The circuit is designed to be unaffected by daylight. That is, the circuit can sense infrared rays with a frequency of only 2.7 kHz thanks to capacitors and resistive filters. In order to operate the relay at the output, 2.7 kHz beams should be sent from the transmitter circuit. A simple 2.7 kHz infrared transmitter circuit is sufficient to operate the circuit.
The 2.7-kHz, square-wave radiation from the transmitter is detected by the phototransistor.
A square wave of the same value of the transmitter frequency is formed in the collector of the phototransistor, which is continuously the emission cutoff. These square waves are rectified by a capacitor of 4.7 mF and 1N4001 diode. With DC current, the T3 transistor is activated.