5V 4Ch Relay Module with Optocoupler is compliant with international safety standards, control and load areas isolation trenches and it has 4 relay channels . A relay is an electrically operated device. It has a control system and (also called input circuit or input contactor) and controlled system (also called output circuit or output contactor). It is frequently used in automatic control circuit. To put it simply, it is an automatic switch to controlling a high-current circuit with a low-current signal. Its optically isolated.