electrical_project_chandresh_report on laser Transmitter and Receiver_ final report matter
1. INTRODUCTION OF PROJECT
1.1 PROJECT DEFINITION
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.
1.2 PROJECT OVERVIEW
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.
The transmitter circuit comprises condenser microphone transistor amplifier BC548 (T1)
followed by an pomp stage built around μA741 (IC1). The gain of the op-amp can be controlled
with the help of 1MΩ potentiometer VR1.The AF output from IC1 is coupled to the base of
transistor BD139 (T2), which, in turn, modulates the laser beam.
The transmitter uses 9V power supply. However, the 3-volt laser torch (after 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.
The receiver circuit (Fig. 2) 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 does
not 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. To avoid 50Hz
humming 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.
BLOCK DIAGRAM AND ITS
2.2 BLOCK DIAGRAM DESCRIPTION
2.2.1 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
2.2.2 TRANSMITTING SECTION
The transmitter section comprises condenser microphone, transistor amplifier BC548
followed by an op-amp stage build around IC1. The gain of the op-amp can be controlled with the
help of 1MΩ potentiometer VR1. The AF output from IC1 is coupled to the base of transistor
BD139, which is turn, modulates the laser beam. The transmitter uses 9V power supply. However,
the 3V 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.
2.2.3 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
2.2.4 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.
2.2.5 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.
There are two main part of the circuit.
3.1 Transmitter Circuit.
3.2 Receiver Circuit.
3.1 TRANSMITTER CIRCUIT
Fig.3.1. Transmitter Circuit
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 650 nm
5mW laser diode used in this project has a typical threshold current of 30 mA and produces its full
output when the current is raised by approximately 10 mA above the threshold to 40 mA.
Further increasing the current will greatly reduce the life of the laser diode, and exceeding the
absolute maximum of 80 mA 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.3.1) 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 via capacitor. This input is held at a fixed DC voltage to give a DC output to bias.
3.2 RECEIVER CIRCUIT
Fig.3.2. Receiver Circuit
The transmitted signal is picked up by the photo detector diode in the receiver (shown in
Fig.3.2). 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.
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. We use condenser
microphone in our project.
3.3.1 CONDENSER MICROPHONE
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.
HARDWARE DESIGN AND DESCRIPTION
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.
4.2 LIST OF COMPONENTS
a. Operational Amplifier.
b. VR (potentiometer/resistance Variac/Trimmer).
d. Digital Multimeter.
e. Battery (9V).
f. Laser Torch.
h. Integrated Circuit.
k. Light Emitting Diode (LED).
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
Fig.5.1 Symbol of Op-Amp
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. (5.1) 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
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. Fig.5.1.1 (a) IC-741
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.
5.1.2 NEGATIVE FEEDBACK CONTROL
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
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.
In more recent times negative feedback has been used extensively in the electronics industry
to confer, among other things, electrical stability to electronic devices. In fact without negative
feedback considerable swathes of modern technology would not be able to function. Given the
ubiquity of negative feedback in man-made devices
Fig.5.1.2 Negative feedback control
It should therefore come as no surprise to discover that living systems employ feedback at
many levels, ranging from gene regulatory network, signaling, network, metabolic networks to
neural networks and hormonal control systems.
It is possible to do a simple analysis which illustrates some of the essential properties
conferred by negative feedback. We can represent a negative feedback system using the following
Block diagram illustrating negative feedback. yo is the output, yi is called the reference or set
point that the output, yo, must match. ‘d’ is a disturbance acting on the controller A. ‘k’ represents
the fraction of output yo returned to yi as feedback. The block diagram shown above can be
expressed in algebraic form:
yo = (A + d)(yi � kyo)
where it is assumed that the disturbance d adds to the controller. By rearrangement we obtain:
yo = yi(A + d) kd + kA
If we assume that the gain in the controller, A, and the feedback, k are strong, that is Ak _ 0,
then the expression is simplified to:
yo = yi k
This equation highlights a number of effects, the first is that the controller A, and any
disturbances d are eliminated from the equation and that the output yo is a linear function of the set
point yi. The performance of the feedback is therefore dependent on the quality of the feedback
mechanism, k and is independent of either the controller or any disturbances.
