Reed Switch
A reed switch is a magnetically-actuated normally-open switch. Two ferromagnetic reeds inside a hermetically-sealed glass envelope flex toward each other when an external magnetic field reaches the threshold (typically 10–60 ampere-turns), bridging the contact gap.
The two reeds are flat blades of nickel-iron alloy with rhodium- or ruthenium-plated tips, sealed inside a 7–20 mm glass envelope back-filled with inert gas (typically nitrogen). An external magnet polarises both blades; opposite poles attract across the gap, the blades flex together, and the plated tips touch. Removing the field lets the blade's elastic spring force pull them apart.
In plain terms
Two iron tongues facing each other inside a glass straw; bring a magnet near and they kiss; take it away and they spring apart.
Why designers use it
- Door- and window-open sensing in burglar alarms and home automation.
- Speed and rotation sensing in bicycle computers and washing-machine drums.
- Float-level sensing where a magnetic float closes a reed at the high or low mark.
Best for
- Alarm sensors
- Speed pickups
- Float-level
Key specifications
- Sensitivity: 10 – 60 AT
- Contact rating: 0.25 – 1 A
- Voltage: 100 – 500 V
- Lifetime: 10⁸ – 10⁹ cycles
- Glass length: 7 – 20 mm
When not to use it
- Anywhere a stronger magnetic field nearby could falsely close the contacts.
- In high-shock applications — the cantilevered reeds can self-actuate.
- Where the contacts must switch significant inductive load — arc damage shortens life dramatically.
Common mistakes
- Cutting the leads too close to the glass — the seal cracks and the contacts oxidize.
- Bending the leads against the glass body and shattering it.
- Magnetising the reeds permanently by switching DC current near saturation — they stick closed.
Where you will find it
- Burglar-alarm window contacts hide a reed switch in the frame and a magnet in the sash; opening the window pulls the magnet away and the reed opens, triggering the alarm.
- Bike computers use a reed switch on the fork and a tiny magnet on a spoke: each rotation closes the reed once and the head unit times the interval to compute speed.
- A washing-machine door-locked sensor uses a reed switch held closed by a magnet in the latch — the controller refuses to spin if the reed isn't closed.
A short history
Standex describes reed switches as hermetically sealed magnetic-field-operated contacts, with multiple contact forms, sizes and power ratings. Its product categories include bistable and latching versions as well as other constructions. The seal protects the contacts, but the selected model's load, lifetime and magnetic operating requirements still determine whether it fits a design.
Good to know
- The reed-relay matrix in a 1ESS telephone exchange contained tens of thousands of reed switches; an operator never wore one out.
- Reed switches can chatter at audio frequencies — hold a magnet just at the threshold and they buzz like a tiny bell.
- A modern Hall-effect sensor often replaces a reed in new designs, but the reed wins on zero quiescent current.