Simplify Your Life: Using a Relay Module

Simple relay

A Relay

Today’s blog is on a simple topic, but one that comes up a lot, and  may be of use, especially to beginners. Many projects involve controlling a higher voltage device or devices based on an external event triggered by a sensor, such as a PIR. And to control the higher voltage device, a relay is used. Along with the relay, you’ll need a transistor to provide adequate current to drive the relay, a diode to protect from reverse currents when the relay switches off, and probably a resistor to limit the current draw. This is simple enough, but another option is to use a relay module. The module combines a relay along with these other components into one simple package.

Depending upon your application, you may also put some sort of timer circuit, or a microcontroller such as an Arduino, or a single board computer (SBC) like a Raspberry Pi between the sensor output and the relay input. This lets you implement more complex logic than simply having the relay echo the output from the sensor. For example, one could have a light toggle on or off each time the sensor is triggered. Or, when triggered, one could have a device stay on for a set amount of time before turning off. Or even  more complex actions. But if you don’t need the added logic, you can use the sensor directly, without a microcontroller or SBC.

The output voltage from the sensor or controller board must be high enough to trigger the relay, but voltage alone is not enough. The driving device must also be capable of supplying enough current. This is often not the case for either sensors or computer boards. They lack the power to drive many relays.

Relay and driver circuit, showing transistor, diode, and resistor

Relay and Driver Circuit

The solution is to use a transistor. The sensor or board switches on and off the larger current flow through the transistor, and this, in turn, drives the relay. In addition, one needs a resistor to limit the current drawn from the sensor or controller and a “flyback” diode to protect both devices and the transistor from reverse currents when the relay turns off. This is shown in the figure, taken from Circuit Digest’s Arduino Relay Control Tutorial.

You can learn more about a relay and the relay driver circuit in the article How Electrical Relays Work at Circuit Basics.


Relay Module Board

Relay Module

But rather than using discrete components, there’s another option which is often quicker and easier: the relay module. A relay module combines all of these elements into a single board. It may include additional features, such as an LED to indicate when the relay is triggered. In this way, you only need your sensor, optionally a controller for more complex logic, and the relay module. No discreet diodes, transistors, or resistors required. I’m using the one discussed in this article, along with a PIR sensor, to trigger the “Try Me” switch on a Halloween prop. Although the signal output for the PIR is 3.3V and the relay is a 5V input signal relay, it seems to work just fine. As you can see, it’s the same relay, mounted onto a circuit board along with the transistor, resistor, diode, and indicator LED. In addition, the signal side inputs are connected to 3 male pins for easier connections. The pins are spaced so that they will fit into a standard breadboard or for connecting a standard 3-wire servo cable.

So the next time you need to trigger a higher voltage AC or DC device, reach for a relay module.

An unseasonal post: Automating Halloween Props

This post is more suited to Halloween then the coming Yule, but I finally got around to writing it, and besides, as I write this, there’s only 327 days left to next Halloween!  There are several options for controlling animated props, including prop controllers specifically designed for this purpose.  A recent addition to the market is the MonsterShield, an Arduino-based prop controller with open source code you can modify.  I haven’t tried one out, but thought it was work a mention.  In my props, I just use a handy micro-controllers, such as an Arduino, along with sensors such as a PIR.  The PIR (or pressure mat, or whatever sensor you choose) sends a signal to the microcontroller when it detects someone, and then your microcontroller can trigger a whole sequence of pre-programmed actions.  I had two such props in my Halloween display this year.  The first is a classic “Monster in a Box“.  I use a PIR sensor to detect when someone comes near.  When this happens,a Teensy sends out an output to a Power Switch Tail to turn on the power to a wall wart that delivers 12V power to the windshield wiper motor, as well as to a green light inside the box.  I use a Power Switch Tail so that I don’t have to worry about any safety issues dealing with house current directly.  The motor sequence has several stops and stop in it, of differing lengths, so that the action is more natural.  After it triggers, there’s a dead time, so that it doesn’t keep restarting while trick-or-treaters are standing in front watching it.

Closed Monster in a box, showing the box with the PIR and Arduino that triggers the box.

Inside view of the Monster in a Box, showing the wiper motor and irregular cam.


The other prop where I use a PIR and an Arduino is my scarecrow skeletons, who do a little talking and singing routine when triggered.  Here, an Arduino controls two Cowlacious audio boards that in turn drive the servo boards that control the jaw servo and lights in the skulls (also from Cowlacious).  I’ve had the audio boards for some time.  They work fine, and have a wide host of control options (including the ability to be triggered directly from a PIR).  However if I was remaking this prop, I’d probably just wire up some really cheap mp3 players. The video shows the two skeletons doing their routine. Sorry for the quality, my video editor is refusing to save the edited version.