In relation to actual devices, such as a stream engine, this is a desirable property. It means
that the performance of the steam engine is independent of the load and any component variation in
the construction of the engine, the only requirement is that the feedback mechanism is reliable.
Classical control theory has an extensive framework for analyzing feedback systems,
however the terminology and sometimes the methodology does not always translate easily to
biological systems. In this section we will examine the use of control coefficients and elasticities to
understand the properties of negative feedback.
5.2 VARIABLE RESISTANCE
Variable resistors consist of a resistance track with connections at both ends and a wiper which
moves along the track as you turn the spindle. The track may be made from carbon, cermet (ceramic
and metal mixture) or a coil of wire (for low resistances). The track is usually rotary but straight
track versions, usually called sliders, are also available.
Variable resistors may be used as a rheostat with two connections (the wiper and just one end of
the track) or as a potentiometer with all three connections in use. Miniature versions called presets
are made for setting up circuits which will not require further adjustment.
Variable resistors are often called potentiometers in books and catalogues. They are specified
by their maximum resistance, linear or logarithmic track, and their physical size. The standard
spindle diameter is 6mm.
The resistance and type of track are marked on the body:
4K7 LIN means 4.7 k linear track.
1M LOG means 1 M logarithmic track.
Some variable resistors are designed to be
mounted directly on the circuit board.
But most are for mounting through a hole drilled in the case containing the circuit with
stranded wire connecting their terminals to the circuit board.
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.
The capacitor plays a crucial role in electronics -- it stores electrons for when they are 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
A capacitor stores electric charge. It does not allow direct current to flow through it and it
behaves as if alternating current does
flow through. In its simplest form it
consists of two parallel metal plates
separated by an insulator called the
dielectric. The symbols for fixed and
variable capacitors are given in fig.
Polarized types must be connected so
that conventional current enters their
positive terminal. Non-polarized types
can be connected either way round.
The capacitance (C) of a capacitor measures its ability to store charge and is stated in farads
(f). The farad is sub-divided into smaller, more convenient units.
1 microfarad (1µF) = 1 millionth of a farad = 10
1 nanofarad (1 nF) = 1 thousand- millionth of a farad = 10
1 picofarad (1pF ) = 1 million-millionth of a farad = 10
In practice, capacitances range from 1 pF to about 150 000 µF: they depend on the area A of
the plates (large A gives large C), the separation d of the plates (small d gives large C) and the
material of the dielectric (e.g. certain plastics give large C).
When selecting a particular job, the factors to be considered are the value (again this is not
critical in many electronic circuits), the tolerance and the stability. There are two additional factors.
a. THE WORKING VOLTAGE
The largest voltage (d.c.or lead a.c.) which can be applied across the capacitor and is
often marked on it, e.g. 30V wkg. It is exceeded, the dielectric breaks down and permanent
damage may result.
b. THE LEAKAGE CURRENT
No dielectric is a perfect insulator but the loss of charge through it as leakage current’
should be small.
5.3.1 FIXED CAPACITORS
Fixed capacitors can be classified according to the dielectric used; their properties depend on
this. The types described below in (i), (ii) and (iii) are non-polarized, those in (iv) are polarized.
Two strips of polyester film (the plastic dielectric) are wound between two strips of
aluminum foil (the plates). Two connections, one to each strip of foil, form the capacitor leads. In
the metallized version, films of metal are deposited on the plastic and act as the plates. Their good
all-round properties and small size make them suitable for many applications in electronics. Values
range from 0.01µF to 10µF or so and are usually marked (in pF) using the resistor colour code.
Polycarbonate capacitors are similar to the polyester type; they have smaller leakage currents and
better stability but cost more.
Mica is naturally occurring mineral, which splits into very thin sheets of uniform thickness.
