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.
By connecting several photocells in series, we can directly activate a sensitive relay, as shown in...
The purpose of this circuit is to provide a constant current to an experimental electromagnet. The...
Found in Linear Technology documentation from the 90s, this circuit uses components from that...