PC817 is a photocoupler with a transistor based on light (photon) control. Therefore, the IC basically has an IR (infrared) LED and a phototransistor. When the IR LED is energized, the light emitted by it falls on the transistor and turns on. It is widely used in the signal transmission between computer terminals,heaters and other circuits to completely isolate the front end from the load. The purpose is to increase safety, reduce circuit interference, and simplify Circuit design.