This is a simple explanation of how to use a triac and opto isolator to switch AC voltages on and off to a device (load). The opto isolator is used to give a physical separation between the low voltage and the high voltage circuits. The Arduino is optional. You can substitute the signal coming from the Arduino with any 5 volt DC signal voltage. This cannot be used for dimming lights. The opto isolator I used is a M0c3041. Take notice that in the schematic the opto isolator has a triac and not just a switching transistor. This is essential piece. It allows current to flow in both directions when the opto isolator is switched on. Please feel free to ask questions.