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 was obtained from an English magazine from 1977. The magazine no longer exists, but...
We found this circuit in a 1997 documentation. Its purpose is to fan an inverter for fluorescent lamp...
Found in a Hands On Electronics of the 80s, this circuit makes use of an integrated circuit of the time...