[sdiy] Linearized Vactrol?

harrybissell at wowway.com harrybissell at wowway.com
Wed Mar 12 20:07:59 CET 2008

Hi Tom

Sounds like an interesating circuit. A technique for 'linearizing'
a photocell exists in National Semiconductor AN-20 "analog multiplier"

It needs matched cells as well, and is shown with an incandescent lamp
which really limits the response at very high resistances.

The best deal would be to use a vactrol that is center-tapped so that the
cell is at the same temperature, and reasonably well matched. Some
vactrols like the ones used in the Mutron III envelope follower have
two independant elements, this might be a good choice if available.

Vactrols are useful for many things at s-diy, even if they are slow. They
are reasonable quiet, have no voltage offset etc.

I'd like to see the spice model you have, btw.

If your circuit is really novel, submit it to EDN magazine as a 'design idea'
or Electronic Design magazine and get paid maybe $150 for it, and be famous.
It usually takes about 6 months for publishing. They will not take it if it
has been published (ir public domain) already...

H^) harry

On Wed, 12 Mar 2008 12:27:44 -0500, Thomas P. Gootee wrote
> Hello, everyone.
> I am new to this list.  My name is Tom Gootee.  I live in
> southwestern Indiana, in the USA.
> I was an EE, originally, until 1984, then did mostly
> C-language software for about 15 years, and have been
> designing (and manufacturing & selling) analog electronic
> systems for the last ten years or so (mostly
> test-and-measurement equipment).
> I hope that my question is not perceived as too unusual, or
> inappropriate, for this group. If so, I apologize.  But I
> don't know of anyplace else where Vactrols are discussed as
> frequently, or by people whom I respect as much as the
> members of this group.
> -----
> I am wondering whether or not there might be any interest in
> a wide-range, 'perfectly'-linearized voltage-controlled
> Vactrol-type device (i.e. a wide-range linear
> voltage-controlled resistance).
> I have developed a circuit that does that, which would also
> work for any other LED/LDR type of device (e.g. Silonex, et
> al).  I'm just not sure what to do with it.  i.e.  Would it
> be worth trying to commercialize it?  Would there be enough
> interest?  Would it be foolish of me to simply 'donate' the
> circuit into the public domain?
> I originally wanted to develop it for use in some of my
> (non-audio) product designs.  And I also have audio hobbies,
> and wanted a series/shunt LED/LDR-based audio attenuator
> with precisely-constant input impedance.
> But, for many applications, the vactrol-type devices are way
> too slow.  So the applicability of a linearized version
> seems somewhat limited.
> Also, I think that there are probably other,
> somewhat-similar circuits already available, having seen the
> (fairly horrible) attempt at it that's published on the
> Silonex website.  So it's probably not unique, or even new. 
> However, my circuit HAS been structured for optimization of
> things like maximum resistance-command step-response slew
> rate, with near-zero overshoot, maximally-wide resistance
> range for a given LED/LDR device, high precision in general,
> etc etc, yada yada yada.
> But, since it relies on an identical LED/LDR device for its
> control mechanism, any mis-matches (other than a constant
> resistance offset) between the target LDR device and the
> control LDR device will negatively affect its accuracy.
> In other words, I guess I'm also thinking that maybe it's
> really just not all that great, and I might be worrying
> about all of this for no good reason.
> So I'm hoping that someone here will be kind-enough to give
> their opinion about whether or not it's worth pursuing
> further, in any way at all.
> I'm sorry to have 'blathered-on', for so long, about all of
> that.
> [P.S.  If anyone wants a fairly-detailed LTspice model of
> the Perkin-Elmer/Vactec VTL5C2 Vactrol, or a model of the
> Silonex NSL-32SR2, just let me know.]
> Thank you.
> Regards,
> Tom
> Tom Gootee
> tomg at fullnet.com
> http://www.fullnet.com/~tomg/index.html
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Harry Bissell & Nora Abdullah 4eva

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