How a motion sensor decides that somebody is there

Everything below about motion detection comes from what actually happens rather than from what is supposed to.

Passive infrared detects change, not warmth

The sensor contains paired elements that produce a signal proportional to the difference in infrared radiation falling on each. A lens divides the field of view into a fan of zones, and a warm body crossing between zones produces a rapid difference between the elements. A person standing still produces no difference and becomes invisible to the sensor, which is why lights switch off while you sit reading.

The paired arrangement also cancels slow uniform changes such as the room warming, which is deliberate.

Why it misses and why it triggers

Movement directly towards the sensor crosses fewer zone boundaries than movement across it, so approaching along the axis is the classic blind approach. Warm air from a heater, sunlight moving across a floor, and pets crossing the field all produce genuine signals.

Pet-immune designs use lens patterns and signal thresholds to ignore small low-mounted sources, with imperfect results. Glass blocks the relevant infrared wavelengths almost completely, so these sensors cannot see through windows in either direction.

Radar sensors work differently

A microwave sensor transmits a signal and detects the frequency shift caused by reflection from a moving object. It detects motion regardless of temperature and can sense through thin walls, plastic and glass, which is a feature and a problem.

At the protocol level, sensitivity high enough to detect breathing exists in some presence sensors, which solves the sitting-still failure of infrared. The cost is more false triggering from movement outside the intended area and higher power consumption.

Presence sensing versus motion sensing

Motion sensors answer whether something moved recently; presence sensors attempt to answer whether somebody is in the room now. The difference matters enormously for lighting, since a motion sensor with a timeout is the source of the classic experience of waving at a ceiling. Combining a radar presence sensor with an infrared motion sensor gives fewer false positives than either alone.

Sensor fusion of this kind is the standard approach in better systems and is why they cost more.

Placement decides performance

Mount across the likely path of travel rather than facing it, at the height the manufacturer specifies, and away from heat sources and vents. Avoid pointing at windows, radiators, kettles and anything that moves in a draught.

Outdoor sensors face sunlight, rain and insects, all of which cause false triggers, and insects inside the housing are a very common cause. A sensor that worked for a year and now triggers constantly is usually dirty, disturbed or full of cobwebs rather than faulty.

Implementations differ, and vendors are not obliged to document the differences.

What this means for security systems

A single sensor is easy to defeat and easy to trip accidentally, which is why alarm systems require confirmation from multiple detectors before escalating. Door and window contacts are far more reliable than motion sensors because they detect a binary physical event.

Battery-powered wireless sensors report their own battery state, and a system that cannot tell you a sensor has gone silent is worth little. Local recording and local alerting keep working when the internet does not, which is worth checking before buying.

The takeaway

It detects change in heat across zones. Everything it misses and everything that fools it follows from that.

Understanding the failure mode tells you more than the feature list does.