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Switch a Load with a Relay (Arduino)

Use a relay module to switch a separate circuit on and off from an Arduino pin. Learn how relays drive bigger loads.

Beginner15 min

What you need

Wiring

Arduino Uno
Relay module

Hover a pin or a wire to trace the connection.

Code

relay.ino
// Click a relay on and off every 2 seconds.
// NOTE: many relay modules are ACTIVE-LOW — if yours switches backwards,
// swap HIGH and LOW below.
const int relayPin = 7;

void setup() {
  pinMode(relayPin, OUTPUT);
}

void loop() {
  digitalWrite(relayPin, HIGH); // relay on (or off if active-LOW)
  delay(2000);
  digitalWrite(relayPin, LOW);
  delay(2000);
}

Steps

  1. Wire the control side: VCC to 5V, GND to GND, IN to D7.
  2. Upload — you'll hear the relay click every 2 seconds.
  3. Put your load on COM + NO (on when activated). Start with a low-voltage load.
  4. ⚠️ Mains voltage is dangerous — only wire AC if you know what you're doing.