[sdiy] matching Vactrols

Ben Bradley ben.pi.bradley at gmail.com
Tue Apr 14 03:42:38 CEST 2020

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:

There's also these from the original manufacturer, a lot of info,
probably more than what most people need:

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
> >
> >
> >
> > The information contained in this e-mail message is privileged and confidential and is for the exclusive use of the addressee. The person who receives this message and who is not the addressee, one of his employees or an agent entitled to hand it over to the addressee, is informed that he may not use, disclose or reproduce the contents thereof, and is kindly asked to notify the sender and delete the e-mail immediately.
> >
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at synth-diy.org
> > http://synth-diy.org/mailman/listinfo/synth-diy
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at synth-diy.org
> http://synth-diy.org/mailman/listinfo/synth-diy

More information about the Synth-diy mailing list