Relay

An electromechanical switch where a low-power coil controls a separate, isolated high-power contact.

How it works

A relay is an electromechanical switch that allows a low-power circuit to control a completely separate high-power circuit. It contains a small electromagnet coil. When a small current energizes the coil, the magnetic field physically pulls a set of mechanical switch contacts closed (or open).

Relays provide total electrical isolation (galvanic isolation) between the control circuit and the load. This allows a delicate 5V microcontroller pin to safely turn on a 120V AC appliance without any direct electrical connection between the two systems.

Key Formulas

Coil CurrentI = V_supply / R_coil

Common Mistakes

Driving the relay coil directly from a microcontroller pin. Even small 5V relays often require 40mA to 80mA to energize the coil, which exceeds the safe limit of most logic pins. A driving transistor is required.

Common Variants

5V Relay Module

A ready-to-wire breakout board built around a 5V relay, an onboard transistor driver, flyback diode, and often an optocoupler — commonly wired active-LOW, which trips up beginners expecting a HIGH signal to mean "on."

Bare 5V Relay

A standalone 5V electromechanical relay with no onboard driver circuitry, requiring the builder to add their own transistor driver and flyback diode — the discrete-component path the pre-built relay module in this category exists to avoid.