONICS [Book].10. NAVAS K.A. ELECTRONICS LAB MANUAL [Book]. - [s.l.] : Rajath publishers, 2008. - Vol. 1&2.11. RAI A. VALLAVE ELECTRONICS DEVICE & CIRCUITS [Book]. - 2007.
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APPENDIX
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General PurposeSingle Opeartional Amplifier
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