How to Control Relay with Raspberry Pi: Simple Guide

To control a relay with a Raspberry Pi, connect the relay module's input pin to a Raspberry Pi GPIO pin and use Python's RPi.GPIO library to set the pin HIGH or LOW. This switches the relay ON or OFF, allowing you to control external devices safely.

Syntax

Use the RPi.GPIO library to set up and control GPIO pins connected to the relay.

• GPIO.setmode(GPIO.BCM): Use Broadcom pin numbering.
• GPIO.setup(pin, GPIO.OUT): Set the GPIO pin as output.
• GPIO.output(pin, GPIO.HIGH): Turn relay OFF (depends on relay type).
• GPIO.output(pin, GPIO.LOW): Turn relay ON (depends on relay type).
• GPIO.cleanup(): Reset GPIO pins after use.

python

import RPi.GPIO as GPIO
import time

relay_pin = 17  # GPIO pin connected to relay input

GPIO.setmode(GPIO.BCM)  # Use BCM pin numbering
GPIO.setup(relay_pin, GPIO.OUT)  # Set pin as output

# Turn relay ON
GPIO.output(relay_pin, GPIO.LOW)  # Relay active low

time.sleep(1)  # Keep relay ON for 1 second

# Turn relay OFF
GPIO.output(relay_pin, GPIO.HIGH)

GPIO.cleanup()  # Reset GPIO

Example

This example turns a relay ON for 2 seconds, then OFF, using GPIO pin 17 on the Raspberry Pi.

python

import RPi.GPIO as GPIO
import time

relay_pin = 17

GPIO.setmode(GPIO.BCM)
GPIO.setup(relay_pin, GPIO.OUT)

print("Turning relay ON")
GPIO.output(relay_pin, GPIO.LOW)  # Relay ON (active low)
time.sleep(2)

print("Turning relay OFF")
GPIO.output(relay_pin, GPIO.HIGH)  # Relay OFF

GPIO.cleanup()

Output

Turning relay ON
Turning relay OFF

Common Pitfalls

• Wrong GPIO numbering: Use GPIO.BCM mode for pin numbers matching Raspberry Pi GPIO labels.
• Relay logic level: Many relay modules are active low, so GPIO.LOW turns relay ON and GPIO.HIGH turns it OFF.
• Power supply: Ensure relay module has proper power (usually 5V) separate from Raspberry Pi 3.3V GPIO pins.
• No protection: Use a relay module with built-in transistor and diode to protect Raspberry Pi from voltage spikes.

python

import RPi.GPIO as GPIO

relay_pin = 17

GPIO.setmode(GPIO.BCM)
GPIO.setup(relay_pin, GPIO.OUT)

# Wrong way: assuming relay is active high
GPIO.output(relay_pin, GPIO.HIGH)  # This may turn relay OFF instead of ON

# Right way for active low relay
GPIO.output(relay_pin, GPIO.LOW)  # Turns relay ON

GPIO.cleanup()

Quick Reference

Action	Code	Description
Set GPIO mode	GPIO.setmode(GPIO.BCM)	Use BCM pin numbering
Setup pin	GPIO.setup(pin, GPIO.OUT)	Set pin as output
Turn relay ON	GPIO.output(pin, GPIO.LOW)	Activate relay (active low)
Turn relay OFF	GPIO.output(pin, GPIO.HIGH)	Deactivate relay
Cleanup	GPIO.cleanup()	Reset GPIO pins

Key Takeaways

Use Raspberry Pi GPIO pins with RPi.GPIO library to control relay input pins.

Most relay modules are active low: setting GPIO LOW turns relay ON.

Always use GPIO.cleanup() to reset pins after your program ends.

Ensure proper power and protection for relay modules to avoid damage.

Double-check GPIO pin numbering mode to match your wiring.