This is a simple motion activated light that uses an attiny45 or attiny85, a PIR sensor, 12 leds, and some other components. Its powered by 4 AA batteries and from my calculations should last around 2 months on one set of batteries. It turns on for 30 seconds when its dark and detects motion. See the videos for more information

A short explanation video
https://www.youtube.com/watch?v=8jszWSCRU8M

Full build video
https://www.youtube.com/watch?v=LJtMK_RjTzQ

part links

PIR sensor
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2082927_-1

attiny45
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_2151267_-1

battery holder
http://www.jameco.com/webapp/wcs/stores/servlet/Product_10001_10001_216187_-1