Version 3:

Version 3 looks similar to two, but I basically redesigned everything.

I replaced the off the shelf sensor with a pyroelectric element and and a dedicated M7612 PIR Controller IC.

I also beefed up the solenoid power circuit by switching the transistor to a FET and adding a large cap.

Here's a better pic of the new electronics.

In addition to the new stuff, I made a bunch of minor improvements. I upped the isolation between systems a lot, added filter caps, put in more efficient regulators, etc.

Version 4:

Another major redesign. I figured out that the PIR IC was just a glorified op amp, so I replaced it with an actual op amp.

I also upgraded the PIC to a version with analog inputs (12F675) and started reading the analog outputs from the op amp directly.

To write the triggering algorithm, I bought a DI-194 data logger and recorded the waveforms for a bunch of different stimuli like swaying trees, walking/running at various distances, jostling etc. I used the data to write a triggering algorithm in assembly.

The algorithm keeps a running average of recent readings, and triggers if the input differs substantially for a few cycles in a row. The multi cycle requirement rejects electrical noise (sharp spikes). The running average rejects low frequency drifting.

Version 4 in action.