Arduino - Relay

In a previous tutorial, we have learned how to turn on/off an LED. In this tutorial, we are going to learn how to turn on/off some kind of devices that use the high voltage power supply(such as a light bulb, fan, electromagnetic lock, linear actuator...).

What are the common and difference between controlling LED and controlling a light bulb?

The common: Just like controlling LED, we use the Arduino's output pin to turn on/off them.

The difference:

Hardware Required

1×Arduino UNO or Genuino UNO
1×USB 2.0 cable type A/B
1×Relay
1×LED Strip
1×12V Power Adapter
1×Breadboard
n×Jumper Wires
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About Relay

A relay is a programmable electrical switch, which can be controlled by Arduino or any micro-controller. It is used to programmatically control on/off the devices, which use the high voltage and/or high current.

It is a bridge between Arduino and high voltage devices.

WARNING

When you are making projects that are connected to mains voltage, you need to know what you are doing, otherwise, you may shock yourself. This is a serious topic, and we want you to be safe. If you’re NOT 100% sure what you are doing, do yourself a favor and don’t touch anything. Ask someone who knows!

Although some kinds of relays support both DC and AC devices, We highly recommend you to use a DC device (≤24V) for testing.

Relay Pinout

Relay Pinout

Relay has two groups of pins: input (low voltage) group and output (high voltage) group.

  • Pins in the input group are connected to Arduino, including three pins:
    • DC- pin: needs to be connected to GND (0V)
    • DC+ pin: needs to be connected to VCC (5V)
    • IN pin: receives the control signal from Arduino
  • Pins in the output group are connected to the high voltage device, including three pins (usually in screw terminal):
    • COM pin: is the common pin. It is used in both normally open mode and normally closed mode
    • NO pin: is normally open pin. It is used in the normally open mode
    • NC pin: is normally closed pin. It is used in the normally closed mode

In practice, we usually do NOT use all of the pins in the high voltage group. We use only two of them:

  • We use only COM pin and NO pin if we use normally open mode.
  • We use only COM pin and NC pin if we use normally closed mode.

Additionally, if the relay supports both LOW and HIGH level triggers, there is usually a jumper to select one of two: LOW level trigger or HIGH level trigger.

※ NOTE THAT:

The order of the relay module's pins can vary between manufacturers. ALWAYS use the labels printed on the relay. Look closely!

How to Connect the High Voltage Device to Relay

How to connect relay

How It Works

Depending on manufacturers and user's installation, a relay can work differently.

The input mode mode (for IN pin): There are two input modes that make relay works oppositely:

  • LOW level trigger mode
  • HIGH level trigger mode

The output mode mode (for output pins): There are two input modes that make relay works oppositely:

  • normally open mode
  • normally closed mode. These modes are the opposite.

The “normally” means “if IN pin is connected to LOW (0V)”.

Before going into detail, let's see some quick information:

  • The normally open and normally closed mode work oppositely
  • The most of relay modules supports both normally open and normally closed mode
  • The LOW level trigger and HIGH level trigger mode work oppositely
  • NOT all of relay modules supports both LOW level trigger and HIGH level trigger mode
  • At a time, The relay module can work at only one of two LOW level trigger and HIGH level trigger mode

The combination of the input modes mode and output modes modes creates many use cases. If you are a beginner, we recommend using HIGH level trigger mode and normally open mode

Because the LOW level trigger and HIGH level trigger mode work oppositely, The next will explain the HIGH level trigger mode in detail. The LOW level trigger works oppositely.

HIGH Level Trigger - Normally Open Mode

To use this mode, we need to connect the high voltage device to the COM pin and NO pin.

  • If the IN pin is connected to LOW (0V), the switch is open. The device is OFF (or inactive).
  • If the IN pin is connected to HIGH (5V), the switch is closed. The device is ON (or active).
  • How Relay Works - Normally Open

HIGH Level Trigger - Normally Closed Mode

To use this mode, we need to connect the high voltage device to the COM pin and NC pin.

  • If the IN pin is connected to LOW (0V), the switch is closed. The device is ON (or active).
  • If the IN pin is connected to HIGH (5V), the switch is open. The device is OFF (or inactive).
  • How Relay Works - Normally Closed

Summary

Input modes Output Modes IN pin (programmable) Output pins Relay state Device state
HIGH Trigger Normally Open LOW COM and NO pin ⇒ open OFF
HIGH Trigger Normally Open HIGH COM and NO pin ⇒ closed ON
HIGH Trigger Normally Closed LOW COM and NC pin ⇒ closed ON
HIGH Trigger Normally Closed HIGH COM and NC pin ⇒ open OFF
LOW Trigger Normally Open LOW COM and NO pin ⇒ closed ON
LOW Trigger Normally Open HIGH COM and NO pin ⇒ open OFF
LOW Trigger Normally Closed LOW COM and NC pin ⇒ open OFF
LOW Trigger Normally Closed HIGH COM and NC pin ⇒ closed ON

There are up to 8 use cases. It may overload you. However, If you are a newbie, you just need to care about the two first cases, where HIGH level trigger and normally open are used. The rest of this tutorial will use those two use cases

Arduino - Relay

Arduino controls a high voltage device by controlling a relay.

Controlling a relay is simple. We just need:

  • Connect an Arduino's pin to the IN pin of the relay
  • Control the relay by programming the pin to LOW or HIGH

Wiring Diagram

Arduino Relay Wiring Diagram

Image is developed using Fritzing. Click to enlarge image

How To Program For Relay

  • Configure an Arduino's pin to the digital output mode by using pinMode() function. For example, pin 3:
pinMode(3, OUTPUT);
digitalWrite(3, LOW);
digitalWrite(3, HIGH);

Arduino Code

/* * Created by ArduinoGetStarted.com * * This example code is in the public domain * * Tutorial page: https://arduinogetstarted.com/tutorials/arduino-relay */ // constants won't change const int RELAY_PIN = 3; // the Arduino pin, which connects to the IN pin of relay // the setup function runs once when you press reset or power the board void setup() { // initialize digital pin as an output. pinMode(RELAY_PIN, OUTPUT); } // the loop function runs over and over again forever void loop() { digitalWrite(RELAY_PIN, HIGH); delay(500); digitalWrite(RELAY_PIN, LOW); delay(500); }

Quick Steps

  • Copy the above code and open with Arduino IDE
  • Click Upload button on Arduino IDE to upload code to Arduino
  • See LED strip state: blinking

Video Tutorial

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Challenge Yourself

  • Automatically turn on the light when you enter into your room and turn off the light after you leave 30 seconds. Hint: Refer to Arduino - Motion Sensor.

Alternative Relays

The above code also works with the following relays:

Function References

The Best Arduino Starter Kit

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