A reed switch is a small electrical switch that opens or closes when a magnet comes near it. Inside a sealed glass tube, two thin metal blades (called “reeds”) overlap with a tiny gap between them. When a magnetic field reaches the blades, they flex toward each other, touch, and complete a circuit. Remove the magnet, and the blades spring apart. That simple interaction is the basis for countless sensors in everyday life, from home security systems to laptop lid detectors.
How a Reed Switch Works
The two metal blades inside a reed switch are made of a ferromagnetic material, meaning they respond to magnetic fields. Each blade is sealed inside a small glass envelope, typically just a few centimeters long. The overlapping ends of the blades sit close together but don’t quite touch.
When a permanent magnet or an electromagnet creates a field near the switch, the blades become magnetized. One end becomes a north pole and the other a south pole, so the overlapping tips attract each other and snap together. This closes the circuit and allows current to flow. Once the magnetic field is removed, the natural springiness of the blades pulls them apart again, breaking the circuit. The whole process takes roughly 1 to 2 milliseconds to close and can be as fast as 0.2 milliseconds to open.
What’s Inside the Glass Tube
The glass envelope isn’t just structural. It creates a hermetic seal that protects the contact surfaces from dust, moisture, corrosion, and even explosive atmospheres. Because the blades never touch outside air, they don’t oxidize the way an exposed metal contact would over time.
The contact surfaces themselves are plated with precious metals to resist wear. The most common combination in commercial production is a ruthenium coating over a gold underlayer. The ruthenium resists erosion from repeated switching, while the gold layer bonds the ruthenium firmly to the blade and acts as a barrier that prevents metal diffusion during manufacturing. Rhodium is another option, particularly suited for circuits carrying low current. These coatings are what allow reed switches to survive millions of switching cycles.
Three Contact Configurations
Reed switches come in three standard forms, each suited to different jobs:
- Form A (Normally Open): The default state is an open circuit. When a magnet approaches, the blades close and current flows. This is the most common type, used whenever you need to detect that something has arrived or moved into position.
- Form B (Normally Closed): The circuit is closed by default. A magnetic field pushes the blades apart, breaking the connection. This is useful for fail-safe designs where you want the system to react when a magnet moves away.
- Form C (Changeover): This version has three terminals: one common, one normally open, and one normally closed. The common terminal is always connected to one of the other two. When the magnet arrives, it switches from one connection to the other. This lets a single switch control two different circuits.
Sensitivity and Magnetic Range
Reed switch sensitivity is measured in ampere-turns (AT), a unit that describes how strong the magnetic field needs to be to flip the switch. Typical commercial reed switches fall between 10 and 60 AT. A lower number means the switch is more sensitive and will respond to a weaker field or a magnet farther away. A higher number means you need a stronger magnet or a closer approach to trigger it.
This matters when you’re choosing a reed switch for a project. If you need the switch to activate when a magnet is several centimeters away, you’d pick a low-AT switch. If you want it to trigger only at very close range to avoid false activations, a higher-AT switch is a better fit. The strength and size of the magnet you pair with it also plays a role: a larger magnet can activate even a less-sensitive switch from a greater distance.
Where Reed Switches Show Up
The most familiar application is in door and window sensors for home security systems. A small magnet is mounted on the moving part (the door or window), and the reed switch sits on the frame. When the door opens, the magnet moves away, the switch changes state, and the alarm system registers the event.
Laptops use the same principle. A magnet embedded in the screen lid passes near a reed switch in the base, telling the computer whether the lid is open or closed. Bicycles have used reed switches for decades in speedometers: a magnet on a spoke passes a sensor on the fork once per wheel rotation, and the computer calculates speed from the timing.
In industrial settings, reed switches detect fluid levels in tanks (a magnet rides on a float), monitor the position of pistons inside pneumatic cylinders, and count rotations in flow meters. They work well in hazardous environments because the hermetic seal means no spark escapes the glass envelope, making them safe around flammable gases and dust. Medical devices, vending machines, and automotive systems all use reed switches in similar proximity-sensing roles.
Lifespan and Durability
Reed switches are remarkably long-lived compared to most mechanical components. When switching light loads (under 10% of their rated capacity), their lifespan closely matches the mechanical life rating on the datasheet, which for many switches runs into the hundreds of millions of cycles. At around 30% of rated load, lifespan drops by roughly a factor of ten. At full rated load, the switch lasts for the number of cycles the manufacturer specifies for that load condition, which is typically in the tens of millions.
The hermetic seal eliminates the gradual degradation that humidity and contamination cause in exposed contacts, so reed switches maintain consistent performance over very long periods. They require no power to hold their state, generate no heat while idle, and have no electronic components that age. This makes them popular in battery-powered devices and installations where replacing a sensor is difficult or expensive.
Limitations to Keep in Mind
Reed switches aren’t perfect for every situation. The glass envelope, while excellent for sealing, is fragile. A hard impact or excessive vibration can crack the tube or damage the blades. In applications where mechanical shock is likely, manufacturers embed the switch in a protective plastic case or a metallic enclosure to absorb impact.
Like all mechanical switches, reed switches experience contact bounce. When the blades snap together, they briefly bounce apart and reconnect several times before settling, all within a few milliseconds. In digital circuits, this can register as multiple signals instead of one clean pulse, so designers typically add a small debounce circuit or handle it in software.
Reed switches also can’t handle high current or high voltage. They’re designed for signal-level loads, typically in the milliamp to low-amp range. For switching heavy electrical loads, you’d use a reed switch to trigger a larger relay or solid-state device. And because they respond to any sufficiently strong magnetic field, nearby magnets, electric motors, or electromagnetic interference can cause false triggers if you don’t account for the magnetic environment during installation.