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Switching Electrical Loads with a Relay Module and Arduino

Engineering illustration: Switching Electrical Loads with a Relay Module and Arduino

A relay lets a low-voltage signal — like Arduino's 5V output — control circuits at much higher voltages, including devices running on household mains power, which makes it one of the most useful and most dangerous components at the same time.

Components needed

An Arduino board, a relay module (usually with one, two, or more channels), and the device to be controlled.

Wiring it up

The relay connects to Arduino through four simple pins: power and ground (VCC and GND), and one or two control pins (IN1 and IN2 on two-channel modules) connected to digital Arduino pins. Most modules include a jumper linking VCC to JD-VCC, letting the relay's electromagnetic coil be powered directly from the Arduino's supply with no separate electrical isolation needed in simple projects.

How it works

The relay contains an electromagnetic coil that opens or closes a mechanical contact when energized, allowing or blocking current through the high-voltage circuit, with no direct electrical connection between the low-voltage control circuit and the controlled circuit.

Important safety note: working with household mains voltage is a real hazard; only connect a device running on this voltage if you fully understand what you're doing — otherwise consult a qualified specialist. Make sure every connection on the high-voltage side is secure, since loose wiring is a genuine fire hazard. Never upload code to the Arduino while it's connected to the relay and the high-voltage circuit; disconnect the high-voltage supply before any upload or code change.

You can find relay modules with various channel counts at Juthour Tech.

Mains hazard: Use an isolated, rated relay module, enclosure, fuse, and strain relief. Only qualified people should work on mains wiring, with power disconnected before access.

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