Relay module with Arduino Uno R3

Wiring for the Relay module 1 channel (5 V) on the Arduino Uno R3 (ATmega328P, 5 V logic), chosen with the same pin rules BoardPilot uses: default buses first, and no pins that are used for flash, USB, the debugger or booting.

Wiring

Relay module pin	Role	Arduino Uno R3 pin	Notes
IN	Input to the part	2 (PD2)	Many modules switch on with IN LOW. Some do not react to 3.3 V: check with a test
GND	Ground	GND
VCC	Power	5V	Needs 5 V for the coil: use 5V
COM	Not wired to the board	not connected	Relay common contact. Mains voltage is dangerous
NO	Not wired to the board	not connected	Normally open contact
NC	Not wired to the board	not connected	Normally closed contact

Check before you power it on

Supply: 5 V from the 5V pin.
Motors, servos, relays and LED strips can draw more current than the Arduino Uno R3’s pins or USB can give. Power them separately and connect the grounds together.

Test code (Arduino)

void setup() {
  pinMode(2, OUTPUT);  // IN on 2
}

void loop() {
  digitalWrite(2, HIGH);
  delay(500);
  digitalWrite(2, LOW);
  delay(500);
}

Arduino board: arduino:avr:uno.

Questions

Which Arduino Uno R3 pins do I use for the Relay module?

IN → 2 (PD2), GND → GND, VCC → 5V.

Does the Relay module work with the Arduino Uno R3 at 5 V?

Yes: the Relay module runs at 5 V and the Arduino Uno R3 uses 5 V signals, so no level shifting is needed.

BoardPilot checks this wiring in 3D as you build it, and finds crossed or missing wires on the real board.

Download BoardPilot

Full Arduino Uno R3 pinout · Relay module 1 channel (5 V): all pins and data

The Relay module on other boards

ESP32 DevKit Arduino Mega 2560 R3 Arduino Nano WeAct Black Pill STM32F411CE ESP32-C3-DevKitM-1 ESP32-S3-DevKitC-1 Nordic nRF52840 DK NUCLEO-F401RE Raspberry Pi Pico 2 Raspberry Pi Pico W Raspberry Pi Pico Teensy 4.1

Other parts on the Arduino Uno R3

BME280 BMP280 DHT22 DHT11 HC-SR04 SSD1306 OLED MPU6050 DS18B20 WS2812B LED strip SG90 servo HC-SR501 PIR LCD 1602 I2C INA219 VL53L0X BH1750 MAX30102 RC522 RFID NEO-6M GPS Capacitive soil moisture sensor ADS1115 TM1637 28BYJ-48 stepper L298N Potentiometer Push button LED Joystick module SD card module AHT20