An opto isolator is a device which consists of a light producing device such as LED and a light sensing element such as photo transistor. When a voltage is applied to LED, a light is produced which strikes the photo detector. The photo detector then changes its output. Optoisolator provides electrical isolation between input and output side. Opto isolator is an important part of I/O module of PLC. Both input and output module have Optoisolator to achieve electrical isolation between outside electrical circuit and PLC circuit.