Relay

Inside a relay, an electromagnet coil moves a set of contacts, so one circuit can switch another that is electrically separate, often at a higher voltage or current. Relay circuits in the community show a low-voltage switch or transistor controlling a lamp, fan or bell, latching and interlock arrangements, car lighting and starter wiring, and alarm circuits, and they are a common way to learn which contacts are normally open and which are normally closed. For example, "Continuity / Fault Detector Circuit - Simple Theft Detection Circuit" uses a relay to switch on a warning lamp when a guarded wire loop is broken.