Mechanisms: The Reed Switch

Author: Kristina Panos / Source: Hackaday

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.