Sensors

Sensors are the part of a smart home that makes automation worth having, and the part most often bought badly. A cheap motion sensor triggers on a cat, on a curtain moving in a draught, and on nothing at all at three in the morning. The result is automations people switch off within a month.

The distinction that matters is motion versus presence. A PIR sensor detects a change in infrared energy, so it notices you walking in and then loses you the moment you sit still — which is why lights go off while you are reading. A millimetre-wave radar measures distance continuously and keeps detecting a person who is barely moving, at the cost of higher power draw and a higher price.

Combining both is the practical answer: radar for the fine movement, PIR to confirm a warm body, and an ambient light sensor so the room only lights when it is actually dark. That combination is what stops a hallway light from firing in daylight, and it is the difference between a sensor you forget about and one you unplug.

Placement decides more than the spec sheet. Radar passes through thin walls and glass, so a sensor aimed at a corridor can trigger on the neighbours. PIR needs line of sight. Both are affected by heating vents and direct sun, which is why the install notes on each product page are worth reading before you drill.