What is a reed switch and how does it work?

If you visit a modern factory and observe the amazing electronics at work in an assembly cell, you’ll see a variety of sensors on display. Most of these sensors have separate wires for positive voltage supply, ground and signal. Applying power allows a sensor to do its job, whether that’s observing the presence of ferromagnetic metals nearby or sending a light beam out as part of the facility’s security system. The humble mechanical switches that trigger these sensors, like the reed switch, only need two wires to do their jobs. These switches activate using magnetic fields.

What is a reed switch?

The reed switch was born in 1936. It was the brainchild of W.B. Ellwood at Bell Telephone Laboratories, and it earned its patent in 1941. The switch looks like a small glass capsule with electrical leads poking out of each end.

How does a reed switch work?

The switching mechanism is comprised of two ferromagnetic blades, separated by only a few microns. When a magnet approaches these blades, the two blades pull toward one another. Once touching, the blades close the normally open (NO) contacts, allowing electricity to flow. Some reed switches also contain a non-ferromagnetic contact, which forms a normally closed (NC) output. An approaching magnet will disconnect the contact and pull away from the switching contact.

Contacts are constructed from a variety of metals, including tungsten and rhodium. Some varieties even use mercury, which must be kept in the proper orientation to switch correctly. A glass envelope filled with inert gas—commonly nitrogen— seals the contacts at an internal pressure under one atmosphere. Sealing isolates the contacts, which prevents corrosion and any sparks that might result from contact movement.


Figure 1: NO reed switch

Reed switch applications in the real world

You’ll find sensors in everyday items like cars and washing machines, but one of the most prominent places these switch/sensors operate is in burglar alarms. In fact, alarms are a nearly perfect application for this technology. A movable window or door houses a magnet, and the sensor resides on the base, passing a signal until the magnet’s removal. With the window open—or if someone cuts the wire—an alarm will sound.

While burglar alarms are an excellent use for reed switches, these devices can be even smaller. A miniaturized switch will fit inside ingested medical devices known as PillCams. Once the patient swallows the tiny probe, the doctor can activate it using a magnet outside of the body. This delay conserves power until the probe is placed correctly, which means onboard batteries can be even smaller, a critical feature in something that’s designed to travel through a human’s digestive tract. Besides its small size, this application also illustrates just how sensitive they can be, as these sensors can pick up a magnetic field through human flesh.

Reed switches don’t require a permanent magnet to actuate them; an electromagnet relay can switch them on. Since Bell Labs initially developed these switches, it comes as no surprise that the telephone industry utilized reed relays for control and memory functions until everything went digital in the 1990s. This type of relay no longer forms the backbone of our communication system, but they are still common in many other applications today.

Advantages of reed relays


Figure 2: Reed switch with both NO (bottom) and NC (top) contacts

The Hall effect sensor is a solid-state device that can detect magnetic fields, and it is one alternative to the reed switch. Hall effects are certainly appropriate for some applications, but reed switches feature superior electrical isolation to their solid-state counterpart, and they face less electrical resistance due to closed contacts. Additionally, reed switches can work with a variety of voltages, loads and frequencies, as the switch functions simply as a connected or disconnected wire. Alternatively, you’ll need supporting circuitry to enable Hall sensors to do their job.

Reed switches feature incredibly high reliability for a mechanical switch, and they’re able to function for billions of cycles before failing. Additionally, because of their sealed construction, they can operate in explosive environments where a spark could potentially have disastrous results. Reed switches may be an older technology, but they are far from obsolete. You can apply packages containing reed switches to printed circuit boards (PCBs) using automated pick-and-place machinery.

Your next build may call for a variety of integrated circuits and components, all of which debuted in the last few years, but don’t forget the humble reed switch. It completes its basic switching job in a brilliantly simple way. After over 80 years of use and development, you can rely on the reed switch’s tried and true design to work consistently.

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