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Contents 1- Introduction:
..............................................................................................................................................
4
2-Objective:
...................................................................................................................................................
4
3-Theory:
......................................................................................................................................................
5
3.1Basic concept of Laser:
............................................................................................................................
5
Spontaneous emission:
..................................................................................................................................
5
Stimulated emission
......................................................................................................................................
5
3.2 Free-space optical communication (FSO):
.............................................................................................
7
Advantages
....................................................................................................................................................
7
Disadvantages
...............................................................................................................................................
7
3.3Some real life
Applications:.....................................................................................................................
8 4-Circuit Components:
....................................................................................................................................
9
4.1 Op-amp LM-741:
..................................................................................................................................
10
4.2 Op-amp LM-386:
..................................................................................................................................
11
4.3 BC548 NPN General Purpose Amplifier:
.............................................................................................
12
4.4 2N5777 Silicon NPN Photo Detector:
..................................................................................................
12
4.5 BC549 NPN General Purpose Transistors:
...........................................................................................
12
4.6 Laser Light(650nm-5mW laser Pointer):
..............................................................................................
12
4.7 Table of components:
............................................................................................................................
13 5 -Theory of operation:
..................................................................................................................................14
5.1The
Transmitter:.....................................................................................................................................
14
5.2The Receiver:
.........................................................................................................................................
15 6-Simulation:
................................................................................................................................................16
6.1 Schematic:
.............................................................................................................................................
16
6.2 Layout:
..................................................................................................................................................
17 8-Problems &improvements:
..........................................................................................................................19
8.1 Problems:
..............................................................................................................................................
19
8.2 Improvements:
......................................................................................................................................
19
8.3Possible Improvements:
.........................................................................................................................
19 9-Hardware Implementation:
.........................................................................................................................18
10-Conclusion:
..............................................................................................................................................19
11-List of references:
.....................................................................................................................................21
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List of Figures:
Figure1: Laser Audio communication system Block Diagram..4
Figure2: Energy state diagram ..6
Figure3: Free space optical network..8
Figure4: Basic op-amp 9
Figure5: Inverting Amplifier ... 9
Figure6: LM-741 .10
Figure7: LM-386 .....11
Figure8 BC548 transistor ...12
Figure9 Transmitter circuit 14
Figure10: Receiver circuit ..15
Figure11: Transmitter circuit .16
Figure12: Receiver circuit ..16
Figure13: Transmitter layout ..17
Figure14: Receiver layout .. ....17
Figure15: Transmitter Hardware implementation .. 18
Figure16: Receiver Hardware implementation ...18
Figure17: Receiver Hardware implementation using stereo speaker
......18
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List of Tables:
Table1: RF (802.11) vs. free-space visible spectrum (LED)8
Table 2: Some Op-amp Parameters.10
Table3: Transmitter components..13
Table4: Receiver components..13
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1- Introduction:
A laser as a communications medium has some unique properties
compared to other forms
of media. A line-of-sight laser beam is useful where wires
cannot be physically connected to
a remote location. A laser beam, unlike wires, also does not
require special shielding over
longer distances. Lasers offer at least an order of magnitude
longer distances compared to
infrared LEDs. Although RF transmitters may offer longer
distances than line-of-sight
lasers, they are subject to interference from other
transmitters. Since the laser medium is
line-of-sight and the beam being only several millimeters in
diameter it is very difficult for
the data stream to be tapped. This offers secure communication
since any attempts to
intercept the laser beam would be detected at the receiver as a
loss in data; also they have the
benefit of eliminating the need for broadcast rights and buried
cables. Laser communications
systems can be easily deployed since they are inexpensive,
small, low power and do not
require any radio interference studies. The carrier used for the
transmission signal is
typically generated by a laser diode.
2-Objective:
The objective of this circuit is to transmit sound wirelessly
over large distance with very
high speed using the laser beam as carrier that changes its
intensity according to the
amplitude of the input sound, the input sound is converted from
analog to digital using the
transmitter.
Figure1: Laser Audio communication system Block Diagram
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3-Theory:
There is only one way that light can be produced: that is,
through the rapid change of state of
an electron from a state of relatively high energy to a (more
stable) state of lower energy.
