Introduction to phototransistor light receiving circuit and its principle

Function of the circuit

In the optical receiving circuit, if the external light causes malfunction, the reliability will decrease. By using pulsed light, a circuit that can distinguish between average external light and pulse waves can be made. The output has not only on and off states, but also pulse train output with the same driving frequency.

How the Circuit Works

The current flowing through the emitter of the phototransistor PT1 is proportional to the intensity of the incident light. If external light is incident, current will flow through the base of TR1, and the collector voltage will decrease, thereby reducing the base bias of the phototransistor and not reaching the saturation state, thus forming a self-biasing circuit. If there is no external light, only modulated light, its intensity is fed back, so the operation is stable. The time constant of the averaging circuit composed of resistor R3 and capacitor C1 is set to be non-responsive to periodic pulse light and only allow external light to pass. For conventional applications, the parameters in the circuit diagram can be used. Because the output is a pulse train, a diode can be used for peak rectification when driving a relay. If a short response time is required, a re-trigger monostable large-scale multivibrator can be used to extend the oscillation output to a period greater than the light-emitting period.