A reed switch is an electrical switch that changes state when a magnetic field is applied. In its common form, two flexible metal contacts sit inside a sealed glass tube; a nearby magnet or an energized coil makes them attract and touch. The moving object does not need to press the switch directly.
How a reed switch works
A common reed switch contains two overlapping, flexible ferromagnetic blades—called reeds—inside a sealed glass envelope. With no actuating magnetic field, the reeds rest in their unactuated position. When a suitable field reaches them, they become magnetized and attract, making or changing an electrical connection. When the field is removed, the reeds’ spring force returns them to their original position.
The field can come from a permanent magnet moved near the switch, or from a coil. When a coil supplies the field, the resulting device is a reed relay. In the simplest reed switch, the sealed contact assembly itself provides both magnetic response and switching; there is no separate electronic sensing circuit. A packaged reed sensor assembly may include mounting or other features, so it is not always synonymous with the bare switch component.
What the contact forms mean
Contact form describes the electrical paths and the switch’s ordinary, unactuated state. The descriptions below apply to conventional, non-latching contacts.
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| Form | Unactuated state | When actuated by a magnetic field |
|---|---|---|
| Form A (normally open) | Contacts are open. | Contacts close. |
| Form B (normally closed) | Contacts are closed. | Contacts open. |
| Form C (change-over, SPDT) | Common contact connects to the normally closed path. | Common contact switches to the normally open path. |
Special biased or bistable designs behave differently from ordinary non-latching contacts. For example, WIKA describes a magnetically biased contact that retains its state until a field of opposite polarity resets it. That behavior is specific to such arrangements, not a general property of reed switches. WIKA’s explanation of reed-contact behavior
Where reed switches are used
Reed switches provide non-contact magnetic position detection. A magnet can move with a door, cover, or other object while the switch remains mounted separately. One documented example is GEZE’s door-position reed switch, which can connect to an external alarm system. That particular product is rated for a switching voltage of 30 V DC, a switching current of 0.2 A, and a service-temperature range of −25 to 70 °C. Those are model-specific specifications, not general limits for reed switches. GEZE door-position reed switch
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What to check before choosing one
- Contact form: Choose normally open, normally closed, or change-over to suit the circuit’s required default and actuated states.
- Electrical load: Check the exact component’s voltage and current ratings against the load. Do not assume a bare reed switch can switch arbitrary voltages or currents.
- Sensitivity and magnet arrangement: Operating distance depends on the switch and the magnet’s placement and geometry. Consult the manufacturer’s sensitivity and application guidance; there is no universal switching distance.
- Mounting and dimensions: Confirm the package and mounting method suit the space and moving part. A packaged door-contact assembly may be more appropriate than a bare component.
- Vibration and impact: Murata’s comparison says reed switches are more sensitive to vibration and impact than AMR sensors. The actual suitability depends on the specific device and application. Murata’s reed-switch and AMR comparison
- Contact bounce: A reed switch is a mechanical contact and is not bounce-free. In PLC or other logic inputs, account for bounce; software debounce may be appropriate. WIKA on reed-contact bounce
Reed switch or AMR sensor?
The practical difference is not simply that both detect magnets: a reed switch makes and breaks a physical contact, while an AMR sensor is a powered magnetic-sensing alternative. Murata identifies greater reed-switch sensitivity to vibration and impact as a tradeoff, and notes that AMR sensors require a power supply. Compare the exact devices for supply availability, contact bounce and wear requirements, vibration conditions, mounting constraints, and the switching interface needed by the circuit. Murata’s comparison of reed switches and AMR sensors
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Standards and specifications
BSI lists BS EN 62246-1:2015 as the generic specification for reed switches operated by an applied magnetic field. Its overview describes test and measurement procedures and says mercury-wetted reed switches are not covered because of their possible environmental impact. BSI summarizes the subject this way: “The reed switch is an electrical switch operated by an applied magnetic field.” For engineering or compliance decisions, consult the standard and the detailed datasheet for the particular product. BSI overview of BS EN 62246-1:2015
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