Light detector responsive only to pulsed light - using phototransistor cross-coupled to second transistor in 12 volt DC circuit

摘要

The pulsed light detection circuit uses a phototransistor (T1) with its base connected via a resistor (R1) to one supply terminal (+V) and directly to the collector of a second transistor T2). The collector of the phototransistor (T1) is connected via a second resistor (R2) to the former supply terminal (+V) and its emitter is coupled to the base of the second transistor (T2). The emitter of the second transistor (T2) is connected to earth via a third resistor (R3), while its base is connected to earth via a capacitor (C1) in parallel with a fourth resistor (R4). The pulsed light falls on the phototransistor (T1), with the output of the circuit taken from the collector of the phototransistor (T1).