We wanted to save energy, and create convenience, by adding motion sensors to our lighting circuits. Maybe you want some notification of an intruder. Both can be done with a PIR Motion sensor. When I walk into a room, the lights come on automatically, and when I leave, shut off after a short period of time. You can choose how long that time delay is in the code. No more fumbling for a light switch in the dark with my arms full of groceries!
http://www.instructables.com/id/DIY-Arduino-Motion-Sensor-Lighting-Control/
