Frequency selective photoelectric relay circuit diagram

This circuit controls a relay with the help of a modulated light signal. The modulation frequency is about 2.7kHz. The selectivity of the receiving circuit is determined by two parallel T-shaped networks (Figure b). The RC component parameters determine a very narrow cutoff band centered at 2.7kHz. For signals outside the frequency band, they are weakened by the strong negative feedback introduced from the emitter of the second transistor BC108 to the base of the first transistor BC108. Figure a shows the transmitter part of the circuit that controls the above circuit. It uses a gallium arsenide light-emitting diode CQY12B. Two transistors form a multivibrator with a resonant frequency of 2.7kHz and outputs a rectangular wave light pulse. The frequency can be accurately adjusted using a 250 ohm potentiometer.

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