How to Place and Aim a Motion Detector

Why this matters

A passive infrared (PIR) motion detector is the cheapest interior protection in a system and the most common source of false alarms. Most of both its failures are placement: a detector aimed straight down a hallway so an intruder walking toward it barely registers, or a detector that can see a sunlit window, a supply register or the curtain that sways over it, and calls the police on a warm afternoon. The detector itself is rarely the fault. Placing one well is a short sequence: work out how someone would actually move through the space, put the detector where those paths cross its field, keep it away from anything that changes temperature or moves, set pet immunity if there is a pet, then prove it with a walk test from every approach and with the pet.

Why crossing beats approaching is owned by a sibling card; in one clause, a PIR responds most strongly to a warm body moving across its zones and weakly to one walking straight at it. See related: Motion Detector Coverage and the Walk-Across Rule.

Start state

You need: the detector and its instruction sheet (mounting height, coverage pattern, pet weight rating, warm-up time, walk-test mode), the zone list from the survey, a step ladder, a drill and the mounting hardware, and the owner, if there is a pet, for the pet test.

Check before you begin:

• Account on test. A walk test trips the zone repeatedly. See related: How to Put an Alarm Account on Test and Take It Off.
• Pets. Weight and habits, from the owner. The pet immunity rating is per the maker and assumes the animal stays on the floor; a cat on a bookshelf or a dog on a couch is closer to the detector than the rating assumed. See related: Pet-Immune Motion Detectors and Where They Still Fail.
• The room's interferences, walked before you choose a spot: windows that take direct sun, supply and return registers, space heaters, fireplaces and wood stoves, ceiling fans, curtains and blinds near drafts, plants or balloons that move in moving air, and anything glass or shiny the sun reflects off.

The steps

1. Draw the approaches before choosing the corner. List every way into the space an intruder might come from: the uncovered or forced openings, the stair from another floor, the hall from the bedrooms. Then trace the path from each to wherever they would go next, usually toward the valuables or the rest of the house. Why: the corner follows from the paths. Choosing the corner first and hoping the paths cross it is how a detector ends up looking down the long axis of a room everyone walks along.

2. Pick the corner that makes the most paths cross the field. A corner mount covers the most floor. Of the corners available, choose the one from which the paths you drew run across your line of sight, not toward or away from you. A path that runs straight at the detector is the one it is worst at.

3. Rule out the corners that can see an interference. A detector must not look at a window that takes direct sun, a register blowing into its field, a heater, a fireplace, or something that moves in drafts. A register mounted below the detector, blowing away from it, is outside its view; one across the room blowing up into its pattern is not. If the best corner for the paths can see a sunlit window, the next-best corner usually wins. Why: a PIR detects a change in infrared across its zones. Sun moving across a floor, a blast of warm air or a curtain swinging in front of a radiator all produce that change, and the detector cannot tell them from a person.

4. Mount at the maker's height, square to the corner. The coverage pattern, the look-down zone under the detector and the pet immunity all assume the stated height. Too high and the zones ride higher off the floor across the room, leaving a gap a crouching intruder can pass under; too low and the upper zones sit near the floor, where pets reach into them, and the range falls short of the far wall. Mount the bracket solid; a detector that wobbles on drywall when a door slams can trip on its own movement. Hazard: stable step ladder, not a chair; check the corner for wiring and plumbing before drilling.

5. Set pet immunity and sensitivity to suit the space, per the sheet. Set the pet setting to cover the heaviest animal that will be in the room while it is armed, if the maker's rating reaches it. Leave the pulse count or sensitivity at the maker's default unless a test below argues otherwise. If the animal is heavier than the highest rating, or climbs, motion in that room is the wrong device for when the pet is loose; say so plainly and plan around it (motion armed only when the pet is crated, or glass-breaks and perimeter instead).

6. Let it warm up, then walk test with its test LED from every approach. Power up, wait the warm-up time the sheet gives, put the detector in walk-test mode so its LED lights on each detection, and walk each path you drew in step 1 at a normal pace, then slowly. Note where along each path the LED first lights. Why every approach: one path tested proves one path. The approach you skip is the one an intruder takes.