When this happens the energy has to go somewhere and it is
often34 emitted in the form of
light. The word "laser" is an acronym for Light Amplification by
Stimulated Emission of
Radiation. Lasers are finding ever increasing military
applications principally for target
acquisition, fire control, and training. These lasers are termed
rangefinders, target
designators, and direct-fire simulators. Lasers are also being
used in communications, laser
radars (LIDAR), landing systems, laser pointers, guidance
systems, scanners, metal working,
photography, holography, and medicine.
The primary wavelengths of laser radiation for current military
and commercial applications
include the ultraviolet, visible, and infrared regions of the
spectrum. Ultraviolet radiation for
lasers consists of wavelengths between 180 and 400 nm. The
visible region consists of
radiation with wavelengths between 400 and 700 nm. This is the
portion we call visible
light. The infrared region of the spectrum consists of radiation
with wavelengths between
700 nm and 1 mm.
3.1Basic concept of Laser:
Spontaneous emission: is really the normal case. When an
electron is elevated to a high energy state this state is usually
unstable and the electron will spontaneously return to a
more stable state very quickly (within a few picoseconds)
emitting a photon as it does so.
When light is emitted spontaneously its direction and phase will
be random but the
wavelength will be determined by the amount of energy that the
emitting electron must give
up.
Stimulated emission is what happens in the operation of a laser.
In some situations when an electron enters a high energy (excited)
state it is able to stay there for a relatively long
time (a few microseconds) before it changes state spontaneously.
When an electron is in this
semi-stable (metastable) high energy state it can be stimulated
by the presence of a photon
of light to emit its energy in the form of another photon. In
this case the incident photon
must have the right energy (wavelength) within quite small
limits.
It is of fundamental importance to understand that when
stimulated emission takes place the
emitted photon has exactly the same wavelength, phase and
direction as that of the photon
which stimulated it. For spontaneous or stimulated emission to
occur, energy must be
supplied to boost the electron from its low energy state to a
higher energy state.
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Figure 2: Energy state diagram showing: (a) absorption; (b)
spontaneous emission;
(c) Stimulated emission. The black dot indicates the state of
the atom before and after a transition takes
place
The energy can come from many sources:
Heat.
Electrical Discharge.
Electric Current.
Chemical Reaction.
Biological Reactions (Bioluminescense).
Absorption of Light.
Nuclear Radiation.
(a) By spontaneous emission in which the atom returns to the
lower energy state in an
entirely random manner;
(b) By stimulated emission when a photon having an energy equal
to the energy difference
between the two states (E2 E1) interacts with the atom in the
upper energy state causing it
to return to the lower state with the creation of a second
photon.
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3.2 Free-space optical communication (FSO):
Over the last two decades free-space optical communication (FSO)
has become more and
more interesting as an adjunct or alternative to radio frequency
communication. Free-space
optical communication (FSO) systems (in space and inside the
atmosphere) have developed
in response to a growing need for high-speed and tap-proof
communication systems. Links
involving satellites, deep-space probes, ground stations,
unmanned aerial vehicles (UAVs),
high altitude platforms (HAPs), aircraft, and other nomadic
communication partners are of
practical interest. Moreover, all links can be used in both
military and civilian contexts. FSO
is the next frontier for net-centric connectivity, as bandwidth,
spectrum and security issues
favor its adoption as an adjunct to radio frequency (RF)
communications.
Advantages
Ease of deployment
License-free long-range operation (in contrast with radio
communication)
High bit rates
Low bit error rates
Immunity to electromagnetic interference
Disadvantages
For terrestrial applications,
Beam dispersion
Atmospheric absorption
Rain
Fog (10~100 dB/km attenuation)
Snow
pollution/smog
These factors cause an attenuated receiver signal and lead to
higher bit error ratio (BER). To
overcome these issues, vendors found some solutions, like
multi-beam or multi-path
architectures, which use more than one sender and more than one
receiver.
http://en.wikipedia.org/wiki/Bit_ratehttp://en.wikipedia.org/wiki/Bit_error_ratehttp://en.wikipedia.org/wiki/Electromagnetic_interferencehttp://en.wikipedia.org/wiki/Dispersion_%28optics%29http://en.wikipedia.org/wiki/Rainhttp://en.wikipedia.org/wiki/Foghttp://en.wikipedia.org/wiki/Snowhttp://en.wikipedia.org/wiki/Bit_error_ratio
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3.3Some real life Applications:
Deliver HD video to individual seats
Airbus holds > 500 people; HD requires 13 Mb/s; short
range
Personal lighting/communication for channel isolation; copper is
heavy. High bandwidth
density (>10 Mb/m3)
Indoor localization
Finding roaming patients and doctors in a hospital; RF
techniques
can be problematic; lights can be uniquely modulated with
ID;
tagging bats; security in downlink channel. Data trickle.
