1Pins
2Ferromagnetic reeds (Ni-Fe)
3Glass tube (hermetic) · Ø 2.2 × 14 mm
4Gap ≈ 0.1 mm · enlarged in the drawing

Thin clear glass tube with two metal reeds inside, legs coming out of both ends
Glass tube from a few mm to a few cm; axial leads

Switches and relays

Reed switch

In simple terms

What is this?

Two thin metal reeds inside a glass tube; they snap together when a magnet comes near.

What is it for?

Closes the circuit without touching the magnet; it is the core of a door-open sensor.

Where do you see it?

Door and window alarm sensors, bicycle speedometers.

How do you spot it?

Thin glass tube with one pin at each end.

Caution:

The glass tube breaks easily; do not bend the pins right at the base of the tube.

Technical definition

It consists of two ferromagnetic metal reeds very close together inside a glass tube. When a magnet comes near, the reeds become magnetized and stick together, closing the circuit. Because the contacts are in a sealed tube, it lasts a long time.

TypeNO (normally open)

TriggerMagnet

CurrentLow

Schematic symbol Same in IEC and ANSI

IEC 60617: the drawing style used in Türkiye and Europe.

Pins 2

1Pin 1Has no direction1

2Pin 2Has no direction2

Features typical

Type: SPST-NO (NC and SPDT types also exist)
Contact: Low current, signal level
Body: Glass, fragile
Triggering: Depends on the magnet’s direction and distance
Lifespan: No rubbing parts, many operations

Common mistakes

Bending a pin at the bodyThe glass tube is fragile. When bending a lead, hold it with pliers a few mm from the body.

Switching an inductive loadSmall contacts can weld from a relay coil or motor spark. Drive the load with a transistor and let the reed only carry the signal.

Test with a multimeter

Test whether this part is good with the multimeter on the measurement screen.

Take a measurement

Where it is used

Door and window alarm sensorBicycle speed counterFloat-type liquid level sensorLid-closed detection