Qwiic Single Relay Hookup Guide

Contributors: Englandsaurus
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The Qwiic Single Relay is SparkFun's easiest to use relay yet. The single relay can handle up to 5.5A at 240 VAC for long periods of time. The Qwiic connectors and screw terminals also mean that no soldering is necessary.

SparkFun Qwiic Single Relay

SparkFun Qwiic Single Relay


Required Materials

To get started, you'll need a microcontroller to control everything. You may not need everything though depending on what you have. Add it to your cart, read through the guide, and adjust the cart as necessary.

SparkFun RedBoard - Programmed with Arduino

SparkFun RedBoard - Programmed with Arduino

SparkFun ESP32 Thing

SparkFun ESP32 Thing


Particle Photon (Headers)

32 Retired

Raspberry Pi 3

92 Retired

Now to get into the Qwiic ecosystem, the key will be one of the following Qwiic shields to match your preference of microcontroller:

SparkFun Qwiic HAT for Raspberry Pi

SparkFun Qwiic HAT for Raspberry Pi

SparkFun Qwiic Shield for Arduino

SparkFun Qwiic Shield for Arduino


SparkFun Qwiic Shield for Photon


You will also need a Qwiic cable to connect the shield to your Qwiic Single Relay, choose a length that suits your needs.

Qwiic Cable - 50mm

Qwiic Cable - 50mm

Qwiic Cable - 100mm

Qwiic Cable - 100mm


Qwiic Cable - 500mm

1 Retired

Qwiic Cable - 200mm



You will need a flush cutter and wire stripper to remove the sheath and insulation from a cable. A Phillips head screwdriver will be required to connect the load's to a screw terminal.

Flush Cutters - Xcelite

2 Retired
SparkFun Mini Screwdriver

SparkFun Mini Screwdriver


Self-Adjusting Wire Strippers

2 Retired

Suggested Reading

If you aren't familiar with the Qwiic system, we recommend reading here for an overview.

Qwiic Connect System
Qwiic Connect System

We would also recommend taking a look at the following tutorials if you aren't familiar with them.

Serial Communication

Asynchronous serial communication concepts: packets, signal levels, baud rates, UARTs and more!


An introduction to I2C, one of the main embedded communications protocols in use today.

Serial Terminal Basics

This tutorial will show you how to communicate with your serial devices using a variety of terminal emulator applications.

Qwiic Shield for Arduino & Photon Hookup Guide

Get started with our Qwiic ecosystem with the Qwiic shield for Arduino or Photon.

Hardware Overview

First let's check out some of the characteristics listed in the relay's datasheet that we're dealing with, so we know what to expect out of the board.

Operating Voltage1.7V-3.6V
Supply Current100mA
Coil Resistance23.5Ω
I2C Address0x18 (Default), (Jumper changes to 0x19)
Max Current (Through Relay) 5.5A (240 VAC)


The following table lists all of the relay's pins and their functionality.

NCNormally ClosedSwitch
NONormally OpenSwitch
COMSwitch CommonSwitch

Optional Features

The Qwiic Relay has pull up resistors attached to the I2C bus; if multiple sensors are connected to the bus with the pull-up resistors enabled, the parallel equivalent resistance will create too strong of a pull-up for the bus to operate correctly. As a general rule of thumb, disable all but one pair of pull-up resistors if multiple devices are connected to the bus. If you need to disconnect the pull up resistors they can be removed by cutting the traces on the corresponding jumpers highlighted below.

Pull-up Jumper

Pull-up Jumper

The Power LED will light up when the board is powered. The Status LED will light up when the relay has been triggered and the switch is closed, both are highlighted in the below image

Power LED

Power LED

The onboard screw terminal should be used to connect your high-power load, it is highlighted below. The middle COM pin should be hooked up to the Live wire (Usually black) coming from the wall, while NO or NC should be connected to the Live wire on the device side of things.

Screw Terminal

Screw Terminal

Hardware Assembly

If you haven't yet assembled your Qwiic Shield, now would be the time to head on over to that tutorial. Depending on the microcontroller and shield you've chosen, your assembly may be different, but here's a handy link to the Qwiic Shield for Arduino and Photon Hookup Guide to get you started!

With the shield assembled, SparkFun's new Qwiic environment means that connecting the relay could not be easier. Just plug one end of the Qwiic cable into the Qwiic Relay, the other into the Qwiic Shield and you'll be ready to upload a sketch and start turning things on and off. It seems like it's too easy too use, but that's why we made it that way!

Connected to Qwiic Shield

SparkFun RedBoard and Qwiic Shield with the Qwiic Relay attached

You'll also need to place the relay in line with the AC powered item you're attempting to control. You'll have to cut your live AC line (usually black or red) and connect one end of the cut wire to COM and the other to NC or NO, depending on what you want the resting state of your device to be. If your AC device is going to be on for most of the time, and you occasionally want to turn it off, you should connect one end to COM and the other to NC. Connect to NO if the device will be off for most of the time. Check out the picture below for a visual aid.

