1

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

2

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

3

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

4

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

5

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.

6

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.

7

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

8

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)

9

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

11

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

12

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

15

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

16

6-Simulation:

By making the circuit in Orcad circuit simulator.

6.1 Schematic:

Figure 14

Figure12: Receiver circuit

Figure11: Transmitter circuit

17

6.2 Layout:

Figure14: Receiver Layout

Figure13: Transmitter layout

18

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

19

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.

21

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.

21

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.