PC817 is an opto coupler which consists of an IRED. IRED is opticaly connected to the transistor and there is no electrical connection between them. It has an internal builtin LED when it glows photo transistor becomes actives and produces out correspondingly. When the LED is off there will be no result at output. PC-817 has several applications e.g. noise suppression in switching circuits, input/output isolation for MCU (Micro Controller Unit).