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This electronics circuit use light from a laser torch to transmit an receive signal from a distance up to 500 meters . This laser transmitter receiver circuit use a laser torch as the carrier in the circuit .This project can be used as an spy device by mounting the transmitter in one building and the receiver in other building , but the phototransistor of the receiver must be oriented towards the laser beam of the torch . In the phototransistor is not accurately oriented to the laser beam the device will not work ( no sound will be heard from the receiver ) .
The first stage of the project is the transmitter circuit that comprises a microphone , transistor amplifier BC548 followed by an 741 op amp . The gain of the 741 op-amp can be controlled with the VR1 1M ohm variable resistor . The AF output from 741 op amp is coupled to the base of transistor bd139 , which modulates the laser beam.
The transmitter stage uses a 9v power supply. The BD139 transistor must be Heatsinked .
The receiver stage circuit uses an NPN phototransistor as light sensor that is followed by a two-stage transistor preamplifier and lm386 audio power amplifier.
The receiver stage circuit does not need any alignment it need just to be ( phototransistor ) oriented towards the transmitter laser point and adjust the volume from the variable resistor Vr2 for a clear sound.
To avoid 50Hz noise in the speaker, keep the phototransistor away from ac light sources ( such as bulbs ) and do not expose the sensor directly in the sun light .
The receiver circuit need to be powered like the transmitter circuit using a 9 volts power supply and the used speaker must be an 8 ohms per 0.5 watts output power .
 
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