The simplest sensor in the system
A contact sensor, sometimes called a door and window sensor, is two small pieces. One holds a magnet and the other contains a switch that responds to that magnet. You mount them next to each other, with the switch half on the frame and the magnet half on the moving door or window. While the two sit close together, the circuit stays closed. When the door or window opens, the magnet moves away, the circuit changes, and the sensor reports it to the control panel.
Most contact sensors use a reed switch, a tiny sealed switch that closes in the presence of a magnetic field. Because there is little to go wrong, these sensors are generally dependable and inexpensive, which is why they are the foundation of most home alarms.
Wired and wireless versions
Wired contact sensors connect to the panel by low-voltage cable. They need no batteries, and are easiest to add during construction or major renovation.
Wireless versions have a small radio in the switch half and run on a battery that often lasts for a few years, with the lifespan depending on the model and how often the door is used. They are simple to install on an existing house, usually with adhesive tape or a few screws. The magnet half normally needs no battery.
Either type reports to the panel, so the system can tell you which opening was triggered.
Why the gap matters
Every contact sensor has a maximum gap, the distance between the two halves at which it still registers as closed. Manufacturers give this figure in the instructions, and it varies by model. Some points follow from it.
- If the halves are mounted too far apart, the sensor reads “open” even when the door is shut, causing nuisance alerts.
- If the door or window moves a little in its frame, such as in wind or when the house settles, a marginal gap can occasionally flicker.
- Swollen wood in humid weather or a sagging door can change the gap by enough to matter.
- Large metal surfaces near the sensor can weaken or interfere with the magnetic field. Some models include special versions for metal doors and garage doors.
When you install one, line up the marks on the housings, keep the halves aligned and parallel, and test by opening the door a small distance. Many people check that the system registers an opening when the door is only slightly ajar.
Placement on doors
Mount the sensor at the top corner on the latch side or the top of the frame, away from where hands and furniture will bump it. For double doors, put a sensor on the active leaf, or use one on each leaf if you want both covered.
For sliding patio doors, mount on the frame where the moving panel meets the fixed edge. If you have a slider, also consider how the door is secured physically, because a sensor reports forced entry but does not stop it. Our notes on sliding door security cover bars and other mechanical measures.
Contact sensors do nothing for a door that is left unlocked and opened quietly while the system is disarmed. They do their job when the system is armed. Good locks and a reinforced strike plate remain the first line, and the sensor adds notice.
Placement on windows
Place the sensor at the top or side of a window sash, near the corner where the sash meets the frame. For double-hung windows, decide which sash opens most often, and that is the one to cover, or cover both if the lower and upper sashes can both be opened.
Covering every window is not always needed. Many owners prioritize ground-floor windows, windows hidden from the street, and any reachable from a porch roof or deck. Basement windows are often forgotten. Upper-floor windows might reasonably be left for later. Simple window locks and limiters help in the meantime.
What contact sensors cannot see
A contact sensor reports only that a door or window moved. It will not notice:
- A pane being broken or cut without the sash opening.
- Someone entering through a spot with no sensor.
- An intruder already inside who does not touch a protected opening.
That is the reason systems add other sensors. Glass break sensors watch for the sound or shock of shattering glass, and motion detectors watch rooms. Used together, they cover each other’s blind spots. Security window film is a mechanical measure, not an electronic one, and pairs well.
Maintenance and testing
- Test each sensor when you arm the system, using the walk-test mode if your panel has one.
- Replace batteries when the panel warns of a low level.
- Check that tape or screws have not loosened, especially after painting.
- Wipe dust from the housing.
- After replacing a door or window, remount and retest the sensors.
Contact sensors are modest devices, yet they carry much of the load in a typical alarm. Mounted with attention to the gap and tested regularly, they are a reliable way to know when an opening has been opened.