Passive Infrared detectors are used to sense motion. I am right now experimenting with such a device from Parallax. For once, this is a sensor which works with 3.3V, so the interface with the AVR32 processor is simple. Whenever movement is sensed, the output of the device goes HIGH and then LOW. Placing an LED on the output pin (buffered by a transistor) makes this `sensing' visible! Watching the LED light up and go off in a dark room as you move around the sensor is kind of spooky! Now, what kind of application can this have on a FreeBird?