Plates are formed by depositing a silver film on the mica or by using interleaving sheets of
aluminum foil. Their tolerance is low ( + 1% ), stability and working voltage high, leakage current
low but they are used in radio frequency tuned circuits where low loss is important and are pictured
in figs. Polystyrene capacitors have similar though not quite so good properties as mica types but are
There are several types depending on the ceramic used. One type has similar properties to
mica and is used in radio frequency circuits. In another type, high capacitance values are obtained
with small size, but stability and tolerance are poor; they are useful where exact values are not too
important. They may be disc, rod- or plate-shaped. A disc-shaped capacitor is shown in fig. Values
range from 10pF to 1µF.
In the aluminum type the dielectric is an extremely thin layer of aluminum oxide which is
formed electrolytically. Their advantages are high values (up to 150 000µF) in a small volume and
cheapness. Their disadvantages are wide tolerance (-20 to + 100% of the value printed on them),
high leakage current and poor stability but they are used where these factors do not matter and high
values are required, e.g. in power supplies. Examples are shown in Fig.
Electrolytes are polarized. Usually their positive terminal is marked with a +VE or by a groove;
often the aluminum can is the negative terminal. The d.c. leakage current maintains the oxide layer,
otherwise reversed polarity (or disuse) will cause the layer to deteriorate.
Tantalum electrolytic capacitors can be used instead of aluminum in low voltage circuits where
values do not exceed about 100 uF. They have lower leakage currents.
5.4 DIGITAL VOLTMETER
A multimeter or a multitester, also known
as a volt/ohm meter or VOM, is an electronic
measuring instrument that combines several
measurement functions in one unit. A typical
multimeter may include features such as the
ability to measure voltage, current and
resistance. Multimeters may use analog or
digital circuits—analog multimeters and
digital multimeters (often abbreviated DMM
or DVOM.) Analog instruments are usually
based on a microammeter whose pointer
moves over a scale calibration for all the
different measurements that can be made;
digital instruments usually display digits, but
may display a bar of a length proportional to
the quantity measured.
Fig.5.4 Digital voltmeter
A multimeter can be a hand-held device useful for basic fault finding and field service work or a
bench instrument which can measure to a very high degree of accuracy. They can be used to
troubleshoot electrical problems in a wide array of industrial and household devices such as
electronic equipment, motor controls, domestic appliances, power supplies, and wiring systems.
Modern multimeters are often digital due to their accuracy, durability and extra features. In a
digital multimeter the signal under test is converted to a voltage and an amplifier with electronically
controlled gain preconditions the signal. A digital multimeter displays the quantity measured as a
number, which eliminates parallax errors.
Modern digital multimeters may have an embedded computer, which provides a wealth of
convenience features. Measurement enhancements available include:
a. Auto-ranging, which selects the correct range for the quantity under test so that the most
significant digits are shown. For example, a four-digit multimeter would automatically
select an appropriate range to display 1.234 instead of 0.012, or overloading. Auto-ranging
meters usually include a facility to 'freeze' the meter to a particular range, because a
measurement that causes frequent range changes is distracting to the user. Other factors
being equal, an auto-ranging meter will have more circuitry than an equivalent, non-auto-
ranging meter, and so will be more costly, but will be more convenient to use.
b. Sample and hold, which will latch the most recent reading for examination after the
instrument is removed from the circuit under test.
c. Current-limited tests for voltage drop across semiconductor junctions. While not a
replacement for a transistor tester, this facilitates testing diodes and a variety of transistor
d. A graphic representation of the quantity under test, as a bar graph. This makes go/no-go
testing easy, and also allows spotting of fast-moving trends.
e. A low-bandwidth oscilloscope.
f. Automotive circuit testers, including tests for automotive timing and dwell signals.
g. Simple data acquisition features to record maximum and minimum readings over a given
period, or to take a number of samples at fixed intervals.
h. Integration with tweezers for surface-mount technology.
i. A combined LCR meter for small-size SMD and through-hole components.
5.5 BATTERY (9VOLT)
An electrical battery is one or more electrochemical
cells that convert stored chemical energy into electrical
energy. Since the invention of the first battery (or "voltaic
pile") in 1800 by Alessandro Volta, batteries have become a
common power source for many household and industrial
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 Battery (9volt)
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.
