>> They tend to work on differences caused by a heat source moving across
>> their field. The plastic fresnel lens enhances the results of objects
>> moving through 'zones'. If the IR source increases relatively evenly
>> across the field of view or part of it it probably wouldn't work.

> But turning on a IR source suddenly in any one position is likely to work.
> The only reason is wouldn't is if the illumination on both ends of the
> sensor just happened to be the same for that particular position, which
> would be quite tricky to arrange on purpose if you tried. =A0Two bulbs a =
few
> inches apart would almost certainly work.

Yes, maybe, sort of.
I wasn't intending to state a sole position but only a comment on that one.
I'd previously said

> Also, if what it wants is a change in one field relative to the other
> it MAY be enough to use one LED or bulb (Olin) occasionally activated.

so Gus and Russell and Olin are all essentially agreed, for a change :-).

The uncertainty lies in how intelligent and/or crude the sensor is.
Dim memory suggests that they use a single sensor with the lens
effectively providing a "grating" which enhances some areas and blocks
or desensitises others. As you move an IR source across the zones you
create a modulation of the level at the sensor.

OK - lets see what MrWpedia says ... time warp ...

   "The sensor is often manufactured as part of an integrated circuit
and may consist of one (1), two (2) or four (4) 'pixels' of equal
areas of the pyroelectric material. Pairs of the sensor pixels may be
wired as opposite inputs to a differential amplifier. In such a
configuration, the PIR measurements cancel each other so that the
average temperature of the field of view is removed from the
electrical signal; an increase of IR energy across the entire sensor
is self-cancelling and will not trigger the device. This allows the
device to resist false indications of change in the event of being
exposed to flashes of light or field-wide illumination. (Continuous
bright light could still saturate the sensor materials and render the
sensor unable to register further information.) At the same time, this
differential arrangement minimizes common-mode interference, allowing
the device to resist triggering due to nearby electric fields."

So, what we all said, or not :-).
And adding processing power to that. as some do, makes it more uncertain.

If using one only light / LED etc a relatively point source would seem
best. This simulates moving a warm body in and out of a single zone
rather than between zones. How well it works seems liable to be quite
dependant on the sensor and it's intelligence,

Trying it would be extremely easy.
I like the curtain flapping in the air conditioner breeze as the
modulator though - seems apposite to use the system to trigger itself
:-).


     Russell

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