An Optocoupler is a combination of LED and a Photo-diode packed in a single package. As we can see in the below-shown circuit diagram, when a high voltage appears across the input side of the Optocoupler, a current start to flow through the LED. Due to this current LED will emit. An optocoupler, also known as photocoupler or opto-isolator, is a device which can transfer an electrical signal across two galvanically-isolated circuits by way of optical coupling. They use light to pass signals between circuits. As we have already learnt about transistors, an ideal transistor will not. Let's understand the term Optocoupler. It can be separated as OPTO + COUPLER.