If you look at any battery, you'll notice that it has two terminals. One terminal is marked
positive (+VE), while the other is marked negative (-VE). In 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
5.6 LASER TORCH
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.5.6 Laser torch
Sound is an amazing thing. All of the different sounds that wehear 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. We use
condenser microphone in our project.
5.7.1 CONDENSER MICROPHONE
A condenser microphone is essentially a capacitor, with one plate of the capacitor moving in
response to sound waves.
Condenser means capacitor, an electronic component which stores energy in the form of an
electrostatic field. The term condenser is actually obsolete but has stuck as the name for this type of
microphone, which uses a capacitor to convert acoustical energy into electrical energy.
require power from a battery or
external source. The resulting
audio signal is stronger signal
than that from a dynamic.
Condensers also tend to be more
sensitive and responsive than
dynamics, making them well-
suited to capturing subtle
nuances in a sound. They are not
ideal for high-volume work, as their sensitivity. Fig.5.7.1 Condenser
A capacitor has two plates with a voltage between them. In the condenser mic, one of these
plates is made of very light material and acts as the diaphragm. 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. When the plates are further apart, capacitance decreases and a discharge current
A voltage is required across the capacitor for this to work. This voltage is supplied either by
a battery in the mic or by external phantom power.
The electrets condenser mic uses a special type of capacitor which has a permanent voltage
built in during manufacture. This is somewhat like a permanent magnet, in that it doesn't require any
external power for operation. However good electrets condenser mics usually include a pre-
amplifier which does still require power.
Other than this difference, you can think of an electret condenser microphone as being the same
as a normal condenser.
5.8 INTEGRATED CIRCUIT
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.
Fig.5.8 Integrated circuit
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 then exposed to
visible light in the same circuit, the diode current will rise, possibly to as 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
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.
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, or between the diode and the
positive supply line.
The human eye is sensitive to a range of light radiation, 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 Photodiode Circuit
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.
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 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 response
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.
a. P-N photodiodes are used in similar applications to other photodetectors, such as
photoconductors, charge-coupled devices, and photomultiplier tubes.
b. Photodiodes are used in consumer electronics devices such as compact disc players smoke
detectors, and the receivers for remote controls in VCRs and televisions.
c. 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.
d. Photodiodes are often used for accurate measurement of light intensity in science and
industry. They generally have a better, more linear response than photoconductors.
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 and functions as follows.
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 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 than that of a photodiode by using only its base and collector terminals and ignoring the
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 than
that of a photodiode by using only its base and
collector terminals and ignoring the emitter.
Fig.5.10.1 Phototransistor Circuit
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
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 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).
5.11 LED (LIGHT EMITTING DIODE)
Light emitting diode (LED ) is basically a P-N junction semiconductor diode particularly
designed to emit visible light. There are infra-red emitting LEDs which emit invisible light. The
LEDs are now available in many colour red, green and yellow.
A normal LED emit at 2.4V and consumes MA of current. The LEDs are made in the form of flat
tiny P-N junction enclosed in a semi-spherical dome made up of clear coloured epoxy resin. The
dome of a LED acts as a lens and diffuser of light. The diameter of the base is less than a quarter of
an inch. The actual diameter varies somewhat with different makes. It is similar to the conventional
rectifier diode symbol with two arrows pointing out.
LEDs often have leads of dissimilar length
and the shorter one is the cathode. This is not
strictly adhered to by all manufacturers.
Sometimes the cathode side has a flat base. If
there is doubt, the polarity of the diode should
be identified. A simple bench method is to use
the ohmmeter incorporating 3-volt cells for
ohmmeter function. When connected with the
ohmmeter: one way there will be no deflection
and when connected the other way round there
will be a large deflection of a pointer. When
this occurs the anode lead is connected to the negative of test lead and cathode to the positive test
lead of the ohmmeter.
