This laser transmitter receiver electronic circuit use light from a laser torch to transmit an receive signal from a distance up to 500 meters . This laser transmitter receiver use a laser torch as the carrier in the circuit  .This laser transmitter receiver 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 .