Motion Detector Coverage and the Walk-Across Rule

Why this matters

A passive infrared motion detector is very good at catching someone who walks across its view and noticeably worse at catching someone who walks straight at it. Mount it above the doorway an intruder will head for, aimed back at the window they came in through, and you have built the slowest possible detection into the one room the system depends on. The same physics explains most motion false alarms: anything that moves heat from one part of the detector's view to another looks like a person, whether it is a person or not.

What a PIR actually sees

A passive infrared (PIR) detector does not see motion, and it does not see a shape. It sees a change in infrared energy arriving at its sensing element. Two pieces make that useful:

• The element, usually a dual (or quad) pyroelectric sensor wired so that equal energy on both halves cancels. A uniform change, like the whole room warming, mostly cancels out. An imbalance between halves produces a signal.
• The lens, a segmented Fresnel lens or mirror that divides the room into a fan of narrow sensing zones (installers call them fingers or beams) with blind gaps between them. Each segment focuses its own slice of the room onto the element.

A warm body standing still in one zone produces a steady input and no signal. A warm body moving OUT of one zone and INTO the gap or the next zone changes which half of the element is lit, and that swing is what the detector counts. Most detectors also require more than one swing (pulse count) or a swing of a certain size before they call it, which is how they reject a single flicker.

The size of the swing depends on the contrast between the target and the background. A person at body temperature against a cool wall is a strong target. The same person in a room that is close to body temperature (an attic room on a summer afternoon) is a weak one, and range shrinks. Many detectors compensate for temperature; none of them remove the effect.

The walk-across rule, drawn

Top view of a corner-mounted detector and two paths through its fan:

 A = PIR in corner, mounted at the maker's height
 A--------------------------------------------------------
 |  \ \   .     .
 |   \  \  o  .        .
 |    \   \   o   .           .
 |     \    \    o    .              .
 | P1---\-----\-----o-----.-----------------.------------>
 |       \      \      o      .                    .
 |        \       \       o       .                       .
 | "\" and "." mark zone edges   "o" marks P2's path to A

P1 walks across the room. In the stretch shown it crosses four zone edges, and each crossing is a swing on the element. Several swings arrive within a few steps, which is exactly what the pulse count is waiting for.

P2 walks straight at the detector, along a line that stays between the same two edges the whole way. It crosses none. The only change the element sees is the target growing as it approaches, which is a slow, mostly balanced change. Many detectors still catch P2 eventually, through the close-in zones or the look-down zone directly below the lens, but later, and closer to the detector, than P1. Maker range ratings are normally established walking across the pattern; the distance at which a detector catches someone walking toward it is shorter.

The rule that follows: aim so the path an intruder must take crosses the fan, not so it runs along a line of sight. That usually means the detector looks across the room from a side wall or corner, rather than from the far end of the path back down it. Placement steps, mounting heights and the walk test are in How to Place and Aim a Motion Detector.

The rule on a real room

A living room 20 ft east to west by 16 ft north to south. A slider sits in the west wall. A hallway doorway sits in the east wall, and the hallway leads to the bedrooms and the keypad. Anyone coming in through the slider has to cross the room west to east.

As found. The previous installer mounted the detector over the hallway doorway on the east wall, aimed west at the slider. Walk-tested at a normal pace from the slider straight toward the hall (the account on test first if the panel's own walk-test mode is used, because some panels report walk-test activity), the detector's test LED stayed dark for the first several steps and lit only when the tech was well past the middle of the room. The intruder path ran almost exactly along a line of sight: P2, every time.

Two more problems came with that position. The detector looked straight at the slider glass, so the afternoon sun patch on the floor inside the slider, and the shadow of a tree moving across it on a windy day, sat squarely in its view. And a ceiling supply register in the middle of the room blew warm air past the detector's face when the furnace ran.

