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I am sorry but: Swapping pots for cv
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author I am sorry but: Swapping pots for cv
I know know know this has been asked.... But couldn't get the search right. I have a pedal manufacturer Friend who is open to me trying to convert one or more of his products to Eurorack, the power issues are clear to me as are input and output but what I don't know is how to replace it's with CV? Will a simple transistor
Work? Op amp? How about normalization? I'd like the pots to work as they do on the pedal when there is no cv and to work as atenuators of the cv when it is patched...

Too many questions? I'm pre-shamed for the "advise on how to do everything" quality of this post...

It all really depends on how the potentiometers are used. As variable resistors or as voltage dividers. As the words tell you, it will be easier when they're used as a voltage divider.
lessavyfav wrote:

How about normalization? I'd like the pots to work as they do on the pedal when there is no cv and to work as atenuators of the cv when it is patched...

Once you sort out how to control the parameter with external voltage, you can normal a DC power supply to the CV input jack (use a jack with a switching contact.) The pot will attenuate the power supply and act like a normal manual control. When a jack is inserted, the normalization will be broken and the pot will attenuate the external supply.
Make Vactrols your friend. Here is a link to a page that has links to many others with good ideas and specs -

Basically you may be able to replace the pots with the vactrol which acts like a pot - a photo resistor and LED in a small container. Feed the CV to the LED and the LED lights up and the resistance changed. There are 4-leg versions (2 for the LED and 2 for the photo resistor) and 5 leg versions (3 for the photo resistor). You can also buy a Doepfer A-101-9 Universal Vactrol Module to test things out with.
There are also some DC controlled potentiometer ICs out there like this one - - do a search on voltage controlled potentiometers and you will get some useful hits. These are cool but not as simple as a vactrol.

Vactrols ot H11F1 FET optoisolators.
Thanks for the info, also I finally realized that a Vactrol is an optocoupler. Somehow I had come to think they were something more "magical"

I'll find out about the resistor Vs voltage divider (divider I'm betting) and proceed from there. If it is a divider do I just insert a switching cv input in-between the voltage source and the pot? That seems reasonable ;-)

Doctorvague just gave you the gold mine there. The dual vactrol part will repace almost any single pot in a circuit. Look at what the pot's outer taps are connected to and it will determine if you need the dual version. The duals are a little more rare and of course more expensive.
Bumping this because it was a crazy interwebz hunt to find the JPEG posted above. I've been looking for this info as it is referenced a lot here and at e-m, but the squarewaveparade host site is dead. I figure that there are likely some other newbie DIYers like me who want to do just this.

Also, I found a grab bag of LDRs in a box of DIY crap I inherited. Woo!
Dr. Sketch-n-Etch
Actually, I just followed the link in the OP and found all the schemos (which I copied and saved to my HD thumbs up )
Oh, it's back up? Good, hopefully the Teaspoon is back on track!
Very timely bump bleeps, just what I was looking for smile
Its really that simple?
Just add that to pretty much any resistor pot that controls something youd like to have CV control on?
Sweet smile
One thing I think you have to keep in mind, or tell me I'm wrong. Certainly a vactrol will respond to fine at speeds a human would have turned a pot, but when you go with CV inputs that also leads to people expecting inputs will respond at audio rates. I know some vactrol designs are fast but will they do changes in the Khz range?
also that you cant completely set a vactrol to 0 resistance so depending on the circuit attached it might not completely close or open
OOPS. Mis-post. help
Hi there! Somewhat a beginner in electronics.

But i ended up here - which is the right place. I basically want to do this mod;

I was wondering if i could make something similar using vactrols? And what would be the most logical of the circuits mentioned?
FWIW, I did follow the ideas in this thread to successfully add voltage control to a few pots.

Some things I discovered along the way in case it helps someone:
  • Coolaudio clones of the VTL5Cx series vactrols are now easily available (SmallBear, & eBay). Still a bit expensive.
  • VTL5C1 is excellent. It is faster and gets to a lower resistance than most vactrols. Still slow enough to be pinged, and of course still no audiorate modulation.
  • Opto FETs (H11F1 series) are more predictable/linear, get down to MUCH lower resistance, are cheaper and are MUCH faster than vactrols. They also draw less current for normal operation. Only downside is that the nice linearity gives way quickly for certain ranges of current, leading to noisy CV modulation in that range.
  • Also messed around with an Arduino interpreting MIDI/CV to control digital pots (MCP4261). This is nice because of the precise control and non-volatility, but the response is stepped. You also lose the isolation aspect of opto-coupling and need to shape your signal to be within 0..5v. Don't recommend for the above purpose.
  • To replace switches rather than pots, an analog switch (DG412) is a nice option. The DG412 has separate analog and digital voltages and can be powered by usual +/-12-15v, +5v modular supplies. MAX335 is a serially-controlled version of the same thing, so many of them can be controlled by an Arduino. I'm building a MIDI-controlled patchbay using this right now.
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