Motion Detector Placement Guide

Most motion detector trouble is placement trouble. A detector aimed down a hallway misses the intruder walking straight at it, and one above a heating vent calls the police when the furnace starts. Good motion detector placement comes from how a passive infrared (PIR) detector actually sees: zones, heat and movement across them. This guide covers direction, corners and height, what to keep out of view, and pets.

How a PIR motion detector sees

A PIR detector does not see a picture. It watches for changes in the infrared that warm bodies give off, through a lens that splits the room into separate detection zones with gaps between them. An alarm needs a heat source moving from zone to zone.

A University of California, Berkeley sensor study describes exactly this: a lens forming a pattern of discrete zones, a sensor that detects a person moving across those zones, but stays insensitive to overall temperature rising or falling in its view, and to a person standing still. That behaviour drives every placement rule below.

Walk across, not toward: motion detector placement by path

Someone walking straight at the detector stays inside one zone for longer and crosses few boundaries, so the detector responds late or not at all. The same person walking across the room passes through zone after zone and trips it quickly.

• Find the path first: the route from the likely entry point to what is worth taking, such as the hallway to the bedrooms or the door to the office.
• Aim across it: mount the detector to the side of that path, so the path cuts across the coverage.
• Use chokepoints: a hallway or the bottom of the stairs that everyone must pass through covers a whole floor with one detector, if it views the hall from the side or end at an angle.

Do not aim a detector straight at the entry door as the only protection for that door. Doors and windows are better watched by perimeter devices; see recessed vs surface door contacts and glass break sensor placement.

Corner mounting and PIR motion sensor height

A corner mount looks across the whole room from one point, and wall detectors generally mount either flat on a wall or across a corner. The coverage drawing on the installation sheet shows the pattern from above and from the side, and it is drawn for one mounting height. That listed height is the right PIR motion sensor height for that detector; it varies by model.

A quick calculation shows why the number matters. Take an illustrative zone, not from any sheet, that reaches the floor 35 ft out from a 7.5 ft mount. It slopes down at arctan(7.5 / 35), about 12 degrees.

• Mounted at 9 ft: the same zone lands about 9 / tan(12 degrees) = 42 ft out, and every zone moves away from the detector, widening the area it overlooks near the wall.
• Mounted at 6 ft: it lands about 28 ft out, shortening reach and dropping the lower zones toward pet height.

Mount flat and level unless the sheet lists a bracket and a tilt. Some detectors add a look-down zone for the floor directly beneath; the data sheet says whether yours does.

Heat sources, windows and vents to keep out of view

Because a PIR reacts to moving heat, anything that warms, cools or moves in its view is a false alarm risk. The City of Kirkland false alarm prevention tips list motion sensors near heating or cooling vents, moving lightweight objects such as helium balloons or curtains, among the top reasons for false alarms.

A PIR cannot watch the yard through a window: ordinary glass blocks most of the long wave infrared that body heat gives off. What it does see is the room the sun is heating.

Pet immune motion detector: what the rating means

A pet immune motion detector uses lens patterns and signal processing to ignore animals below a weight listed on its data sheet, as long as the animal stays on the floor. The rating assumes the listed height, a flat mount and a pet walking at floor level.

• Climbers defeat it: a cat on a couch back, a dog on the stairs or a pet on a counter is up in the zones meant for people.
• Furniture near the detector: some installation sheets set a minimum distance between the detector and anything a pet can climb; follow the one in the box.
• Several pets together moving side by side may not read as one small animal; check what the data sheet says about more than one pet.

Kirkland’s sheet also advises keeping pets where they will not activate motion sensors when the system is armed. Where that is not possible, an area of the home can be left off the armed motion zones and covered by perimeter devices instead.

Dual-technology detectors in brief

A dual-technology detector pairs a PIR with a microwave sensor and alarms only when both detect motion. Heat from a vent alone, or movement without a heat signature, does not trip both, which is why dual-tech suits garages, sunrooms and rooms that defeat a plain PIR. Microwave energy passes through some walls and glass, so aim it into the room, away from outside walls and moving doors.

Dual-tech still needs the placement rules above: across the path, at the listed height, clear of heat and fans.

Walk test and zoning before you leave

1. Put the panel or detector in walk test mode, so its indicator shows each detection.
2. Walk the expected paths at normal speed, from the far end of the room inward, and note where it trips.
3. Run the heating or cooling and watch for trips with the room empty.
4. Assign interior detectors to zones that stay off in stay mode, so the household can move around at night.

Hardwired detectors need power as well as an alarm loop, covered in 22/2 vs 22/4 alarm wire; choosing wired or wireless is covered in wireless vs hardwired alarm sensors.

Where to find motion detectors

The intrusion alarm category lists motion detectors, contacts and panels, and the security and surveillance division lists the wire and accessories that go with them. For pricing and availability, ask your sales representative through the Bursma contact page, reach the Grand Rapids low voltage branch or the Columbus Ohio low voltage branch, or open a Bursma dealer account.

Frequently asked questions