27-07-2012, 02:33 PM
Laser Based Communication System
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1. INTRODUCTION
Laser as a communication medium can provide a good substitute for the present day communication systems as the problem of interference faced in case of electromagnetic waves is not there and high deal of secrecy is available. Laser communications offers a viable alternative to RF communications for inter satellite links and other applications where high-performance links are a necessity. High data rate, small antenna size, narrow beam divergence, and a narrow field of view are characteristics of laser communications that offer a number of potential advantages for system design. The present paper involves the study of wireless, open channel communication system using laser a carrier for voice signals.Using this circuit we can communicate with your own neighbours wirelessly. Instead of RF signals, light from a laser torch is used as the carrier in the circuit. The laser torch can transmit light up to a distance of about 500 meters. The phototransistor of the receiver must be accurately oriented towards the laser beam from the torch. If there is any obstruction in the path of laser beam, no sounds will be heard from the receiver.
3. BLOCK DIAGRAM EXPLANATION
3.1 CONDENSER MICROPHONE
It is also called a capacitor or electrostatic microphone. Condenser means capacitor, which stores energy in the form of an electric field. Condenser microphones require power from a battery or external source. Condenser also tends to be more sensitive and responsive than dynamic, making them well suited to capturing subtle nuances in a sound.
The diaphragm vibrates when struck by sound waves, changing the distance between the two plates and therefore changing the capacitance. Specifically when the plates are closer together capacitance increases and a charge current occurs and this current will be used to trigger the transmitting section.
3.2 TRANSMITTING SECTION
The transmitter section comprises condenser microphone, transistor amplifier BC548 followed by an op-amp stage built around IC1. The gain of the op-amp can be controlled with the help of 1-mega ohm pot meter VR1. The AF output from IC1 is coupled to the base of transistor Bd139, which in turn, modulates the laser beam.The transmitter uses 9V power supply. however, the 3-volt laser torch ( after the removal of its battery) can be directly connected to the circuit--with the body of the torch connected to the emitter of BD139 and the spring-loaded lead protruding from inside the torch to circuit ground.
3.3 LASER TORCH
Here we use the light rays coming from laser torch as the medium for transmission. Laser had potential for the transfer of data at extremely high rates, specific advancements were needed in component performance and systems engineering, particularly for space-qualified hardware.Free space laser communications systems are wireless connections through the atmosphere. They work similar to fibre optic cable systems except the beam is transmitted through open space. The laser systems operate in the near infrared region of the spectrum. The laser light across the link is at a wavelength of between 780 - 920 nm. Two parallel beams are used, one for transmission and one for reception.
3.4 RECEIVING SECTION
The receiver circuit uses an NPN phototransistor as the light sensor that is followed by a two stage transistor preamplifier and LM386-based audio power amplifier. The receiver doesn't need any complicated alignment. Just keep the phototransistor oriented towards the remote transmitter's laser point and adjust the volume control for a clear sound.
3.5 LOUD SPEAKER
A loudspeaker (or "speaker") is an electro acoustic transducer that converts an electrical signal into sound. The speaker moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such as air or water.
5.2 CAPACITOR:
The capacitor plays a crucial role in electronics -- it stores electrons for when they're needed most. Capacitors consist of two conducting plates placed near each other. Inside the capacitor, the terminals connect to two metal plates separated by a dielectric. The dielectric can be air, paper, plastic or anything else that does not conduct electricity and keeps the plates from touching each other..
They can store electric charge for later discharge. Direct current through a capacitor will charge the capacitor for a short time, and then stop flowing. Alternating current, because of the changing electric fields it generates, can “flow” across a capacitor.
5.3DIGITAL MULTIMETER (DMM)
The DMM is an instrument that is able to measure voltage, current, and resistance in a circuit, or across circuit components and displays its measurements on a digital display.
5.4 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.
5.5MICROPHONE
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 mic in our project.
5.5.1 CONDENSER MICROPHONES- A condenser microphone is essentially a capacitor, with one plate of the capacitor moving in response to sound waves.
5.6INTEGRATED CIRCUIT (IC)
An integrated circuit is a pre-made circuit shrunk down to small size and put on a chip. IC’s save circuit makers time by serving common purposes like amplifying a signal which would otherwise have to be done by a new circuit built from scratch every time.
5.7 PHOTODIODES
If a conventional silicon diode is connected in the reverse-biased circuit , negligible current will flow through the diode and zero voltage will develop across R1. If the diode casing is now carefully removed so that the diode's semiconductor junction is revealed, and the junction is them exposed to visible light in the same circuit, the diode current will rise, possibly to as
high as 1 mA, producing a significant output across R1. Further investigation will show that the diode current (and thus the output voltage) is directly proportional to light intensity, and that the diode is therefore photosensitive.
In practice, all silicon junctions are photosensitive, and a photodiode can be regarded as a conventional diode housed in a case that lets external light reach its photosensitive semiconductor junction. Fig. shows the standard photodiode symbol.
In use, the photodiode is reverse biased and the output voltage is taken from across a series-connected load resistor. This resistor may be connected between the diode and ground, as in fig. 1, or between the diode and the positive supply line, as in fig.
The human eye is sensitive to a range of light radiation, as shown in fig. 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. shows typical response curves of a general-purpose photodiode, and infrared (IR) photodiode.
Photodiodes have a far lower light-sensitivity than cadmium-sulphide LDRs, but give a far quicker response to changes in light level. Generally, LDRs are ideal for use in slow-acting direct-coupled light-level sensing applications, while photodiodes are ideal for use in fast-acting AC-coupled signaling applications. Typical photodiode applications include IR remote-control circuits.
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.