Laser Torch Based Voice Transmitter and Receiver

In this paper we designed the circuit, using that circuit we can communicate with our neighbors wirelessly. Instead of RF signals, light from an IR LED 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 beam from the LED. If there is any obstruction in the path of the beam, no sound will be heard from the receiver. The transmitter circuit comprises of condenser microphone transistor amplifier BC548 followed by an op amp stage built around µA741. 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 beam. The transmitter uses 9V power supply. The receiver circuit uses an NPN phototransistor as the light sensor 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 transmitters beam. The phototransistor is kept 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.