Yeah, I have vague recollections of taking apart an analog electric organ (like https://en.wikipedia.org/wiki/Hammond_organ https://en.wikipedia.org/wiki/Hammon

Yeah, I have vague recollections of taking apart an analog electric organ (like https://en.wikipedia.org/wiki/Hammond_organ https://en.wikipedia.org/wiki/Hammond_organ) and seeing an "optocoupler" with an incandescent bulb and an LDR in a little enclosure.