The office light that switches off while you are still working, the security scene that never triggers because the intruder froze, the bathroom fan that runs for nobody — all of these trace back to one confusion. Motion detection and presence detection are different physical measurements, and a sensor built for one cannot be re-tuned into the other. Understanding the difference explains most "faulty sensor" complaints that are actually correct behavior. It also explains why wireless sensors and alarms now ship in two distinct families.
What a PIR actually sees
A passive infrared sensor has no emitter. It watches the thermal pattern in its field of view through segmented Fresnel lenses, and it reports a signal only when that pattern changes. A person walking across the room changes the pattern; a person sitting still does not, because the pattern is constant. Warm air from a heater moving through the zone also changes it, which is the classic false trigger. The sensor answers one question — did something move — and it answers it with low power, which is why battery PIR devices last a year or more on coin cells.
What mmWave radar sees
A millimeter-wave sensor emits a low-power radio signal and analyzes the reflections. Because it samples continuously, it can detect micro-motion: the rise of a chest during breathing, a hand lifting a cup, a person shifting weight while standing. From the radar's point of view, a still-but-alive human is a moving target. This is what presence means in datasheets — not that the sensor is more sensitive, but that it measures a different physical quantity. Many units also report rough distance zones, which lets one ceiling sensor treat someone at the desk differently from someone near the door.
Why sensitivity cannot fix it
Turning a PIR's sensitivity up does not make it detect a still person, because there is nothing to detect. The thermal pattern is static and the sensor has no signal to amplify. Turning it up instead makes the sensor trigger on smaller changes: a cat at greater distance, sunlight crossing the wall, a draft from the vent. Every sensitivity complaint about lights turning off on seated occupants is this mismatch. The correct fix is a presence-class sensor for occupied-but-still zones and a motion-class sensor for transit zones like hallways.
The mounting geometry is different too
PIR detection depends on segmented zones, so coverage is a fan of beams — strongest walking across the beams, weakest walking toward the sensor. Radar coverage is a cone, with best results when the person's torso stays inside it; a radar on the ceiling looking down covers a desk well, while a radar on a side wall can lose a seated person behind a metal monitor arm. Neither sensor cares about the room's light level, which is an advantage over camera-based occupancy, but both have opinions about geometry that the installer must respect.
Power draw and reporting
The continuous transmitting and processing in a mmWave sensor costs power, which is why presence sensors are usually wired or at least battery-hungry, while PIR devices idle at microamps. On ZigBee networks the presence sensor also reports state changes and heartbeats more often, so expect more frequent battery replacement even on efficient platforms. A practical split seen in real deployments: PIR for battery-powered hallways and stairwells, radar presence for the desk, sofa, and bed zones where people linger.
Where each one belongs
Use motion sensing where the question is whether someone passed through: corridors, doorways, garages, and alarm perimeter zones. Use presence sensing where the question is whether the space is in use: home offices, living rooms, bathrooms with long occupancy, and any automation that turns things off — heaters, screens, lights — after people leave. For security the pairing is deliberate: a PIR catches the mover, a radar catches the freezer. Anyone evaluating platforms for a smart home line can compare both families through the door and window sensor programs that address detection geometry in detail.
Questions worth asking a supplier
Ask what physical principle the sensor uses before comparing price, because two products both named occupancy sensor may answer different questions. Ask about the detection cone angle and the minimum detectable motion — breathing-rate micro-motion for radar, walking speed at a given distance for PIR. Ask how the device handles a heater, a fan, or curtains in the field of view, since that answer separates tested firmware from optimistic marketing. More technical guidance is collected at Xingliao, and volume sourcing discussions start on the contact page.
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