What a motion detector actually senses
Most home motion detectors are passive infrared sensors, usually shortened to PIR. “Passive” means the sensor sends out nothing. It simply watches for changes in infrared energy, which is the heat that people, animals and objects give off.
Inside the unit, a lens divides the room into zones of view. A person walking across the room moves from one zone to another, and the sensor sees a change in heat between them. If the change fits a pattern the detector treats as human movement, it signals the control panel. A person walking straight toward the sensor tends to be harder to detect than one walking across its field of view, so the best positions face a likely path from the side.
Some detectors, called dual-technology units, combine PIR with a second method, such as microwave. The idea is to require both to agree before raising an alert, which can reduce false triggers. These tend to cost more, and microwave elements can occasionally detect motion through thin walls, so check the manual.
Coverage
A typical detector covers a fan-shaped area, described by an angle and a distance. The numbers differ by model, so rely on the specification for the unit you buy. Mounting height matters, because the lens pattern is designed for a certain height, commonly given as a range. Mount outside that range and the coverage may change or develop gaps.
Motion detectors see only what is in line of sight. Furniture, plants and half-open doors can block the view, so a sensor behind a large sofa may miss movement.
Pets
Pets are the most common source of complaints. A dog or cat gives off heat and moves, so a basic detector may trigger.
Pet-friendly detectors handle this in several ways:
- Weight or size thresholds: the sensor is tuned to ignore movement below a certain size. Manufacturers state a limit, which varies by model and is not a guarantee. A larger dog, or a pet that jumps onto furniture, can still be seen.
- Lens design: some lenses reduce sensitivity in the lower part of the field of view, so that something on the floor is missed while a standing adult is seen.
- Sensitivity settings: lowering sensitivity can reduce false alerts but may also make the sensor miss a person.
- Signal processing: some units look at the pattern of movement rather than only its presence.
Even with these features, a determined or unlucky pet can trip a sensor. Cats climbing shelves and dogs on the sofa are classic problems. If your pets roam freely, pet-friendly models are the first step, then careful placement, and then a quick test with your own animals walking around. Some owners decide to keep motion detectors off in the pets’ areas, relying on door and window contact sensors, and arm the system in a mode that disables interior motion when pets are home alone.
Good places to put them
- Corners, facing across the room so that movement crosses the field of view.
- Hallways and stairs that an intruder would need to pass through to reach bedrooms.
- Rooms with valuables, such as an office, or an entry hall near a door.
- Ground-floor rooms with few other sensors.
A detector in a main passage is often more useful than several in rarely used rooms.
Spots to avoid
Because the sensor responds to changing heat, anything that changes temperature quickly can fool it.
- Direct sunlight through a window, which can warm surfaces unevenly.
- Heating and cooling vents, fireplaces, space heaters and wood stoves.
- Over radiators or above a kitchen range.
- Facing a window, where headlights or moving shadows may create changes.
- Near ceiling fans, hanging plants or curtains that move in drafts.
- Outdoors, unless the model is rated for outside use.
- Directly above furniture a pet can climb onto.
Note also that glass blocks infrared, so a PIR sensor cannot see through a window to the outside. Detecting activity in the yard is a job for outdoor cameras, floodlights or fence-based equipment, and false motion alerts from cameras have their own tips.
Testing
Most detectors have a walk-test mode with an indicator light that blinks when motion is seen. Walk across the room at different distances and speeds, then repeat with pets present. Many also have a warm-up period after power is applied, so test after it has settled.
Where motion detectors fit
Motion detectors guard the interior. They are a backup if someone gets past a door or window, and a useful complement to contact sensors and glass break sensors. They do not stop entry, and they are normally disarmed or bypassed while you are home. Understanding that limit, and not leaning on a single sensor type, makes a layout more dependable. Reducing accidental alerts also depends on habits, such as remembering to arm in the right mode.