A relay is an electromechanical device that can be used to make or break an electrical connection. It consists of a flexible moving mechanical part that can be controlled electronically through an electromagnet. Basically, a relay is just like a mechanical switch, but you can control it with an electronic signal instead of manually turning it on or off. There are two types of relay technologies, the electromechanical and the solid-state. An electromechanical relay is essentially a combination of an inductor (coil) and a switch (contact), where the electromagnetic force of the inductor causes the switch to change position. A solid-state relay accomplishes the same function as semiconductor devices changing impedance to effectively energize or de-energize a circuit (i.e., open or closed). Figure 6.1 shows different types of relays.