by Chris Woodford. Last updated: December 18, 2014.
If you've got a laptop computer or a cellphone that flips open like a clamshell, you've probably noticed that it senses when you open and close it and switches on or off accordingly. But how does it know? Some kind of switch wired to the hinge so it can detect the opening and closing movement? If that's what you think, you're at least half right! Think about it more carefully and you'll see a standard switch would be quite tricky to wire up in that way—and probably quite unreliable too: all that opening and closing would quickly wear it out. So, instead, many laptops and phones use an inexpensive and very reliable device called a reed switch that turns on or off when a magnet is nearby. Intruder alarms and model railroads often use them too. Let's take a closer look at how they work!
Photo: A typical reed switch. You can just see the two overlapping metal contacts (reeds) inside the glass envelope. The contacts spring together and touch when the switch is "on"; they spring apart and interrupt the circuit when the switch is "off".
Switches that work as detectors
A switch is like a drawbridge in an electric circuit. When the switch is closed, the "bridge" is down and electric current can flow around the circuit; when the switch opens, the "bridge" is up and no current flows. So the purpose of a switch is to activate or deactivate a circuit at a time of our choosing.
Most of the electrical switches we encounter are ones we control ourselves. If you want light in a room, you flick a switch on the wall. Want to watch TV? Turn on the switch. Want to listen to your iPod? Push the wheel at the front and that activates a switch that turns on the power. But sometimes we want electrical and electronic circuits to be activated in other ways.
Suppose you want to wire up a bank safe so it triggers an alarm whenever the door opens. How would that work in practice? You'd need electrical contacts on both parts of the door frame so when the door opened the circuit would be broken, triggering the alarm. But think how tricky it would be to make a reliable electrical connection on a door frame. What if you painted over it? What if it got dirty? And wouldn't it be so obvious to a thief that they'd be able to disable it quite easily? There are lots of ways in which the electrical contact could be rendered inactive and useless. This is where reed switches can help.
Photo: A "push-to-make" switch makes a connection and completes a circuit when you push it in; a spring makes it pop back out again when you take your finger away. A reed switch switches a current on the same way, but a magnet provides the "pushing pressure" instead of your finger.
What is a reed switch?
An ordinary switch has two electrical contacts in it that join together when you push a button and spring apart when you release it. Rocker switches on wall lights (like the one in the photo up above) push the two contacts together when the switch is in one position and pull them apart when the switch flicks the other way.
How does a reed switch work?
Reed switches come in two varieties called normally open (normally switched off) and normally closed (normally switched on).
In a reed switch, the two contacts (which look like metal reeds) are made from magnetic material and housed inside a thin glass envelope. (You can see this quite clearly in our top photo.) As you bring a magnet up to a reed switch, it magnetizes the contacts in opposite ways so they attract and spring together and a current flows through them. A reed switch like this is normally open (NO) (normally off), unless a magnet is positioned right next to it, when it switches on:
Take the magnet away and the contacts—made from fairly stiff and springy metal—push apart again and return back to their original positions.
You can also get reed switches that work the opposite way. The two contacts are normally snapped together. When you bring a magnet up to the switch, the contacts magnetize, repel one another, and split apart, opening the switch and breaking the circuit. Reed switches like this are called normally closed (NC) (normally switched on), and they switch off when you bring a magnet up to them.
How do you use reed switches in practice?
You can probably see now how a clamshell phone switches on and off when you open or close it. It has a normally closed reed switch in the lower part of its body (where the keypad is) and a magnet in the upper part (where the screen is). When the phone is open, the reed switch and the magnet are relatively far apart. The contacts on the reed switch are pushed together and the power flows through the phone. However, if you close the case, you swing the magnet close to the reed switch and that pushes apart the contacts inside the switch. A circuit inside the phone senses this and switches off the power in an orderly way.
Ebook readers, such as Kindles and Sony Readers, use a similar trick. When you fit them inside a protective leather jacket, you'll find they switch off automatically when you close the cover—and switch on again when you open it up. There's no magic here, of course: there's simply a reed switch in the corner of the ebook device and a magnet in the corresponding part of the cover (test it for yourself by holding a paperclip nearby).
Photo: Some flip-style cellphones, like this one, are switched on and off by magnetic reed switches. There's a magnet in one part of the case and a reed switch in the other. The phone switches off when the reed switch is near the magnet (when the case is closed) and switches on when the reed switch and magnet separate (when the case is open again).
You can see how the same idea would work in our bank safe door: you'd simply have a reed switch on the door frame and a magnet on the door. Opening the door would separate the magnet and the reed switch, causing the switch's contacts to spring together and trigger the alarm. You can get reed switches built inside little pieces of plastic so you can't even see they're there—perfect for all kinds of security applications.
You can use reed switches in lots of other ways too. LEGO® enthusiast Bill Ward, who runs the superb Brickpile blog (and a Flickr photo page), has built these ingenious robotic cows for his model railroad. Whenever a train moves past, they swivel their heads to watch it go by. The whole thing is worked by a reed switch. Each cow's head is operated by a small electric motor that's connected to a circuit in which there's a normally open reed switch. The reed switch is positioned next to the train track and a little magnet is fitted to the side of the train. As the train passes by the reed switch, the magnet forces the contacts to close and activates the circuit that turns the cows' heads. How neat is that? Some people are just so ingenious!
Who invented reed switches?
Like many other great inventions, reed switches were born at Bell Laboratories, invented there in the mid 1930s by Walter B. Elwood. His original patent application for an Electromagnetic switch was filed on June 27, 1940 and officially granted on December 2, 1941. Reading through Elwood's patent, it's very easy to recognize the reed switch that's still in common use today: "When an external magnetic force is applied to this unit the two magnetic members which form part of the magnetic circuit... are moved together... since the external magnetic force acts to diminish the air-gap between the two said magnetic elements."
Artwork: Walter Elwood's original reed switch design taken from US Patent: 2264746: Electromagnetic switch. This is a slightly different design from up above, switching back and forth between two different circuits with one of them always on. We have two nonmagnetic contacts on the left (1,2) and a magnetic contact (3,4) on the right, which snaps back and forth between them when a magnet approaches. The contacts are kept apart by an insulating spacer (5). Original artwork courtesy of US Patent and Trademark Office. (Please note that I've colored and simplified the original slightly to make it easier to follow.)
Find out more
On this website
You'll find quite a few examples of how to use reed switches on the ever-excellent Instructables website and in the popular Evil Genius books; here are a handful to start you off:
- Instructables: Quick and dirty homemade reed switch by Ryan. How to make a simple reed switch (a good way to understand exactly how they work).
- Instructables: Make a magnetic reed switch work backwards by Pronteon. An easy way to reverse the function of a reed switch by using a second magnet.
- Instructables: A versatile and low-cost digital counter by Trebuchet03. Make a simple distance counter (odometer) using a reed switch and an old calculator.
- 50 Model Rocket Projects for the Evil Genius by Gavin Harper. McGraw-Hill, 2006. Project #37 (model rocket video camera) uses a couple of normally closed reed switches to trigger the camera automatically.