This optical coupler for short distances is powered by 12 V which is the voltage of the lamp and the relay. The circuit can also use emitting LEDs and the phototransistor can be of any type.
This circuit using a SIDAC as a relaxation oscillator was found in the ON Semiconductor manual for...
We found this circuit in a 1974 documentation. Upon receiving an audio signal, it activates a...
This circuit was obtained from an American publication of 1982, but can be assembled with ease...