Hi all, a circuit and a question

I've attached the schematic of a Passive Infra-Red detector I've been
tinkering with. I'm using it to wake the PIC up from SLEEP by a port
change interrupt. As shown, it works well. Current draw is about 50uA
(TLC271 in LF, LP mode) and with a proper plastic Fresnel lens, as
found on security lights, will detect movement at 10m indoors. If you
use this without a lens the distance is a lot shorter. To increase the
angle of view I use two of these at 60 degrees, output of a each
through a diode to b.4.

The PSU is 5V from F84 pin a.2, as once the movement is detected
I shut  the PIR down for 3 minutes. When turned back on, I ignore
any IRQ from it for about 15 seconds while it stabilises. This isn't a
security application, so picking up every trigger is not important. BC548,
if you aren't familiar with it, is a general purpose npn signal transistor.
The PIR chip must be orientated to detect movement effectively, the two
piezo elements inside should be perpendicular to the axis of movement
so they can make a difference signal. (I think I remembered that
correctly, my notes are somewhere else).

The question I have is - is there some feedback or auto-regulation
I could add to this circuit to eliminate the 10M preset or make it
"tighter" but without significantly increasing the current drain ? It's
just a 0.5 - 4Hz amplifier and comparator - can it be made "smarter"
or less hands-on ?

The PSU for the whole circuit is a 6V gel cell + LP2951, and the only
two parts alive most of the time are PIC and the PIR. Everything else
is powered off by the PIC switching the LP2951. The PIR circuit is
stable, no problems in the 6 weeks it's been on so far, but if I could
make it self-settling that would be a big advantage when it comes
to installing a number of them. ie set and forget, no need to worry
about any drift, if it happens. Putting a smaller preset between two
resistors does make it easier to set, but it does still need to be set
manually.

TIA, Jinx




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