Reed Switch
Switch
Two ferromagnetic contacts sealed in a glass tube that snap together when a magnet comes near. No power needed, no wear from a mechanism — the classic door/window sensor.
Schematic symbol
Switch (SPST)
How reed switch appears in a circuit diagram.
Types & variants
Key specs
Pull-in sensitivity
ATAmpere-turns of field needed to close — lower = more sensitive.
Switching rating
V / ASmall: typically 100–200 V at 0.5 A max — it's a signal switch.
Contact resistance
mΩ~100 mΩ when closed.
Operate time
ms~0.5 ms — fast for a mechanical contact.
Laws that govern it
- Gauss's law for magnetismΦ(B) = 0 (∮ B · dA = 0)
The magnet's flux lines close through the two ferromagnetic reeds, and it is that flux that pulls them together.
- Ampère's circuital law∮ B · dl = μ₀ × I(enc)
It can equally be operated by a coil's field rather than a magnet — ampere-turns around the glass envelope do the same job.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
None on the bare glass tube; encapsulated versions label NO/NC on the housing.
Standard values
Standard 14 mm glass NO reeds and white plastic door-sensor pairs (switch + magnet).
How to choose
NO for most sensing. Respect the tiny current rating — switch a signal or a transistor, never a load directly. Encapsulated for anything that gets handled.
Pinout & package
2 axial leads through glass. Don't bend leads close to the glass seal — it cracks.
Example circuits
- Door/window alarm sensor to an MCU pin
- Bicycle wheel speed sensor (magnet on spoke)
- Float-and-magnet liquid level switch
Common failures
Cracked glass from lead bending, welded-shut contacts after switching too much current (often a capacitor's inrush), sticking near strong permanent magnets.
How to test
Continuity mode; bring a magnet near — beeps as the magnet approaches, stops as it leaves. No beep ever = broken; beep always = welded shut.
Substitutes
Hall-effect sensor (no contacts, needs power, unlimited life) or a microswitch where physical contact is acceptable.