Thanks. I'll take a look.

--Bob

Peter P. wrote:
> Bob Axtell <engineer <at> cotse.net> writes:
>   
>> Just incidentally: sources tell me that these PIR sensors 
>> have been improved 10,000 fold, and some can pick up long 
>> range IR from the "Big Bang". 
>>     
>
> I don't think so. PIR chips have always worked very well against an 'ice'
> cold background (and the +20K ?) temperature of the black sky is certainly
> cold enough for that. But they do not really detect the sky's IR, instead
> they 'see' the contrast between objects in their bandwidth and the absolute
> black of the sky.
>
> The problem with most PIRs made for commercial applications is noise. The
> FET preamp and the crystal itself create enough noise that any use outside
> the designated purpose is nearly impossible. The S/N ratio for the
> designated area of application (detecting nearby passing people through a
> cheap plastic segmented Fresnel lens, at a hot/cold target ratio of about
> 1-5 degrees K - the difference between a clothed person's surface
> temperature vs. room ambient) is something like 10dB-ish. The crystal
> itself drifts like crazy and shows bumps and peaks in the output that have
> nothing to do with IR (maybe they detect some radiation particle or single
> photon impact events). This is easy to test. Put a PIR in a thermostat
> (empty thermos bottle Dewar works) and connect the output to a DVM and
> analog scope (the preamp analog output). You will be surprised at what
> comes out of there. The FET that has a lot of 1/sqrt(Hz) noise at LF to dc
> is also not very helpful in this situation. Chopping is limited to <10Hz
> before the gain decreases sharply (as specified by the datasheet, likely
> due to the crystaland die capacitance), a frequency that does not pull the
> signal out from the horrible low frequency noise domain of the built-in FET
> transistor.
>
> I have made experiments with commercial PIR sensor chopping and other
> similar devices and the results were not good. I believe that using a raw
> (sans FET) sensor and a better quality preamplifier may yield better
> results, if the PIR crystal is of a low noise type (I don't think that the
> types used in commercial PIRs are low noise at all - they seem to make
> noises reminiscent of damaged carbon microphones in an otherwise quiet
> environment).
>
> Peter P.
>
>
>   

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