> IR Motion Sensors operates by changes in the IR energy
> level received during a short period of time.
> At power on, it takes few seconds to develop the average
> IR received and store that level in a capacitor with a
> long RC constant (seconds).

The nett effect is the same, but this isn't what happens in the
sensor.

The sensing element is a semiconducting crystal (Lithium tantalate
from memory) which develops a voltage depending on its absolute
temperature. It absorbs IR radiation which causes it to heat up. It
also quite "leaky" and the charge across the crystal equalises fairly
quickly so it performs its own averaging. Motion sensors come with an
inbuilt JFET to amplify the minute signals and there is only an
output if the rate of change in IR is faster than the decay rate of
the crystal. Obviously it is sensitive to thermal noise so a bit of
signal conditioning is required before triggering.
The multi-faceted lens (which is a different thing to a Fresnel lens)
ensures that a slow moving object causes relatively rapid transitions
to the IR sensor by moving in and out of the sensors field of view.

If you take the same sensor and put a mechanical chopper in front of
it you can make your own non-contact thermometer (although that
isn't what the commercial ones do). Get lots and make a thermal
imaging camera. :-)

Steve.

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Steve Baldwin                Electronic Product Design
TLA Microsystems Ltd         Microcontroller Specialists
PO Box 15-680, New Lynn      http://www.tla.co.nz
Auckland, New Zealand        ph  +64 9 820-2221
email: steveb@tla.co.nz      fax +64 9 820-1929
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