I have been reverse- or re-engineering some inexpensive Passive InfraRed (PIR) motion detectors I picked up, with the idea of making them work on low voltage AC or DC supplies, and providing a contact closure for output.

The first parts I experimented with were the BC8008 and BC8010 models, then available from Target and Orchard Supply Hardware for between $5 and $10. Unfortunately, these are no longer commonly available, and are probably being closed out.

There are still some bargain PIR units out there, the Regent MS-401 for one. This one is just a sensor head, without the usual floodlamp sockets, turned out to be very easy to modify, and works well on batteries.

Another current model is the Heath-Zenith SL-5408. This is also amenable to hacking, but will not work on battery power. Should work fine on low voltage AC, however.

I tried modifying a (used) Regent MS35. It looks like it should be particularly easy to modify, both electrically and optically (to customize the field of view.) Unfortunately, the unit I was working with seems to be dead, so I will have to update the notes later.

Modification notes and schematics for BC8000 series PIRs
Modification notes for Regent MS401 PIR
Modification notes for Heath-Zenith SL-5408
Modification notes for Regent MS35 PIR

There are many instances where we want to "filter" trigger pulses, usually to prevent an effect from being re-triggered continuously, or to add a "hold-off" period, where it cannot be triggered, say until the guest leaves the area. One fairly simple circuit to do this is below. It uses a single 556C dual timer chip to provide a 10 second output pulse, with an extra 20 seconds of hold-off time, with the parts shown.