This tutorial gives an introduction to the HY-M154 / 817 optocoupler module. Moreover, a simple application is programmed that shows how to wire and how to program an Arduino when working with the module. Optocouplers are very useful when you need to isolate different sections of a circuit, for example in power. An optocoupler (also called an opto-isolator or photocoupler) is a component that transfers an electrical signal between two isolated circuits using light.