Fig.5.11.2 Working Of LED
An LED consists of a junction diode made from the semiconducting compound gallium
arsenide phosphide. It emits light when forward biased, the colour depending on the composition
and impurity content of the compound. At present red, yellow and green LEDs are available. When
a p-n junction diode is forward biased, electrons move across the junction from the n-type side to
the p-type side where they recombine with holes near the junction. The same occurs with holes
going across the junction from the p-type side. Every recombination results in the release of a
certain amount of energy, causing, in most semiconductors, a temperature rise. In gallium arsenide
phosphide some of the energy is emitted as light which gets out of the LED because the junction is
formed very close to the surface of the material. An LED does not light when reverse biased and if
the bias is 5 V or more it may be damaged.
2. EXTERNAL RESISTOR
An LED must have a resistor connected in series to limit the current through the LED;
otherwise it will burn out almost instantly.
The resistor value, R is given by:
R = (VS - VL) / I
VS = supply voltage
VL = LED voltage (usually 2V, but 4V for blue and white LEDs)
I = LED current (e.g. 10mA = 0.01A, or 20mA = 0.02A)
Make sure the LED current you choose is less than the maximum permitted and convert the current
to amps (A) so the calculation will give the resistor value in ohms ( ).
To convert mA to A divide the current in mA by 1000 because 1mA = 0.001A.
If the calculated value is not available choose the nearest standard resistor value which is greater,
so that the current will be a little less than you chose. In fact you may wish to choose a greater
resistor value to reduce the current (to increase battery life for example) but this will make the
LED less bright.
If the supply voltage VS = 9V, and you have a red LED (VL = 2V), requiring a current I = 20mA =
R = (9V - 2V) / 0.02A = 350 , so choose 390 (the nearest standard value which is greater).
3. DECIMAL DISPLAY
Many electronic calculators, clocks, cash registers and measuring instruments have seven-
segment red or green LED displays as numerical indicators. Each segment is an LED and
depending on which segments are energized, the display lights up the numbers 0 to 9. Such
displays are usually designed to work on a 5 V supply. Each segment needs a separate current-
limiting resistor and all the cathodes (or anodes) are joined together to form a common connection.
a. Efficiency; LEDs emit more light per watt than incandescent light bulbs. Their efficiency is
not affected by shape and size, unlike fluorescent light bulbs or tubes.
b. Color; LEDs can emit light of an intended color without using any color filters as traditional
lighting methods need. This is more efficient and can lower initial costs.
c. Size: LEDs can be very small (smaller than 2 mm2
) and are easily populated onto printed
d. On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full
brightness in under a microsecond. LEDs used in communications devices can have even
faster response times.
e. Cycling: LEDs are ideal for uses subject to frequent on-off cycling, unlike fluorescent lamps
that fail faster when cycled often, or HID lamps that require a long time before restarting.
f. Dimming: LEDs can very easily be dimmed either by pulse-width modulation or lowering
the forward current.
g. Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR
that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat
through the base of the LED.
h. Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt failure of
i. Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000
hours of useful life, though time to complete failure may be longer. Fluorescent tubes
typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of
use, and incandescent light bulbs at 1,000–2,000 hours.
j. Shock resistance: LEDs, being solid state components, are difficult to damage with external
shock, unlike fluorescent and incandescent bulbs which are fragile.
k. Focus: The solid package of the LED can be designed to focus its light. Incandescent and
fluorescent sources often require an external reflector to collect light and direct it in a usable
5.11.2 SEVEN SEGMENT DISPLAY
A seven-segment display, or seven-segment indicator, is a form of electronic display device
for displaying decimal numerals that is an alternative to the more complex dot-matrix displays.
Seven-segment displays are widely used in digital clocks, electronic meters, and other electronic
devices for displaying numerical information.
A seven segment display, as its name indicates, is composed of seven elements.
Individually on or off, they can be combined to produce simplified representations of the Arabic
Often the seven segments are arranged in an
oblique (slanted) arrangement, which aids
readability. In most applications, the seven segments
are of nearly uniform shape and size (usually
elongated hexagons, though trapezoids and
rectangles can also be used), though in the case of
adding machines, the vertical segments are longer
and more oddly shaped at the ends in an effort to
further enhance readability.
