Optocoupler: the Best Electronics Tutorial

Optocoupler is an electricity-light-electricity conversion device used for electrical signal transmission with light as the medium. It is composed of a light-emitting source and photoreceiver, both of which are assembled in the same closed casing and are isolated from each other by a transparent insulator. The pin of the light-emitting source acts as the input terminal and the pin of the photoreceiver acts as the output terminal. The commonly used light-emitting source is LEDs and the commonly used photoreceiver is photodiodes, phototransistors, etc.