A reed switch is a small electromechanical device that opens or closes an electrical circuit in response to a magnetic field. It consists of two thin, flexible metal blades (called “reeds”) sealed inside a tiny glass tube. When a magnet comes close enough, the reeds bend toward each other, touch, and complete the circuit. Remove the magnet, and the reeds spring apart. That simplicity is what makes reed switches so widely used: they show up in everything from home security sensors to automotive systems, often lasting billions of switching cycles.
How a Reed Switch Is Built
The construction is remarkably straightforward. Two flat blades made of a nickel-iron alloy sit inside a hermetically sealed glass capsule, with their tips overlapping but not touching. The glass envelope is matched to the thermal expansion rate of the nickel-iron reeds, so the seal stays intact across a wide temperature range. The sealed environment, often filled with an inert gas or a partial vacuum, keeps moisture, dust, and corrosive contaminants away from the contact surfaces.
This sealed design is the single biggest reason reed switches last so long. Because the contacts never touch the outside atmosphere, they don’t oxidize or corrode the way exposed mechanical switches do. High-quality reed switches have been tested past 100 million cycles without a single failure under moderate electrical loads.
How It Works
The reeds are ferromagnetic, meaning they respond to magnetic fields. When you bring a permanent magnet (or an electromagnet) near the glass capsule, the magnetic flux flows through the reeds and magnetizes them with opposite polarities at their overlapping tips. That turns the tips into tiny magnets that attract each other. The reeds flex, make contact, and current flows through the circuit.
Pull the magnet away and the magnetic force drops below the threshold needed to hold the reeds together. The natural springiness of the metal blades pushes them apart, breaking the circuit. No power source is needed for the switch itself. It’s entirely passive, activated only by the presence or absence of a magnetic field.
Contact Configurations
Reed switches come in three standard configurations:
- Form A (Normally Open): The contacts stay apart until a magnetic field brings them together. This is the most common type, used whenever you need to detect that something has moved close to a magnet.
- Form B (Normally Closed): The contacts are touching by default and separate when a magnetic field is applied. Useful when you want a circuit to break in the presence of a magnet.
- Form C (Changeover): This version has three leads. One common contact is normally connected to a closed circuit. When a magnetic field appears, it switches over to an open circuit, giving you both states in a single device.
Sensitivity and Electrical Ratings
Reed switch sensitivity is measured in ampere-turns (AT), which describes how much magnetic force is needed to pull the contacts closed. A lower AT number means a more sensitive switch that responds to weaker magnetic fields. As a rough guideline, about 0.1 millitesla per ampere-turn will activate a switch, so a 20 AT reed switch needs roughly 2 millitesla at its location to close.
Standard miniature reed switches typically handle up to 10 watts of switching power. Voltage ratings range from about 120 to 200 volts DC (or 120 to 140 volts AC, depending on the model), with current ratings between 0.25 and 0.5 amps DC. These are not high-power devices. They’re designed for signal-level switching and low-current circuits, which suits the vast majority of sensing applications.
Where Reed Switches Are Used
The most familiar application is the door and window sensor in 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 reeds separate, and the alarm system registers the change. The same principle works for detecting whether a laptop lid is open or closed.
In automotive systems, reed switches serve as position sensors for trunks, hoods, doors, gear selectors, and brake pedals. They’re also used in liquid level sensing (such as fuel tank float gauges) and current monitoring in electric vehicles. Advanced driver-assistance systems use reed-based proximity sensors for collision prevention and automated parking features.
Industrial and medical devices rely on reed switches too. Their sealed construction makes them well suited for harsh environments where dust, chemicals, or moisture would destroy an exposed switch. Because they contain no electronics, they can operate in places where active sensors would be impractical.
Key Advantages
Reed switches require no power to operate. They work with both AC and DC circuits. The hermetic glass seal protects the contacts from oxidation, corrosion, and contamination, giving them mechanical lifespans that can reach into the billions of operations. Because the magnet never physically touches the switch, there’s no mechanical wear on external parts, and the activation is contactless from the outside.
They’re also extremely small, inexpensive, and simple to integrate into a design. There’s no software, no calibration, no power supply needed for the switch itself.
Limitations to Know About
The glass capsule is fragile. Mechanical shock, vibration, or bending the wire leads too close to the glass seal can crack the enclosure and destroy the switch. Careful handling during installation matters.
Strong external magnetic fields from nearby motors, speakers, or other magnets can trigger a reed switch accidentally or hold it jammed in one position. In environments with lots of magnetic interference, this can be a serious reliability concern. And because reed switches are mechanical devices with moving parts, they can’t match the switching speed of solid-state alternatives.
Reed Switches vs. Hall Effect Sensors
Hall effect sensors are the main alternative for magnetic sensing, and the choice between them comes down to the application. Hall effect sensors are semiconductor devices with no moving parts, giving them a virtually infinite mechanical lifespan and the ability to switch at very high speeds. However, they require a power supply and continuously draw current. They also work only with low-voltage DC circuits.
Reed switches, by contrast, need zero power, work with both AC and DC, and provide a true mechanical contact (a “dry contact”) that many control systems prefer. They’re the better choice for low-power or battery-operated applications where you can’t afford constant current draw. Hall effect sensors make more sense in high-speed applications, environments with severe vibration, or designs where the glass fragility of a reed switch is a dealbreaker.