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 of a publication from the 70's uses a different type of power that we had already...
This circuit, from Motorola's 1993 Linear Interface ICs. The circuit causes a lamp to blink at a...
This frequency corresponds to musical note C3. Frequency accuracy depends on the accuracy of the...