sanjeez,

The advantage PIR has over Ultrasonic is that it is passive and doesn't give away its position.

At Tandy, (Radio Shack's design lab) we designed a line of such detectors that looked for changes in the level of IR intensity in the target area. Note that it didn't IR illuminate; it didn't need to do so and that would make it active instead of passive detection.

Obviously the trick is to allow slow variations over time without triggering the alarm threshold or else as the sun crossed the sky, shadows and room heating patterns would trigger it.

The sensitivity is on the scale of human movement. However, I don't think its anything as sensitive and accurate as I've heard users image. After all, its only dealing with the SUM of the IR intensity in its viewing area, not a pixelation.

aka j