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.