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 small unregulated variable power supply was found in an American book from the 60's. The...
This circuit indicates when the voltage of a 12 V battery is low. With changes, for example, the...
This basic circuit is from 1984 documentation. This circuit can be used as an efficient burglar alarm...