presented by:
G.srinivas, Ch.hemanthkumar, J.n.k.sagar

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1.ABSTRACT
Using this circuit we 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. The transmitter circuit comprises condenser microphone transistor amplifier BC548 followed by an pomp stage built around µA741. The gain of the op-amp can be controlled with the help of 1-mega-ohm potmeter 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 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 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.
2. 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.
Use of laser in communication systems is the future because of the advantages of the full channel speeds, no communication licenses required at present, compatibility with copper or fiber interfaces and no bridge or router requirements . Besides this there are no recurring line costs, portability, transparency to networks or protocols, although range is limited to a few hundred meters. Also the laser transmission is very secure because it has a narrow beam (any potential evesdropping will result in an interruption which will alert the personnel. Also it cannot be detected with use of spectrum analyzers and RF meters and hence can be used for diverse applications including financial, medical and military. Lasers can also transmit through glass, however the physical properties of the glass have to be considered. Laser transmitter and receiver units ensure easy, straightforward systems alignment and long-term stable, servicefree operation, especially in inaccessible environments, optical wireless systems offer ideal, economical alternative to expensive leased lines for buildings. The laser can also be commissioned in satellites for communication, as laser radar requires small aperture as compared to microwave radar. Also there is high secrecy and no interference like in EM waves. Further, potential bandwidth of radar using lasers can translate to very precision range measurement. For these reasons, they can be used as an alternative to present modes of communication. laser communication, which is both wide-band and high-speed .
3.BLOCK DIAGRAM
4.BLOCK DIAGRAM EXPLANATION
i.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.
ii. 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.
iii. 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 page link is at a wavelength of between 780 - 920 nm. Two parallel beams are used, one for transmission and one for reception.
vi. 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.
v. LOUD SPEAKER
Aloudspeaker (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