MICROCONTROLLERS

Passive IR is always an interesting tech for me.

The gist is that human-beings are constantly emitting IR. In the infrared spectrum, we glow like light-bulbs, and this is in fact one of the secrets to advanced night-vision (aka: Thermal Vision).

PIR goes the other direction, making the bare minimum sensor that barely costs anything to produce but only provides a "yes/no" to the "is a human there?" question. They seem reasonably accurate in my experience ("hand wave" sensors to dispense water from faucets, or paper towels, etc. etc.), as well as home-detection alarms and the like.