Digitally Attenuating the Minimoog Mod Wheel circuit

[Calphool]

Hello,

I've been building a Minimoog clone for the past 18 months or so, and it's been floating around 98% done for about a year... lol... I want to finish it completely, but I need a little help.

I decided to get rid of the left hand controller and control everything in it digitally (glide switch, decay switch, pitch bend, and mod wheel). I decided to do that because I don't have an actual keyboard, and I built a Midi2CV board (Midibox STM32) to interface to the keyboard.

Anyway, long story short, I've got everything moved out of the left hand controller and working except one thing: the mod wheel pot. Please see this schematic:

http://2078578442551681271-a-1802744773 ... wiring.gif


The whole site is at: http://sites.google.com/site/minimoogwiki/

In particular I'm looking at the relationships between the Oscillator board (which both produces the mod signal via oscillator 3 and also consumes the signal via the mod input line). As you can see, the mod circuit takes a very 'circuitous' route from the oscillator through the power supply board (which contains an isolated mixer amplifier to mix noise and oscillator signal), through the left hand controller, up through a switch (SW1), and finally back into the oscillator.

What I want to be able to do is replace the potentiometer in the left hand controller with something I can digitally control. It _appears_ that the pot is only there to attenuate the signal by driving it toward ground as the pot is opened up. Does anybody have any ideas how I might accomplish this attenuation with a digital device connected to my microprocessor?

Thanks!

Joe R. (Calphool)