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.

Circuit Classics’s Homage to Forrest Mims III

Electronic Hobbyists (at least those of us of a certain age), have undoubtedly encountered one or more of Forrest Mims III‘s wonderful introductory books on electronics, probably through their local Radio Shack. According to Wikipedia, Radio Shack commissioned 36 books from Mims between 1972 and 2003 (and have continued to carry several of his books). His books had a unique style, with hand-printed text and schematic diagrams, written using a mechanical pencil, as shown in the figure below:

Star Simpson’s Circuit Classics project brings three of his circuits to life in simple, elegant, well-crafted kits. The three circuits are a dual LED flasher, a stepped tone generator, and a bargraph voltage indicator. Each is a kit consisting of a blue circuit board and the parts to solder onto the board. The front of the board has a copy of the Forrest Mims’ hand drawn circuit diagram, and the back has a copy of his handwritten description of the circuit.  Also included is a copy of his signature. The kits are very well done, with a slotted wooden base for display and even four rubber feet to either use on the wooden base or on the bottom of the board if you prefer.

I purchased and put together the dual LED flasher kit. The kit is an easy to solder kit for beginners, consisting of a little over a half dozen through-hole parts to solder. It would be a good kit to give to a beginner, or (like me), to someone who has fond memories of getting started with one of Mims’ books, and likes the homage. Below is the front of the finished kit:

As mentioned above the back has a copy of Mims’ original hand-lettered circuit description. the circuit can either be powered from coin cell batteries using the battery holder on the bottom right, or by clipping power leads to the two copper corners.

Reverse side of the Dual LED Flasher

As of this writing, Mims is still going strong, conducting research and publishing books and articles in Make magazine. Many of his books are available on Amazon and you can learn more about him at his website:

The Circuit Classics project was fully funded Crowd Supply project, and the kits can be ordered through Crowd Supply.