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Unformatted text preview: removal of its battery) can be directly con-nected to the circuit—with the body of the torch connected to the emitter of BD139 and the spring-loaded lead protrud-ing from inside the torch to circuit ground. The receiver circuit (Fig. 2) 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 ad-just 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. LASER TORCH-BASED VOICE TRANSMITTER AND RECEIVER SANI THEO...
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- Spring '10