I've been working on this project off and on for a couple of years. It's a directional mine with a PIR motion trigger. The shooting part is an airsoft m203 grenade.

On the left is my most recent working prototype.

Version 1:

My first shot at this used an off-the-shelf motion sensor, and a way-oversized solenoid I had lying around.

Buried in that rats-nest of wires is a PIC12F509 that handles the timer and trigger logic.

This version worked, but it had a lot of problems with false triggering. Swaying trees in particular were a big problem.

The system diagram is pretty simple. The shell's trigger is basically a push button on the back. This is pushed by the solenoid. The solenoid's controlled by a motion sensor and microcontroller.

A quick demo.

Version 2:

My next major rebuild involved a totally new case, and a real lens assembly. As it turns out, it's really hard to get directional infrared Fresnel lenses, even on the internet. I finally found this awesome guy. He takes orders via fax.

Besides focusing, the lens material acts as a bandpass filter for the IR that humans emit. This improved the trigger reliability a lot, but trees, birds, etc still caused false positives occasionally.