Motion Sensors and Door Contacts | Falcon Security Systems

How each sensor type behaves in a real house

Alarm sensors are simple devices whose reliability depends almost entirely on where you put them. Understanding what each one actually reacts to explains every false alarm you will ever have.

A passive infrared motion detector responds to a moving heat difference across its field. That is why it triggers on sunlight moving across a floor, on air from an air conditioner, and on a cat crossing a room. It also explains why it detects movement across its view far better than movement straight toward it — a detector facing down a corridor is much less sensitive than one facing across it.

A magnetic door contact is two parts that must stay aligned within a few millimetres. Doors settle, frames swell in humidity, and a contact that worked in winter starts reporting the door open in summer. It is the most common nuisance fault in any system and takes minutes to adjust.

Glass-break detectors listen for the specific frequency signature of breaking glass and cover an area rather than a single pane. They are worth adding where a large window or a shop front is the obvious entry point, and they respond before anyone is inside — which a motion detector cannot do.