Relay Example Connection

Relay Example Connection

Each relay takes just over 100 mA to power Daisy chaining these boards together on the same bus will result in pretty large power spikes, you may need a separate 3.3V source depending on what else is included in your project

Arduino Library

We've written a library to make it even easier to get started with the SparkFun Qwiic Single Relay. The library will give you the full functionality of the Qwiic Single Relay without the hub bub of the I²C data transactions. You can click the link below to download the file or navigate through the Arduino Library Manager by searching SparkFun Qwiic Relay. You can also go the Github page and get it directly.

This library also works with our Qwiic Quad Relay board. If you need more relays on one PCB, then go check it out!

Note: This example assumes you are using the latest version of the Arduino IDE on your desktop. If this is your first time using Arduino, please review our tutorial on installing the Arduino IDE. If you have not previously installed an Arduino library, please check out our installation guide.

Library Functions Overview

The list below outlines the functions of the Qwiic Relay Arduino Library designed to work with the Qwiic Single Relay along with short descriptions of what they do. The examples cover nearly all of these functions so take a look at them for help integrating them into your own code.

  • bool begin(TwoWire &wirePort = Wire); - Initialize the Qwiic Single Relay on the I2C bus
  • float singleRelayVersion(); - Returns the firmware version present on the Qwiic Single Relay.
  • void turnRelayOn(); - Turn the relay on.
  • void turnRelayOff(); - Turn the relay off.
  • void toggleRelay() - Toggles the relay to the opposite state. The function first checks the status of the relay and is toggled to either on or off depending on what the status check returns.
  • uint8_t getState(); - Returns the status of the relay. Returns 1 if on or 0 if off.
  • bool changeAddress(uint8_t newAddress); - Changes the I2C address of the Qwiic Relay to the value set for newAddress. Valid newAddress values can be between 0x07 and 0x78. The new address is written to the memory location in EEPROM that determines the address.

Example Code

The Qwiic Relay is pretty simple, so all of the functions to control it are simply contained in the first example of the SparkFun Qwiic Relay Arduino Library.

Example 1 - Single Relay Basics

Go ahead and unzip the folder to a directory of your choosing and open up Example1_Single_Relay_Basics.

Starting at the top, we have #include-ed the path to the library's header file as well as to Arduino's I²C library: Wire.h. To use the functions in the SparkFun Qwiic Relay Library we create a version of it, and name it relay. You'll notice that in parentheses we have given it the board's address. If you have changed this address or closed the address jumper, than change the RELAY_ADDR to your address.

#include <Wire.h>
#include "SparkFun_Qwiic_Relay.h"

#define RELAY_ADDR 0x18 // Alternate address 0x19

Qwiic_Relay relay(RELAY_ADDR); 

Let's look at the setup. First we check that we can communicate correctly with the Single Qwiic Relay with the relay.begin() function call. If there are some connection issues we'll find out about them here. Note that the code will not stop if there is an error, so keep an eye out for an eror message. Next we check the version of the firmware that is on your product with the relay.singleRelayVersion() function.

void setup()

  // Let's see
    Serial.println("Check connections to Qwiic Relay.");
    Serial.println("Ready to flip some switches.");

  float version = relay.singleRelayVersion();
  Serial.print("Firmware Version: ");

Now we turn on and off the relay by using two functions. First the more obvious function turnRelayOn() turns on the relay, which can then be turned off with turnRelayOff. Next the relay is toggled on and off, with the toggleRelay() function. The difference here is that the relay is turned off or on depending on the current state of the relay: if off ---> turn on and vice versa.

  // Let's turn on the relay...
  // Let's turn that relay off...
  // Let's 'toggle' the relay; if it's off turn it on and vice versa.
  // Toggle the relay back off.

You're probably wondering why I'm doing this entirely in the setup? In the chance that you're running this for the first time without reading, you won't have a relay constantly turning on and off. Now at the end of the setup we see the final feature of the SparkFun Qwiic Arduino Library.

  Serial.print("The Relay is now: ");
  // Is the relay on or off?
  int state = relay.getState();
  if(state == 1)
  else if(state == 0)


Using the getState function we can check whether or not the relay is on or off without looking at the board. This can help you programmatically make decisions based on your project's needs.

Resources and Going Further

Now that you've successfully got your Qwiic Single Relay up and running, it's time to incorporate it into your own project!

For more information, check out the resources below:

Need some inspiration for your next project? Check out some of these related tutorials using relays. Be sure to check your current rating when handling the Qwiic Single Relay when browsing some of the other tutorials using relays.

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