Moved. A corner looked tempting, but from either north corner the second half of the walk runs away from the detector nearly along a line of sight, which is P2 in reverse. The detector went to the middle of the north wall instead, looking south. From there the west-to-east path (8 ft out from the north wall) crosses the line of sight at a right angle in the middle of the room, and even at either end it meets the line of sight at about 39 degrees (10 ft across, 8 ft down). From the northwest corner, the far end of the same path meets the line of sight at only about 22 degrees (20 ft across, 8 ft down). The walk is now P1. On retest the LED lit within the first two steps inside the slider. The slider glass is now at the side edge of the pattern rather than straight ahead, and the supply register is no longer between the detector and the room.

The sun patch did not entirely leave the view: it lies on the floor just inside the slider, and the detector's west-side zones look across that floor. That was handled the way the maker allowed (masking the lens segments that land on that patch, per the maker's instructions, and confirming the customer closes the slider blinds on summer afternoons when armed Away) and checked by watching the detector's LED for a quiet half hour with the sun on the floor. One position rarely fixes every risk; the job is to know which risks remain and test those specifically.

What causes false trips, by mechanism

Each of these produces the same thing a person does: infrared energy moving from one zone to another.

• Heat sources that cycle. Baseboard heaters, radiators, space heaters, a fireplace insert, a wood stove. When they heat up or cool down they change the energy from that patch of the view. Keep them out of the pattern; a unit that comes on and warms quickly is worse than one that changes slowly.
• Sunlit surfaces. Window glass is largely opaque to the far-infrared band a PIR reads, so the detector does not look through a window at the yard. What it does see is what the sun heats inside: a floor patch, a couch arm, a wall. A cloud passing turns that patch warm-cool-warm in seconds, and car headlights swinging across a room at night can do it too.
• Air that moves things. Moving air itself is a poor infrared target, but a supply register blowing on curtains, a plant, a hanging decoration or a helium balloon moves warm or cool objects across zones. Air blowing directly onto the detector also changes the element's own temperature quickly. Keep detectors away from supply and return registers and do not aim them at anything a register moves.
• Things on or in the detector. An insect crawling on the lens is a target the size of a person to the near segments. A detector mounted on an exterior wall that gets sun, or on an uninsulated wall, can see its own housing temperature swing.
• Pets. A separate problem with its own limits; see related: Pet-Immune Motion Detectors and Where They Still Fail.

Dual-technology detectors, which pair the PIR with a microwave sensor and alarm only when both agree, cut false trips from heat sources considerably. The microwave half has its own blind spot in reverse: it passes through drywall and some doors and sees motion in the next room or outside, so it must be ranged down to the room per the maker, or the microwave half agrees with every heat flicker and the detector is effectively a PIR alone.

What changes the answer

• Hot rooms. Where the room approaches body temperature, range falls, and a detector that covered the room in winter may not in August. Test in the season that is worst for the room, or choose a detector the maker rates for it.
• Long, narrow spaces. A hallway is the one place walking toward the detector is unavoidable. That is why hallway detectors are long-range lens patterns mounted at the end, accepting the P2 geometry because the walls force every path down the same line, and why they belong as followers rather than the room's only protection (see related: Zone Types and What Each One Does When Tripped).
• Glass-heavy rooms. A room that is mostly windows has sun patches all afternoon. A glass-break detector protecting the glass, with the motion as backup, may serve better than a motion detector fighting the sun (see related: Glass-Break Detectors: Placement, Range and Testing).

How to verify you got this right

Walk test every path an intruder could take into the room, at a normal pace, crossing from each entry point toward each exit, and note the step at which the LED lights. Any path where it lights late is a P2 path, and the fix is the detector's position, not its sensitivity. Then stand still and let the room run: furnace on, sun on the floor, ceiling fan running if there is one. The LED should stay dark with nobody moving. A detector that passes the walk and fails the quiet watch will generate false dispatches; one that passes the quiet watch and lights late on the walk will miss a burglar.

References

• Detector manufacturer's installation instructions (mounting height, rated pattern, masking, pulse count, dual-technology adjustment)
• Panel manufacturer's programming guide (walk-test mode and whether it reports)
• See related: How to Place and Aim a Motion Detector; Pet-Immune Motion Detectors and Where They Still Fail; Security + Alarm Systems Reference