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

[cheater cheater]

Interesting. I guess a scaling error could be taken account of similarly?

On Sat, Jul 10, 2010 at 22:51, Amos <controlvoltage at gmail.com> wrote:
> 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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