[sdiy] Vactrol tempco was: Vactrols subbed for dual-gang pot

[Amos]

I think one solution would look something like providing your bias not
from a fixed rail voltage but via a transistor with a similar thermal
response to your vactrol.  That way the bias tracks with temperature
and should reduce the effect.

On 7/10/10, cheater cheater <cheater00 at gmail.com> wrote:
> Dave,
>
>> I'm pretty sure that the transfer (LED) or response (LDR) curves vary
>> widely
>> between different types/makes of LED *and* LDR making it hard to draw any
>> general conclusions, so specific datasheets for the exact product(s) are
>> needed I guess.....
>> With the same LDRs mentioned in the original post, regular red LEDs
>> performed the worst (in terms of response time and "off" resistance) with
>> superbright white LEDs coming out best.
>> Interestingly, different LEDs of the the same colour from different
>> sources
>> gave very widely varying results....
>>
>> All superbrights are not equal - my uncle ordered what were claimed to be
>> superbright green 10mm LEDs from the net for a marine application,
>> (navigation light)  but having seen them I then showed him a proper (small
>> 3mm) one driven from only +5V, and the response was "get that f***ing
>> thing
>> out of my eye" !.... :-)
>>
>> Efficiency seems very varied, and I would bet the transfer curves vary
>> tremendously between types.
>>
>> From what I've read/absorbed, the LED/LDR pair is not temperature stable -
>> no problem in a noise maker, but may have some problems if tuning the
>> cutoff
>> FREQ of a filter to provide a specific frequency.
>>
>
> Hmm, good point. I wonder how you temp-compensate that :) could be an
> interesting problem in creativity, imo. Definitely not your usual
> worked-out-in-a-book example problem.
>
> Cheers,
> D.
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