Reed switches

Last updated: February 6, 2009.

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". A hi-res version of this image is available for commercial use from our Flickr page.

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.

Photo: A normal light switch turns a circuit on or off when you choose. A reed switch is designed to switch things on and off when a magnet is nearby, whether you're around or not. That makes it really useful for all kinds of circuits that operate automatically.

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.

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?

Normally open

In a reed switch, the two contacts (which look like metal reeds) are made from magnetic material and housed inside a thin glass envelope. 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.

Normally closed

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.

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!

Photo: LEGO® cows operated by a reed switch. Photo by courtesy of Bill Ward, published on Flickr under a Creative Commons Licence.