Figure 2
Figure 3:
Table1: RF (802.11) vs. free-space visible spectrum (LED)
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4-Circuit Components:
OP-AMP BASICS: An operational amplifier is a very high gain
amplifier having very high input impedance
(typically a few Mega ohms) and low output impedance (less than
100 ).
The basic circuit is made using a difference amplifier having
two inputs (plus and minus)
and at least one output. the plus (+) input produces an output
that is in phase with the signal
applied, while an input to the minus (-) input results in an
opposite polarity output. As shown
in Figure 8
Feedback:
*There are two types of feedback
Negative feedback allows high-precision signal processing.
Positive feedback makes it possible to build oscillators
To make the op-amp works as amplifier we must connect the output
terminal to the inverting
input terminal which is a negative feedback.
Some Op-amp Applications:
1- Non-Inverting Amplifier.
2- Inverting Amplifier.
3- Voltage follower (Buffer).
1-Non- Inverting Amplifier:
Figure1.2.2
Figure4 :
Figure 5: Inverting Amplifier
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The output is obtained by multiplying the input by a constant
gain equal [(R2/R1)+1] in
phase with input
Some Op-amp Parameters:
PARAMETER ABBV UNITS DEFINITION
Bandwidth BW MHz The upper frequency limitation or useful
frequency range
Slew rate SR V/s The rate of change in the output voltage
with respect to time
for a step change at the input.
4.1 Op-amp LM-741:
The LM741 series are general purpose operational amplifiers
which feature improved
performance over industry standards the lm741is reliable and
required no frequency
compensation.
Above all, it was much easier to manufacture and had good
yields.
Features:
Short-Circuit Protection.
Offset-Voltage Null Capability.
Large Common-Mode and Differential Voltage Ranges.
No Frequency Compensation Required.
Low Power Consumption.
No Latch-Up
Figure 6: LM-741
Table 2: Some Op-amp Parameters
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4.2 Op-amp LM-386:
Description:
The LM386 is a power amplifier designed for use in low voltage
consumer applications. The
gain is internally set to 20 to keep external part count low,
but the addition of an external
resistor and capacitor between pins 1 and 8 will increase the
gain to any value up to 200. The
inputs are ground referenced while the output is automatically
biased to one half the supply
voltages. The
Quiescent power drain is only 24 mw when operating from a 6 volt
supply, making the
LM386 ideal for battery operation.
Features
Battery operation
Minimum external parts
Wide supply voltage range: 4V12V or 5V18V
Low quiescent current drain: 4mA
Voltage gains from 20 to 200
Ground referenced input
Self-centering output quiescent voltage
Available in 8 pin MSOP package Applications
AM-FM radio amplifiers
Portable tape player amplifiers
Intercoms
TV sound systems
Line drivers
Ultrasonic drivers
Small servo drivers
Power converters
Figure7: LM-386
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4.3 BC548 NPN General Purpose Amplifier:
This device is designed for use as general purpose amplifiers
and switches requiring
collector currents to 300 mA.
4.4 2N5777 Silicon NPN Photo Detector:
Features:
High sensitivity.
Economical TO-92 compatible.
4.5 BC549 NPN General Purpose Transistors:
Features:
Low current (max. 100 mA).
Low voltage (max. 45 V).
4.6 Laser Light(650nm-5mW laser Pointer):
Laser light is very different from normal light. Laser light has
the following properties:
The light released is monochromatic. It contains one specific
wavelength of light (one specific color). The wavelength of light
is determined by the amount of energy
released when the electron drops to a lower orbit.
The light released is coherent. It is organized -- each photon
moves in step with the others. This means that all of the photons
have wave fronts that launch in unison.
The light is very directional. A laser light has a very tight
beam and is very strong and concentrated. A flashlight, on the
other hand, releases light in many directions, and the
light is very weak and diffuse.
