Installing Matter Door & PIR Sensors: Placement That Actually Stops Intrusions

Why placement, not the spec sheet, decides detection

Sensor placement is the physical act of choosing where on a door, window or wall a device is mounted, and it determines the detection geometry more than any firmware setting does. Certification proves the hardware behaves to spec; it says nothing about whether the sensor is pointed at the intruder's path or at a curtain. We treat placement as a design deliverable, not an afterthought, because a perfectly built PIR mounted in the wrong corner is functionally a decoration. The two sensor types in a Matter security kit fail in different ways, so they need different siting rules.

Door & window contact placement: the magnetic gap

A door/window contact sensor is a two-part magnetic switch that reports open or closed depending on whether a magnet fixed to the moving part is near the sensor fixed to the frame. The device itself is simple; the failure mode is distance. The two halves must be aligned on the same plane and stay within the rated magnetic gap when the door or window is closed — and, just as importantly, separate cleanly when it opens.

The gap rule

The magnetic gap is the maximum distance at which the contact sensor still detects the magnet as present; beyond it the sensor reports open, which is exactly what you want when a door is ajar but a false trip when the halves simply drift apart in use. In our own contact design we publish the rated gap from the reed-switch and magnet combination, then build the install guide around keeping the two halves concentric. A common mistake is mounting the sensor on the frame and the magnet on the door but leaving a visible offset — the gap looks tiny yet sits outside tolerance after paint, weather stripping or a warped frame. We recommend verifying the closed state with the app's open/closed readout before walking away.

What breaks a contact install

The install mistakes that break a contact sensor are an oversized gap from misalignment, a metal frame that shields the magnetic field, and a moving part that flexes so the halves separate during normal use without the door actually opening. Metal-framed doors and windows are the quiet killers: steel attenuates the field, so the same gap that works on a wooden frame fails on aluminium. Where the frame is metal, the magnet and sensor need to sit proud of the surface or use a design rated for the material. We flag this in the product brief rather than leaving the installer to discover it at 2 a.m.

PIR placement: height, angle and blind spots

A PIR motion sensor detects changing infrared radiation from a warm body moving through its field of view, not the motion itself, which is why a person standing still can vanish from a PIR while a curtain in a draft cannot. The device watches a cone or fan of space defined by its Fresnel lens; placement decides whether that space overlaps the entry path.

Mounting height and look-down

Most wall-mounted PIRs are designed to be installed roughly 2 to 2.4 metres above the floor and aimed across the room, because the lens geometry is tuned to that height and aims its coverage outward and slightly downward to catch a person walking in. Mounting higher than the lens was designed for pushes coverage into the floor; mounting lower or tilting up creates a dead zone right under the device where an intruder can stand undetected. The exact height is a property of the lens, so the install guide must state the figure we tuned the optics for, not a generic "about waist high".

Avoiding the sun and the heater

A PIR is an infrared device, so any source of changing heat in its field of view — a window with direct sun, a radiator, a tumble dryer vent — can trigger or blind it, which is why siting away from heat sources matters as much as aiming at the doorway. We have watched PIRs false-alarm on a sunset through a patio door and go silent next to a space heater. The fix is placement: keep the lens off direct thermal paths and out of line with windows that swing between sun and shade. This is also where firmware helps but cannot rescue a bad site — signal processing suppresses gradients, it does not invent a clean view of a boiling radiator.

Covering the entry path, not the room

The goal of intrusion placement is to cover the path an intruder must take — a doorway, a hall, a stair — rather than to flood a whole room with detection, because a narrow covered path is both more reliable and easier to explain to a customer. A single PIR at the head of a hallway protects every room beyond it better than one PIR per room placed arbitrarily. For contacts, the rule is to protect the openings that matter: ground-floor doors and accessible windows first, upper floors second. We give brands a per-room-type coverage sketch rather than a one-size floor plan, because a studio flat and a two-storey house are different installation jobs.

Battery and radio placement realities

Battery life and radio range are both affected by where a sensor sits, because cold locations shorten battery life and metal or thick masonry between the sensor and the network degrades the radio link that Matter over Thread or Wi-Fi depends on. A contact on an unheated garage door in a cold climate draws down faster than one indoors, and a sensor tucked inside a metal-clad cabined hates the enclosure that hides it. The connectivity itself comes from a standard CSA-certified Matter connectivity module we integrate, but the module's real-world link is still governed by the enclosure and the wall it sits behind. We advise placing battery devices where the temperature stays within rating and where at least one Thread device or access point is within a reasonable distance.

Factory stance: placement is part of the deliverable

Shenan Yangguang designs, prototypes, validates and mass-produces these sensors; we do not relabel a third party's product. That ownership means placement guidance is ours to get right. We publish the magnetic gap from the actual reed and magnet we specify, and the mount height from the actual lens we tune — not rounded numbers copied from a catalogue. A brand selling a white-label security kit should be able to hand its customers an install guide that matches the hardware in the box, and we treat that guide as part of the product, not an afterthought. Where we are unsure how a specific building type will behave, we say so and suggest a site check rather than overpromising coverage.

The deeper engineering behind these two sensors is covered in our notes on contact-sensor design and the PIR technology deep dive, and the battery and radio realities are discussed in Matter sensor battery and low-power design.