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 triggers an LED when the input signal exceeds the peak, or the value set in RV1. The...
Suggested by Radio Electronics from the 90s, this circuit is suitable for NiCAd batteries with a...
This interesting idea of a radio detector for a medium-wave radio is from a 1966 documentation. The critical...