Relay

A relay is an electromechanical device where one voltage source is used to control another. In its simplest form, a relay is an electromagnet and spring loaded armature. When the electromagnet is energized by a control voltage, it pulls the armature in, when de-energized, it releases the spring loaded armature. The end of the armature has a contact that forms a circuit when energized. A small control voltage can control a much larger voltage, and conversely, a large voltage can be monitored by a low voltage monitoring circuit input.