Just about everywhere you go, there’s a reed switch nearby that’s quietly going about its work. Reed switches are so ubiquitous that you’re probably never more than a few feet away from one at any given time, especially at home or in the car. You might have them on your doors and windows as part of a burglar alarm system. They keep your washing machine from running when the lid is open, and they put your laptop to sleep when you close the lid. They know if the car has enough brake fluid and whether or not your seat belt is fastened.
Reed switches are interesting devices with a ton of domestic and industrial applications. We call them switches, but they’re also sensors. In fact, they only do the work of a switch while they can sense a magnetic field. They are capable of switching AC or DC at low and high voltages, but they don’t need electricity to work. Since they’re sealed in glass, they are impervious to dirt, dust, corrosion, temperature swings, and explosive environments. They’re cheap, they’re durable, and in low-current applications they can last for about a billion actuations.
What is a Reed Switch?
The simplest reed switch consists of two thin, ferromagnetic contacts suspended axially inside a glass tube. The tube is filled with an inert gas, usually nitrogen, and hermetically sealed at the ends. The business ends of the reeds are coated with a non-magnetic material like iridium or tungsten to add strength and durability.
The ends of the reeds overlap slightly with a small gap between them. Whenever a permanent magnet or an active electromagnetic coil comes near the glass body, the magnetic field will cause the reeds to attract and touch, closing the circuit. When the magnet is removed, the flexible reeds spring apart and re-open the circuit.
It Came from Bell Labs
The reed switch was patented in 1941 by Walter B. Ellwood, a Bell Labs engineer. Ellwood was looking to design a cheap and easily replaceable relay unit for switching telecommunications traffic, some kind of simple switch that could withstand heavy use.
The device in Ellwood’s patent drawing is only slightly different from the reed switches out there today. In his design, an actuating reed on one end fluctuates between a magnetic reed and a non-magnetic one on the other, and the two are kept separate by a physical insulator.
Over the next 40 or so years, Bell System and TXE telephone exchanges would use millions of reed relays, first as memory modules and then in speedy crossbar configurations that left the old switches in the dust. Digital telephone exchanges have since put millions of reed relays out of work, but Ellwood’s versatile invention quickly spread to many other industries, like household goods.
House of Reeds
Most of the jobs for reed switches are in sensing or pulse counting, and there is much of both happening in the average household appliance. Any system that requires sensing fluid levels or knowing the positions of parts likely uses a reed switch to do so. Consider the washing machine, which does all of these jobs. When you start a load of laundry, the tub fills with water for the wash cycle. The amount of water it needs is going to be different every time, depending on what you’re washing and in what quantity. Instead of pumping in some set amount of water (and possibly overflowing the tub), there’s a reed switch near the top and a floating magnet that rides the rising tide to meet it and shut off the water.
So you’ve got a load started, but there’s a sock on the stairs that must have fallen out of the basket. You lift the lid and the machine immediately stops filling up with water. Why? Because of another reed switch. This one works by detecting a magnet embedded in the lid. Lifting the lid takes away the magnetic field, so the machine pauses until you throw in that sock and shut the lid. Yet another reed switch is used to measure speed of the drum, taking pulses from a magnet affixed to the tub.
Reed Switches Forever
Reed switches have a pretty solid pedigree of durability and reliability. Even so, they have been replaced by Hall effect sensors in situations that call for a higher degree of precision. Hall effect sensors are faster than reed switches because they’re solid-state devices—no need to wait those few milliseconds for the magnetic field to overcome inertia.
The downside of Hall effect sensors is that they need constant power to operate. That’s because they have to be ready at all times to sense the presence of a magnetic field. They’re also finicky about the position of the magnet and won’t register the Hall effect if the magnet is orientated improperly.
As long as there is a need for simple, non-contact switches, reed switches will be around. Don’t have one around to play with? You can build one in a few minutes with a little bit of copper, some magnet wire, and of course, a magnet.