Arduino - Relay | Arduino Tutorial - Arduino Getting Started

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 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 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 output 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).


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).


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