Marco Genovesi wrote:
> Maybe simple but I haven't any experience of this..
> Is it possible to use a common PIR surveillance sensor to detect a
> warm->cold->warm temperature transition?
> I know taht the "normal" use in surveillance is to detect an "hot"
> body crossing the sensor area.

Actually they work on sensing changes in IR.  They don't specifically detect
hot or cold things, just the pattern moving.  These sensors are really two
sensors inside with the temperature to voltage relationship flipped for the
two.  For any fixed ambient IR hitting both sensors, the result eventually
nulls out.  As the IR field changes, there will be some differential on the
two sensors creating other than the steady state output voltage.  A lens in
front of the sensor roughly focuses the IR image in bands, so that something
of a different temperature moving around hits the two individual sense
points with alternating bands.  The circuit in these things is just a window
detector.

> A possible complication is that the cold "object" isn't a solid but
> really a localized flux of COLD air that rapidly cross the sensor
> area and that may be very near ( from 5 to 1 feet from the PIR).

That's a problem as air is transparent to IR and doesn't exhibit any
significant black body radiation compared to hard stuff in the room.


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