Photocell Problems

[Hairy Harry]

Hi Justin...

First of all... dark adapted photocells (that have been in the dark
a VERY long time...) often will have different ULTIMATE resistances.

A more valid test would be to measure in some standard amount of light, and 
then compare.

You can always slam more photons into the cell, as fast as you want...
but for them to "go away" recombination is required and this takes
place at a exponentially decaying rate.

The widest range (highest dark resistance) cells invariably are the
slowest in decay time...

If you have many cells you can try to select a "matched" pair. Or
possibly you might parallel a fixed resistor but that solution would
only be valid at one point... (naw.. don't go there...)

H^)  harry


>From: Justin Herrmann <herrmann at eecs.ukans.edu>
>To: synth-diy at node12b53.a2000.nl
>Subject: Photocell Problems
>Date: Wed, 23 Aug 2000 13:57:29 -0500 (CDT)
>
>I have two RCA 4404 photocells, and they have a 2cm  (13/16ths
>inch) diameter quartz window.  In the dark, one measures greater than 40
>megohms, but the other only measures about 10 megohms.  Is there any way
>to fix this?  I was planning to use them because of the greater
>sensitivity in Tom G's Therman project.
>
>				-Thanks-
>				-Justin Herrmann-
>

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