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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The energy can come from many sources: Heat. Electrical Discharge. Electric Current. Chemical Reaction. Biological Reactions (Bioluminescense). Absorption of Light. Nuclear Radiation.
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
(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.
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Table1: RF (802.11) vs. free-space visible spectrum (LED)
Figure 3:
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.
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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 Ω). 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
Figure4 :
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:
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 Bandwidth
ABBV BW
UNITS MHz
DEFINITION The upper frequency limitation or useful frequency range
Slew rate
SR
V/μs
The rate of change in the output voltage with respect to time for a step change at the input.
Table 2: Some Op-amp Parameters
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
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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: 4V–12V or 5V–18V 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.
Figure8: BC548 transistor
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.
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4.7 Table of components: 1-Transmitter: part
value 8.2 k Ω 1.8 MΩ 15 k Ω 10 k Ω 82 Ω 1 MΩ 1 uF 0.1 uF 470 uF 1000 uF _ _ _ _ _
Resistor Resistor Resistor Resistor Resistor Variable Resistor capacitor capacitor Capacitor Capacitor LM741 BC548 Condenser MIC Laser torch BD139
quantity 2 1 2 1 1 1 1 1 1 1 1 1 1 1 1
Table3: Transmitter components
2-Receiver: part Resistor Resistor Resistor Resistor Resistor Variable Resistor capacitor capacitor Capacitor Capacitor Capacitor Capacitor Capacitor LM386 BC549 2N5777 (photo transistor) Speaker
6.8 4.7 2.2 470 1 10 1 0.1 470 100 10 0.01 47
value kΩ kΩ kΩ kΩ kΩ kΩ uF uF uF uF uF uF PF _ _ _ _
Table4: Receiver components
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quantity 1 1 2 1 1 1 1 2 1 1 1 1 1 1 2 1 1
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 μA741 (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 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.
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 twostage 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:
Figure11: Transmitter circuit
Figure12: Receiver circuit
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6.2 Layout:
Figure13: Transmitter layout
Figure14: Receiver Layout
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8-Hardware Implementation: By connecting the circuit as shown in figures.
Figure15: Transmitter Hardware implementation
Figure16: Receiver Hardware implementation
Figure17: Receiver Hardware implementation using stereo speaker 18
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 it’s 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.
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