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. A basic optocoupler uses a led and a phototransistor, the brighter the led the more current is allowed to pass through the phototransistor. The Electrical signal transfers between an input and an output side optically without any physical connection between both sides. Optocouplers are very useful when you need to isolate different sections of a circuit, for example in power. The PC817X series optocoupler IC is comprised of an IRED (Infrared Emitting Diode, or IR LED) and a phototransistor optically coupled to it. These two parts are not hard-electrically connected; thus, it. This User Guide Covers: 2-Channel, 4-Channel, and 8-Channel Modules | Input 3.