Getting motion sensors to ignore pets is mostly about placement, sensible settings, and a bit of trial and error. I use Home Assistant examples here because that is what I run.
Start with the sensor. Passive infrared, PIR, is the usual choice. PIR spots heat and motion. Some PIR models are sold as pet-immune or have an adjustable pet setting. Dual-technology units pair PIR with microwave or ultrasonic sensing and can cut false positives, but they can be fussier to tune. Check the datasheet for pet immunity or adjustable sensitivity before you buy.
Mounting matters more than most people expect. Aim for around head height, about 2.0–2.3 metres, angled slightly down. That keeps the main detection cone away from a cat’s usual roaming band. Do not point a sensor straight at a floor area where pets run, at vents, at heating sources, or at reflective surfaces that throw heat signatures around.
Set sensitivity to medium first. Run a walk test and a pet test. For the walk test, stand where the motion sensor should trigger and move through its field. For the pet test, get the cat to move through the lower half of the detection cone while you watch the sensor or the Home Assistant binary sensor state. If it triggers too easily, drop sensitivity in small steps and test again.
Common problems are low mounting, windows in the detection field, and drafty vents. Fix those first before messing with advanced settings.
If false alarms still happen, try low-impact physical fixes next. One simple trick reported by users is painting the lower half of the monitored wall. The idea is to change the surface the sensor sees in the lower part of its cone. Paint can absorb or disperse the heat and contrast signature enough to stop small animals showing up as full motion.
- Paint the lower 60–100 cm of the wall with a matt emulsion.
- Test detection for 24–48 hours.
- If false triggers continue, apply a second coat and test again.
Users on Home Assistant forums said one coat sometimes reduced triggers, two coats stopped them. Keep the painted band horizontal and even, not patchy. Where painting is not practical, use a physical barrier or baffle: a low transparent panel or a strip of textured fabric fixed to the wall can do the same job. You can also fit a small cone or collar around the sensor to narrow the field of view. Those little sensor hoods are cheap and reversible.
Tune the automation logic as well as the hardware. In Home Assistant, use conditional logic to ignore short, low-priority motion events. For example, require the sensor to be active for 30 seconds before logging an alarm, or trigger a notification only when nobody is home.
A pragmatic rule set I run: motion must be present for at least 20–30 seconds, then check that an authorised person is not present, and only then trigger an alarm. Make the change, then validate by watching the binary_sensor state and by letting the pet move through the area for a day. If automatic camera snapshots are set, check them to confirm the event was real before escalating to alarms. If a sensor offers adjustable retrigger time, increase that slightly to avoid repeated events from the same pet crossing back and forth.
Keep a maintenance routine. Check batteries monthly for battery-powered sensors and dust or insect screens for wired ones. Re-test after any furniture move or seasonal change like heating coming on, because thermal patterns change. Log each tweak and its result. A short table in your notes with columns for change made, date, test length, and result gives you a quick way to roll back a tweak that makes things worse.
Choose a sensor with pet options, mount it higher and angle it down, test with the actual pet, change one variable at a time, and try a painted lower band or a small physical baffle before buying more kit. That usually gets motion sensors behaving without turning the house into a nuisance alarm.
