[ladyada] has a freshly-published and amazingly thorough tutorial on passive infrared (PIR) motion sensors. Most often seen in security floodlights and automatic doors, in creative hands these sens… ...

Passive InfraRed sensors (PIRs) are electronic devices which are used in some security alarm systems to detect motion of an infrared emitting source, usually a human body.

Get yourself some passive infrared sensors (PIRs) and maybe a smart camera just to be sure. ... The Arduino-based instrument measures DC voltage, current, power, energy, capacity, and temperature.