A reed sensor is a simple electrical switch that opens or closes when a magnet comes near it. It has no batteries, no circuit boards, and no moving parts in the traditional sense. Two thin metal blades sealed inside a tiny glass tube do all the work, responding to magnetic fields to complete or break an electrical circuit. You’ll find reed sensors in door alarms, medical devices, automotive systems, and countless industrial machines.
How a Reed Sensor Works
Inside the glass tube are two flat, flexible strips (called “reeds”) made from a ferromagnetic metal, meaning they respond to magnets. These reeds overlap slightly in the center of the tube but don’t quite touch. A small gap separates them.
When you bring a magnet close to the sensor, the magnetic field flows through the reeds and turns one into a north pole and the other into a south pole. Opposite poles attract, so the flexible reeds bend toward each other and snap together. That physical contact completes an electrical circuit, letting current flow through the sensor’s leads. Pull the magnet away, and the natural springiness of the reeds pushes them apart again, breaking the circuit. The whole process is purely mechanical and magnetic. No electricity is needed to make the switch itself work.
Why the Glass Tube Matters
The glass envelope isn’t just packaging. It’s hermetically sealed, meaning no air, moisture, dust, or corrosive gases can reach the metal contacts inside. This makes reed sensors extremely durable in environments that would destroy exposed switches: underwater equipment, dusty factories, chemical plants, and even potentially explosive atmospheres where a tiny spark from an exposed contact could be dangerous. The sealed design is one reason reed sensors have stayed relevant for decades despite competition from newer electronic alternatives.
Contact Types: Normally Open, Closed, and Changeover
Reed sensors come in three basic configurations, labeled with industry shorthand:
- Form A (normally open): The reeds are separated when no magnet is present. Bringing a magnet close completes the circuit. This is by far the most common type.
- Form B (normally closed): The reeds touch by default, keeping the circuit complete. A nearby magnet forces them apart and breaks the circuit.
- Form C (changeover): This version has three leads. One circuit is closed while the other is open, and a magnetic field flips them. It’s useful when a system needs to switch between two different circuits depending on magnet position.
Electrical Range
Reed sensors handle a surprisingly wide range of signals. At the low end, specialized reed relays can detect voltages as tiny as 10 nanovolts and currents down to femtoamperes (millionths of a billionth of an amp), which makes them useful in sensitive laboratory instruments. At the high end, most pressurized reed switches top out around 250 volts because the electrical arc that forms when contacts separate becomes too strong to break above that level. Vacuum-sealed versions can handle up to 10,000 volts.
A typical reed switch might be rated for something like 200 volts, 0.5 amps, and 10 watts. The key rule is that you can use any combination of voltage and current as long as multiplying them together doesn’t exceed the watt rating. In the 5 to 12 volt range, which covers most consumer and industrial electronics, reed contacts can last tens of millions of switching cycles.
Common Applications
The most familiar use is in home and commercial security systems. A small reed sensor is mounted on a door or window frame, and a matching magnet sits on the door or window itself. When the door is closed, the magnet holds the reed switch in one state. Open the door, the magnet moves away, the switch changes state, and the alarm triggers. It’s reliable, cheap, and nearly impossible to defeat without physically accessing the sensor.
In medical devices, reed sensors monitor fluid levels in equipment like infusion pumps. A small float with an embedded magnet rises or falls with the fluid, triggering the reed sensor when the level hits a critical point. This kind of simple, sealed detection is ideal for medical environments where reliability and contamination resistance matter.
Automotive systems use reed sensors for position detection in seat belts, transmission selectors, and brake fluid reservoirs. Industrial machinery relies on them for proximity sensing, speed measurement, and safety interlocks. Their sealed construction means they tolerate the vibration, temperature swings, and grime of a factory floor without degrading.
Reed Sensors vs. Hall Effect Sensors
The main electronic alternative to a reed sensor is a Hall effect sensor, which uses a semiconductor chip to detect magnetic fields instead of physical contacts. The two technologies overlap in many applications, but they differ in important ways.
Reed sensors require no power to maintain their state. The contacts are either touching or not, and that physical connection draws zero standby current. Hall effect sensors need continuous power to operate, typically drawing around 20 milliamps depending on sensitivity. In battery-powered devices or systems where low energy consumption matters, this difference adds up.
Reed sensors are also more sensitive to magnetic fields. They can respond to fields as weak as 5 gauss and detect a magnet up to about 40 millimeters away. Hall effect sensors generally need at least 15 gauss and work within about 20 millimeters. Reed sensors are also indifferent to magnetic polarity: either pole of a magnet will activate them. Hall effect sensors require specific pole orientation to function correctly.
Where Hall effect sensors have the edge is in switching speed and size. Because they have no moving parts, they can respond faster and fit into tighter spaces. They also don’t suffer from contact bounce, a brief chattering that happens when reed blades make and break contact rapidly. For high-frequency switching or applications where vibration might cause false triggers, Hall effect sensors can be more practical. Reed sensors, on the other hand, are naturally resistant to electromagnetic interference without any additional shielding, while Hall effect sensors need external protection circuitry to filter out electrical noise.
Limitations to Know About
Reed sensors are mechanical devices, and that comes with trade-offs. The glass envelope is fragile. Rough handling, excessive vibration, or careless soldering during installation can crack it, destroying the hermetic seal that makes the sensor reliable. Once the seal is broken, the contacts are exposed to air and moisture, and performance degrades quickly.
Contact bounce is another consideration. When the reeds snap together, they can vibrate and momentarily separate several times before settling, producing a rapid series of on-off signals instead of one clean transition. In digital circuits, this can register as multiple triggers. Most designs solve this with a small debounce circuit or software filtering, but it’s something to account for.
Strong external magnetic fields from nearby motors, transformers, or other magnets can also interfere with operation, either holding the switch closed when it shouldn’t be or preventing it from activating. Proper placement and magnetic shielding solve this in most installations, but it means reed sensors need a bit of thought about their environment during design.