Magnetic Reed Switches

A reed switch is a simple electrical switch operated by a magnetic field. It consists of two thin ferromagnetic blades, or reeds, sealed inside a small glass envelope with their ends overlapping but not touching. When a magnetic field is brought near, the reeds become magnetised with opposite polarities at the overlap, so they attract one another, bend and make contact, completing the circuit. When the field is removed, the springiness of the reeds pulls them apart and the switch opens.

The glass envelope is usually filled with an inert gas or sealed under vacuum, protecting the contacts from oxidation and contamination and giving reed switches long service life and reliable operation. Because they are operated without physical contact to the actuating magnet, reed switches are well suited to position and proximity sensing, for example detecting whether a door or window is closed in a security system, or sensing the level of a float in a tank. A reed relay places a reed switch inside a coil so that an electric current, rather than a permanent magnet, operates it.

Like any mechanical switch, reed switches exhibit contact bounce: when the contacts close they may briefly open and re-close several times before settling. In circuits that count pulses or drive logic, this bounce must be removed by debouncing, using hardware filtering or software timing. Reed switches handle modest currents and voltages and are valued for their simplicity, hermetic sealing and contactless operation.

Frequently asked questions

How does a reed switch close?

A nearby magnetic field magnetises the two ferromagnetic reeds with opposite poles at their overlap, so they attract, bend together and make electrical contact.

What is the difference between a reed switch and a reed relay?

A reed switch is operated by an external magnet, whereas a reed relay surrounds the switch with a coil so an electric current operates the contacts.

Why does a reed switch need debouncing?

Its mechanical contacts can bounce briefly when they close, producing extra pulses that must be filtered out in counting or logic circuits.