7. Test with the pet, on the floor and on the furniture it uses. Have the owner bring the pet through the room, and let it climb onto the couch or chair it uses. The LED must not light.

8. Take it out of walk-test mode and confirm the zone reports. Many detectors disable the LED after testing so it does not show an intruder where the field is. With the account still on test, arm the system away, wait out the exit delay, walk one path, and confirm the alarm arrives on the central station's history with the right zone and name. Then disarm, and take the account off test once the station confirms it is back to normal.

Worked example: a living room with a slider and a dog

A living room 20 ft wide by 16 ft deep. A slider on the north wall, the hall doorway on the west wall, the stair up in the northeast corner. A south window that takes afternoon sun. A supply register low on the east wall. A 35 lb dog that sleeps on the couch along the south wall. The detector's sheet gives a mounting height of 7 to 7.5 ft for this model and pet immunity rated to 40 lb, both as stated for this unit.

LIVING ROOM, PLAN, 20 ft x 16 ft (north up)

             slider (N wall), curtains
 +----------[==========]----------------------+
 |                                     stair |
 |  hall    <---- path A: slider to hall     |
 |  door                                     |
 |  (W)     path B: hall to stair ------>    |
 |                                           |
 |                                           |
 |          couch, dog sleeps here           |
 |                                      PIR  | SE corner,
 +---------[ sunlit window, S wall ]---------+ aimed NW
                    supply register low on E wall,
                    below and behind the PIR

Choosing the corner.

• NE corner, by the stair: path A runs from the slider straight away from it toward the hall, and the curtains on the slider sway in the lens's near field. Rejected.
• NW corner, by the hall: path A runs straight at it, the weakest geometry, and from there it looks directly at the sunlit south window. Rejected.
• SW corner: looks at the slider curtains and down path B lengthwise. Rejected.
• SE corner, aimed northwest: path A runs west across the north half of the room and path B runs east across the middle, both across its line of sight. The sunlit window is on the same wall, behind the lens, outside its view. The register is below and behind it, blowing away. Chosen.

Mounting. Bracket at 7 ft 3 in to the bottom of the detector, inside the 7 to 7.5 ft band. Pet immunity set to its 40 lb setting, which covers the 35 lb dog.

Walk test, LED in walk-test mode after the warm-up:

Every path lit the LED. The crawl test was added because an intruder crossing under the field is exactly the case the look-down zone exists for.

Pet test. The dog walked the room, then lay on the couch, then jumped down and trotted to the slider. LED stayed off on the floor. On the couch, the LED lit once as the dog jumped up.

What the couch result means. On the couch, the dog's back is a foot and a half higher than it is on the floor, and much closer to the detector's upper zones than the 40 lb rating assumes. The detector is doing what PIRs do. Three options were put to the owner: keep the dog off the couch when armed away, arm the motion zone only when the dog is crated, or move the detector to a spot where the couch is outside its pattern, which in this room meant the NE corner already rejected for the curtains. The owner chose to crate the dog when the house is armed away, and the zone was set to arm in away mode only. The decision went on the work order.

So the finish test as run: the LED lit on every intruder path tested, and the pet tripped nothing while on the floor. The single trip on the couch was resolved by when the zone arms, not by turning down the detector, and the record says so.

What failure looks like a month later

False alarms on sunny afternoons. The detector sees sun moving across a floor or a reflective surface. Check what it can see at the time of day the alarms happen, not at the time of the install visit.

False alarms when the heat comes on. A register blowing into the pattern. Usually invisible in summer and the first cold week is when the calls start.

No alarm when an intruder walked through. The history shows the perimeter opening and then nothing. The path ran straight at the detector or under a pattern mounted too high. Walk-testing every approach at install is the fix that cannot be applied after.

Sensitivity turned down to stop false alarms. It trades the false alarm for a miss. Fix the cause, the placement, not the setting.

References

• Detector manufacturer's instruction sheet: mounting height, coverage pattern, pet weight rating, warm-up and walk-test mode
• See related: Motion Detector Coverage and the Walk-Across Rule
• See related: Pet-Immune Motion Detectors and Where They Still Fail
• See related: How to Put an Alarm Account on Test and Take It Off
• See related: How to Survey a House for an Intrusion System