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 simple metronome, from 1985 documentation uses only transistors and has two settings. For...
This circuit is suitable for modulating AM transmitters. The circuit is from a 1977 radio amateur...
This flasher was found in an old Hands On, but uses common components. The optocoupler can be...