Motion Sensors

Motion sensors are the trigger behind most useful home automation, switching lights, alarms and routines the moment someone enters a room.

The choice that matters most is the detection technology. Passive infrared (PIR) sensors read body heat against the background and are cheap, battery-efficient and reliable for on/off triggers, but they need actual movement — sit still on a sofa and a PIR will decide the room is empty. mmWave presence sensors detect the micro-movement of breathing instead, so they hold a room "occupied" while you read. They cost more and draw enough power that most want a USB supply.

Check the reporting delay before buying. Many battery sensors enforce a cooldown of 30 to 180 seconds before reporting "clear" to preserve battery, which is fine for a hallway light and frustrating for anything you want to respond instantly.