Relay Modules

A relay lets a low-power Arduino or ESP32 pin switch a much bigger load, such as a lamp, fan, pump or heater. Inside, a small coil pulls a metal armature that opens or closes the contacts with an audible click.

Terminals:

  • COM is the common contact.
  • NO (normally open) connects to COM only while the relay is on.
  • NC (normally closed) is connected until it switches.

Ready-made relay modules add the driver transistor and the flyback diode the coil needs, so you can drive them straight from a GPIO. Many add an optocoupler on the input. Multi-channel boards are often active-LOW: the relay turns on when the pin goes LOW. Some have a JD-VCC jumper that lets the coils run from a separate supply.

How to choose:

  • Match the coil voltage to your supply (5V is the most common).
  • Pick a contact rating above your real load.
  • Get enough channels for your build.

Switching 220V mains is dangerous. Follow the safety rules in our guides, use an enclosure, and never touch the load side while it is powered.

Frequently asked questions

What do COM, NO and NC mean?

COM is common. NO connects to COM when the relay is on; NC is connected when it is off.

Why does my relay turn on when the pin is LOW?

Many multi-channel modules are active-LOW. Write LOW to switch the relay on.

Do I need a flyback diode?

A bare relay coil does. Relay modules already include one, along with the driver transistor.

Can an Arduino switch 220V?

Through a relay rated for the load, yes. Follow mains safety rules and enclose the wiring.

Guides