"Pet immune" is printed on motion sensors across the market, and homeowners read it as a promise: the alarm will ignore animals. What the label actually describes is a design window — a weight and height band within which moving heat sources are deliberately filtered out. The sensor does not know a pet from a person. It knows that a warm object moving below a certain height and within a certain thermal pattern is statistically more likely to be an animal, and it suppresses that pattern.
How the filtering actually works
The common implementation splits detection zones by lens geometry: upper zones respond to a tall human silhouette crossing them, while lower zones are masked, weighted down, or required to trigger in combination with the upper zones. The published band — commonly up to roughly twenty to twenty-five kilograms and below about a meter of height — is the envelope in which this logic holds. It is engineering statistics, not recognition. Anything outside the assumptions can defeat it in either direction.
The band has edges, and animals find them
A large dog standing on its hind legs at a window puts a tall hot mass into the upper zones — exactly the pattern the sensor is built to alarm on. A cat is the opposite edge case: light enough to be filtered by weight, but a jumper. A cat on a counter or shelf crosses the upper zones at low mass, and weight alone cannot exclude it, which is why honest vendors tell cat owners that pet immunity is weakest exactly where cats live. Running animals add motion speed, which some detection logic also weighs.
Mounting geometry does most of the real work
The band assumes the sensor is mounted where the design expects — typically two to two and a half meters, looking level across the room. Mount one low or aim it down a staircase, and a pet's path fills the same zones a person would. Aim the sensor away from furniture pets climb, away from stairs, and away from heat sources that warm the floor and shrink the contrast between animal and background. In summer, sun-warmed floors reduce the thermal difference a PIR depends on, which is why the same install can behave differently across seasons.
Match the promise to the job
For security alarming, false triggers cost real money and trust, so pet immunity deserves its full engineering: correct height, correct aim, and a walk test with the actual animal. For home automation — turning lights on when someone walks in — the stakes are lower, and a stray trigger from a pet is an annoyance rather than an event, so a cheaper non-immune sensor can be the better buy. Using an alarm-grade pet-immune sensor to switch lights, or a bare PIR to guard a room with two dogs, is misbuying in both directions.
What to ask and how to test
Ask for the weight and height band and what lens implements it. Ask the recommended mounting height and whether the band is configurable. Ask how the logic treats fast motion and elevated animals. Then test with the actual animal, not the brochure: walk the pet through the coverage area at the heights it actually uses, and watch the event log. A sensor that survives your dog's zoomies is the only pet-immune sensor you own.
I work on the manufacturing side of this category, and most pet false-alarm complaints we see resolve to mounting geometry rather than sensor quality — the band was right, the install was not. Our sensor and alarm range lists the pet-immune models with their bands, and our documentation as a door window sensor manufacturer covers detection geometry in depth. The broader line is at XingLiao and detection questions go through our contact page
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