A door sensor is a two-part magnetic contact device that reports a door as open or closed by detecting when a magnet mounted on the door pulls away from a switch mounted on the frame.
Two plastic blocks face each other across a gap — one screwed to the frame, one to the door. When the door opens, the blocks separate, a circuit changes state, and your panel or app flips from “closed” to “open.” Almost all entry-point sensors work this way, so once you understand the magnet-and-switch pairing, the installation quirks and alignment problems behind false alarms stop being mysterious.
What Is Inside A Door Sensor?
The frame-side housing holds a reed switch — a tiny sealed glass tube with two metal contacts that snap together in a magnetic field. The door-side housing holds only a permanent magnet.
When the door is closed and the two pieces line up, the magnet holds the contacts together and the circuit stays closed; the panel reads that as “secured.” Separate the magnet by a few inches — exactly what opening the door does — and the contacts spring apart, the loop breaks, and the panel registers an open condition. No camera, no radar, no light beam. A magnetic field doing mechanical work on two bits of metal.
Door Sensor vs. Motion Sensor vs. Glass-Break Sensor
These devices get lumped together constantly, but they detect different things. A door sensor only knows about the gap it straddles. A motion sensor watches for movement using passive infrared or microwave. A glass-break sensor listens for the frequency of shattering glass. Neither tells you whether a specific door is open right now, and a door sensor can’t tell you about movement in an empty room. Optex, which builds sensing hardware for automatic pedestrian doors, draws the same line — those doors use purpose-built presence sensors rather than magnetic contacts, because they need to detect a person standing in a threshold, not simply an open leaf.
| Sensor Type | What It Detects | Best Placement |
|---|---|---|
| Magnetic contact (door/window sensor) | Separation between two aligned parts | Door and window frames, gates, cabinets |
| Passive infrared motion | Body heat moving across a field of view | Room corners, hallways, ceilings |
| Microwave motion | Movement, including through thin walls | Open areas needing wide coverage |
| Glass-break | Sound and vibration of shattering glass | Windows, glass doors, storefronts |
| Photoelectric beam | An interrupted infrared beam | Perimeter lines, driveways |
| Presence sensor (commercial doors) | People standing or moving in a threshold zone | Automatic sliding and swing doors |
| Tamper switch | Removal of the sensor housing from its mount | Built into many contact sensors |
Mounting: Which Piece Goes Where
Installation sheets agree: the sensor body goes on the fixed frame, the magnet on the moving door or window. Reversing them works with some systems, but the frame side holds the electronics and often the battery, so keeping it stationary avoids flexing wires and rattling batteries.
Johnson Controls’ 433 Door/Window Sensor instructions call for the housing on the frame and the magnet on the moving leaf, with tamper protection on both wall side and cover so a pry attempt reports as an event. Allegion’s door-mounted sensor instructions follow the same fixed-versus-moving split.
Alignment decides reliability. The two pieces should sit square with a narrow, even gap — typically no more than the manufacturer specifies, often around half an inch. Angle them off-center, widen the gap, or mount the magnet on a door that sags in humidity, and the panel may read the door as open while shut, or miss an opening entirely.
For specific models, our tested door sensor roundup and buying guide covers range, hub compatibility, and battery life.
What Happens When The Door Opens
In standby, the sensor draws a trickle of current and reports nothing. The moment the magnet pulls away, the circuit opens and the device transmits. Hardwired sensors close a loop the panel polls constantly; wireless models wake a radio and send an event to a hub, which triggers an alarm, pushes a phone notification, or fires a smart-home automation — turning on a hallway light when the back door opens at night, for example.
The gap problem cuts both ways. Too wide and the closed state never registers, causing nuisance alarms or a door that shows as permanently open. Some panels let you adjust magnet gap tolerance; others don’t, which is why mounting position matters more than any setting.
FAQs
Do door sensors need a magnet to work?
Yes. The frame-mounted unit contains a reed switch that only closes in a magnetic field. Without a properly aligned magnet on the moving door, the switch stays open and the panel cannot distinguish a closed door from an open one.
Can a door sensor detect motion in a room?
No. A magnetic contact sensor only reports the gap between its two parts. Detecting a person moving through a room requires a separate motion sensor using passive infrared or microwave, which is why most security kits pair both device types.
Why does my door sensor show open when the door is closed?
The two halves are almost certainly misaligned or spaced too far apart. Check that the sensor and magnet sit square with an even, narrow gap, and look for a sagging or swollen door that has widened the distance since installation.
References & Sources
- Optex. “Technological Information for Automatic Doors.” Distinguishes entry-point contact sensors from presence sensing used on automatic pedestrian doors.
- Johnson Controls. “433 Door/Window Sensor Installation Sheet.” Specifies frame-mounted sensor body, magnet on the moving part, and wall and cover tampers.
- Allegion. “LCN 8310-804 Door-Mounted Sensor Installation Instructions.” Confirms the fixed-versus-moving mounting pattern for door-mounted sensors.