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It's an optoelectronics device that focuses a light emitter upon a light-sensitive detector, the two devices separated by a transparent, non-conductive barrier. They're used primarily to isolate two circuits that are at drastically different voltage levels or to isolate two circuits that must have independent grounds. Typically, it's an LED as the emitter and a phototransistor as the detector. Others have PIN diodes or light dependent resistors (LDRs) as the detector. Before the advent of the LED, folks used to "roll their own", using an incandescent lamp as the emitter and an LDR as the detector. There were some early electronic organ manufacturers that used them in the "swell" (volume) pedal so that the pedal controlled the incandescent lamp intensity and the LDR detector was part of the volume control voltage divider. They did that to eliminate the problems caused by noisy pots as the slow response of the lamp filtered all the scratchiness out.