Figure8: BC548 transistor
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4.7 Table of components:
1-Transmitter:
2-Receiver:
part value quantity
Resistor 8.2 k 2
Resistor 1.8 M 1
Resistor 15 k 2
Resistor 10 k 1
Resistor 82 1
Variable Resistor 1 M 1
capacitor 1 uF 1
capacitor 0.1 uF 1
Capacitor 470 uF 1
Capacitor 1000 uF 1
LM741 _ 1
BC548 _ 1
Condenser MIC _ 1
Laser torch _ 1
BD139 _ 1
part value quantity
Resistor 6.8 k 1
Resistor 4.7 k 1
Resistor 2.2 k 2
Resistor 470 k 1
Resistor 1 k 1
Variable Resistor 10 k 1
capacitor 1 uF 1
capacitor 0.1 uF 2
Capacitor 470 uF 1
Capacitor 100 uF 1
Capacitor 10 uF 1
Capacitor 0.01 uF 1
Capacitor 47 PF 1
LM386 _ 1
BC549 _ 2
2N5777 (photo transistor) _ 1
Speaker _ 1
Table3: Transmitter components
Table4: Receiver components
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5 -Theory of operation:
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 electrets 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. Using
this circuit you can
communicate with your neighbors 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
the laser beam, no sound will
be heard from the receiver.
5.1The Transmitter:
The transmitter circuit comprises condenser microphone
transistor amplifier BC548 (T1)
followed by an op-amp stage built around A741 (IC1). The gain of
the op-amp can be
controlled with the help of 1-mega-ohm 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 circuitwith the body
of the torch connected to the
emitter of BD139 and the spring-loaded lead protruding from
inside the torch to circuit
ground.
Figure9: Transmitter Circuit
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5.2The Receiver:
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 does not
need any complicated alignment. Just keep the phototransistor
oriented towards the remote
laser pointer and adjust the volume control for a clear sound.
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.
Figure10: Receiver circuit
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6-Simulation:
By making the circuit in Orcad circuit simulator.
6.1 Schematic:
Figure 14
Figure12: Receiver circuit
Figure11: Transmitter circuit
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6.2 Layout:
Figure14: Receiver Layout
Figure13: Transmitter layout
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8-Hardware Implementation: By connecting the circuit as shown in
figures.
Figure16: Receiver Hardware implementation
Figure15: Transmitter Hardware
implementation
Figure17: Receiver Hardware
implementation using stereo speaker
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9-Problems &improvements:
9.1 Problems:
1- Low quality components.
2- Weak laser source (only 50mW).
3- Low performance photo-Diode.
9.2 Improvements:
1- Using laptop connected to the circuit via an Aux cable.
2- Using Stereo Speaker.
3- Lowering the noise coming from the different light sources
(florescent bulbs).
The system performs better in a dark room than it does in a room
with the lights on. In a
dark room there is little or no environmental noise (light) to
interfere with the signal
produced by the laser transmitter.
9.3Possible Improvements:
Due to time and equipment constraints, we were not able to
employ any of the following
modifications which could possibly have led to an improvement in
one or more areas of the
system:
1-By using a signal amplifier, the signal intensity reaching the
receiver would have
increased, thus increasing the range producing a louder, if not
clearer, audio signal.
2-If the bandwidth of the laser transmitter signal were known,
the use of a notch or matched
filter would help in removing much of the unwanted noise.
3-The use of a specially-designed setup to hold both the laser
transmitter and photo resistor
receiver steady while sending the signal would produce a more
steady output, thus
improving the sound quality.
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10-Conclusion:
The project shows that its possible to make audio communication
using laser by simple and
inexpensive components. It will easily give a communication
distance of several hundred
meters, and with a parabolic light reflector, up to several
kilometers. It transmits high quality
audio and the link is virtually impossible for anyone else to
tap into.
An important feature of transmission by laser beam is privacy.
Because a laser beam is
intentionally narrow, it is virtually impossible for someone to
tap into the link without you
knowing. If someone intercepts the beam, the link is broken,
signaling the interception. Fiber
optic cables also have high security, as it is very difficult to
splice into the cable without
breaking the link.
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11-List of references:
1- Optical Fiber Communications Principles and Practice Third
edition John M. Senior.
2- Understanding Optical Communications Harry J. R. Dutton
3-http://www.bu.edu/smartlighting/files/pdf/May808_slides_Little_FSO_Commun1.pdf.
4-http://radioeng.cz/fulltexts/2010/10_02_203_212.pdf
5- Lm-386Data sheet by Fairchild.
6- lm-741 Data sheet by Fairchild.