Magnetic Reed Switch Door/Window Sensor
Simple, reliable open/close detection for DIY alarms.
A reed switch is a simple electromechanical switch made of two ferromagnetic metal reeds sealed inside a glass tube. The reeds snap together to close the circuit whenever a magnet passes near them, and spring apart when the magnet moves away. With no power draw and no moving parts beyond the reeds themselves, it's the classic low-cost sensor for detecting whether a door or window is open or closed in DIY security systems, and for simple proximity or rotation-counting triggers.
Specifications
| Switch type | Normally-open (NO) magnetic reed switch — normally-closed (NC) variants also exist |
| Operating voltage | Passive component — rated for typical logic-level or low-voltage switching, commonly up to 100-200V depending on part |
| Switching current | ~0.5A max typical for small glass reed switches |
| Actuation distance | ~10-15mm typical, depends on magnet strength and reed sensitivity rating |
| Response time | Sub-millisecond mechanical closure, effectively instantaneous for most microcontroller polling rates |
| Interface | Simple digital I/O — read as a plain HIGH/LOW switch input with a pull-up or pull-down resistor |
| Package | Often sold pre-wired in a two-part door/window sensor housing with the matching magnet included |
Pinout
| Pin | Name | Description |
|---|---|---|
| 1 | Lead 1 | One side of the switch contact — wire to a digital input pin |
| 2 | Lead 2 | Other side of the switch contact — wire to GND (or VCC, depending on pull configuration) |
Wire one lead to a digital input pin and the other to GND, then enable the microcontroller's internal pull-up resistor (INPUT_PULLUP) so the pin reads HIGH when open and LOW when the magnet closes the switch — this avoids needing an external resistor for a simple door/window sensor build.
Variants
Pick the pre-wired door/window sensor for a fast alarm-system build since the magnet spacing is already matched. Use a bare reed switch when embedding into a custom enclosure, and consider a Hall effect sensor instead if you need a fully solid-state, higher-cycle-life alternative.
| Variant | Temp range | Hum range | Accuracy | Protocol | Price |
|---|---|---|---|---|---|
| Door/window reed sensor (pre-wired pair) | ~$1-3 | ||||
| Bare glass reed switch | ~$0.20-1 | ||||
| Hall effect sensor | ~$0.50-2 |