[sdiy] matching Vactrols

[Dave Brown]

I have a page on vactrols at
https://modularsynthesis.com/vactrols/vactrols.htm
It's just some measurements. Some people criticize it since I didn't use a
lot of samples. It wasn't intended as an in-depth scientific study. It's
simply a few measurements and comparisons to the datasheets which to me seem
totally worthless. Use it for whatever suits your fancy.

Dave

> -----Original Message-----
> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of Ben
> Bradley
> Sent: Monday, April 13, 2020 6:43 PM
> To: *SYNTH DIY
> Subject: Re: [sdiy] matching Vactrols
> 
> How much testing and selection you need to do definitely depends on
> your application. Regardless, it seems to me you would want to measure
> current into the LED rather than voltage across it, for several
> reasons. An online datasheet says the maximum LED current is 40mA, and
> I'd want to be measuring that if for no other reason than to make sure
> I don't go over it. Also the LED current is much closer to a linear
> relationship with the light output than the voltage, which is
> basically an exponential curve, where most of the change is in a small
> voltage range near the max voltage that gives 40mA. I'd use a resistor
> in series with the LED so that the LED current would be limited to
> 40mA at the full output voltage of the circuit driving it.
> 
> The next question might be what is the output resistance at various
> LED currents, say 0mA, 1mA and 10mA. Then there's the speed of change
> between these currents. The response is much faster to an increasing
> amount of light (LED current) than to a decreasing amount. Notice that
> the "off" resistance is after 10 seconds with zero LED current. The
> time response is taken advantage of in audio compressors, but of
> course for stereo all parameters of two units must be closely matched
> to keep a consistent stereo image.
> 
> A bit of Googling brings up much useful info. Here's a couple of
> datasheets for several parts of the original Vactrol brand:
> https://www.tube-town.net/info/datenblaetter/silicon/datasheet-vtl-
> opto.pdf
> https://datasheet.octopart.com/VTL5C4-Excelitas-Technology-datasheet-
> 21629238.pdf
> 
> There's also these from the original manufacturer, a lot of info,
> probably more than what most people need:
> http://denethor.wlu.ca/pc300/optoisolators/analogoptoisolatorintroduction.
> pdf
> https://home.roboticlab.eu/_media/kit/datasheets/ldr.pdf
> 
> On Mon, Apr 13, 2020 at 7:38 PM Crys <crystals at sonic.net> wrote:
> >
> > gee seems like a lot of work. i made a jig with clips and 2 meters (one
> > for voltage in to the LED and one to measure the resistance of the LDR.
> > and a switch for voltage in (which sorta makes the meter redundant). i
> > tested and categorized hundreds of unmarked siliconex vactrols.
> >
> > On 4/10/2020 11:03 AM, Ullrich Peter wrote:
> > > Hi Pete!
> > >
> > > To show curves from an Arduino you don't need a display, you can do it
> with the serial interface via USB and the Arduino IDE itself by using the
serial
> plotter!
> > >
> > > See here: https://www.instructables.com/id/Ultimate-Guide-to-
> Adruino-Serial-Plotter/
> > >
> > > Ciao
> > > Peter
> > >
> > > http://www.ullrich.at.tt
> > >
> > > ________________________________________
> > > Von: Synth-diy <synth-diy-bounces at synth-diy.org> im Auftrag von Pete
> Hartman <pete.hartman at gmail.com>
> > > Gesendet: Freitag, 10. April 2020 17:17
> > > An: synth-diy at synth-diy.org DIY
> > > Betreff: Re: [sdiy] THAT 2155 on sale
> > >
> > >
> > >
> > > On Fri, Apr 10, 2020 at 8:43 AM Oren Leavitt
> <obl64 at ix.netcom.com<mailto:obl64 at ix.netcom.com>> wrote:
> > >
> > > I have an idea for "quick matching" photo cells.
> > >
> > > Basically one op-amp - a comparator with hysteresis (like that of a
basic
> LFO)
> > >
> > > 1) Put the photo cell side in a resistor divider and connect to the
> comparator input.
> > >
> > > 2) Connect comparator output to to LED side thru appropriate current
> limit resistor.
> > >
> > > The circuit should self oscillate at an amplitude/frequency determined
by
> the LDR's light/dark resistance and lag time characteristics. Plug
different
> photocells into same circuit and sort them by matches in
> amplitude/frequency shown on oscilloscope.
> > >
> > > An idea..
> > >
> > >
> > > I rather like it.  I've occasionally started putting on paper ideas
for using an
> Arduino where an analog output drives the LED and steps from 0 to 5V (with
> appropriate current limiting of course) and an analog input reads from a
> voltage divider including the resistive side.  The idea being to get a
graph.  But
> then I got stuck on how to display the graph meaningfully without going
full
> blown OLED.  Considered simply sending the output to the serial port and
> letting a computer read the details.  And that really omits the time
factor,
> although I could do a 0V -> 5V step and observe the time it takes, I
guess.
> Overall, too complicated, so I never tried to implement it.
> > >
> > > I do have a bunch of Silonex Vactrols that would be nice to
characterize....
> > >
> > > Pete
> > >
> > >
> > >
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