Relay

A switch that turns a big circuit on and off with a small current.

Schematic letter: K / RL
Unit / key rating: Coil voltage (V), contact current (A)

Pinout

Coil (2 pins): The control side; comes in 5 V, 12 V or 24 V types.
COM: The common terminal of the switched line.
NO (normally open): Connects to COM when the coil is energised.
NC (normally closed): Connected to COM while the coil is off.

Real photos

What is it for?

A relay is an electrically operated switch. Current through its coil creates a magnetic field that pulls a contact and closes (or opens) a circuit. The coil circuit is isolated from the contact circuit.

It's used to control a 230 V lamp, a motor or a boiler from a small circuit such as an Arduino.

Things to watch out for

⚠ Always put a reverse diode (e.g. 1N4007) across the coil; the spike when the coil turns off destroys the transistor.
⚠ Don't drive the coil straight from a processor pin: use a transistor or a relay module.

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