Each of the numbers 0, 6, 7 and 9 may be represented by two or more different glyphs on
The seven segments are arranged as a rectangle of two vertical segments on each side with
one horizontal segment on the top, middle, and bottom. Additionally, the seventh segment bisects
the rectangle horizontally. There are also fourteen-segment displays and sixteen-segment displays
(for full alphanumerics); however, these have mostly been replaced by dot-matrix displays.
The segments of a 7-segment display are referred to by the letters A to G, as shown to the
right, where the optional DP decimal point (an "eighth segment") is used for the display of non-
The animation to the left cycles through the
common glyphs of the ten decimal numerals and
the six hexadecimal "letter digits" (A–F). It is an
image sequence of a "LED" display, which is
described technology-wise in the following
section. Notice the variation between uppercase
and lowercase letters for A–F; this is done to
obtain a unique, unambiguous shape for each letter
(otherwise, a capital D would look identical to a 0
(or less likely O) and a capital B would look
identical to an 8).
Fig.5.11.2 (b) Seven Segment Display Showing 16 Hex Digits
6.1 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.
6.2 PCB FABRICATION
PCB fabrication involves the following steps
a. 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.
b. Then the layout is obtained using layout plus. This layout is printed on a paper.
c. This printed layout is transferred to a Mylar sheet and touched with black ink.
d. 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.
e. The exposed copper board is put in hydrogen peroxide solution. It is then put in hot water;
shook till unexposed region becomes transparent.
f. 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.
g. The plastic sheet of the five - star is removed leaving the pattern on the screen.
h. A copper clad sheet is cut to the size and cleaned. This is then placed under the screen.
i. 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.
j. The dried sheet is then etched using ferric chloride solution till all the unwanted copper is
k. The unwanted resist ink is removed using sodium hydroxide solution , holes are then drilled.
The components are soldered neatly on the board without dry soldering.
LIST OF REQUIRED TOOLS AND
7.1 Following tools and instruments are used for preparing the project
a. Soldering iron.
b. Desoldering pump.
c. Drill Machine.
g. Screw driver.
h. Dual power supply.
j. Desoldering wick.
m. Soldering Wire.
CONSTRUCTION AND TESTING
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 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.
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 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.
SETTING UPLINK AND PRECAUTION
10.1 SETTING UPLINK
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.
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.
a. 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 marrows 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.
b. 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).
c. 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.
d. 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.
ADVANTAGES AND DISADVANTAGES
a. Less costly.
b. Circuit can be easily constructed.
c. High data rate.
d. No communication licenses required.
e. The laser transmission is very secure because it has a narrow beam.
f. There are no recurring line costs.
g. Compatibility with copper or fiber interfaces and no bridge or router requirements.
h. Lasers can also transmit through glass, however the physical properties of the glass have to
i. Narrow beam divergence.
To avoid 50Hz hum (humming) 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.
12.1 PROBLEM 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
All these things are to be taken care of, for the efficient working of the project.
a. Using this circuit we can communicate with our neighbors wirelessly.
b. It can be used in inaccessible areas.
c. In future it can be commissioned in satellites for communication.
d. It can be used in conference halls.
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
Provisions have to be made for cases when there is no heavy traffic.
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Publishers, Third Edition 2009.
b. Gupta, J.B; Electronics Device & Circuits; S.K. Kataria & Sons, First Edition Dec 2000. -
c. Kumar, N. Suresh; Electronics Device & Circuits.
d. Mehta, V.K; Principles Of Electronics.
e. Navas, K.A; Electronics Lab Manual; Rajath Publishers, 2008. - Vol. 1&2.
f. Rai A.; Vallave Electronics Device & Circuits- 2007.
g. Wilson, J. & Hawkes; J.F.B. (1987). Lasers: Principles and Applications, Prentice Hall
International Series in Optoelectronics.
h. Siegmen, Anthony E.; Lasers, University Science Books U.S.