It’s been about a year since I said I’d make this, but finally got around to it today! Going to test now, fingers crossed…
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So much fun! It worked… until it didn’t. Stopped dead while playing with it, can’t resuscitate it. I think I put a feedbacking circuit into it… Changed the op-amp, no luck. Just want my new toy back. 
Could it be the LED? or the pot?
EDIT: on the walk home, I think I figured it out. The issue is mechanical on the switched jack…
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I built up the dual version in an evening and just wanted to say this thing is awesome! It’s super fun to have something other than my hands that I can use to influence sounds.
My expression pedal is a bit different than the M-Audio one, so I had to make some modifications. I didn’t know much about expression pedals going in to this, but I found ExpressionPedals.com to be helpful in figuring out the various options. I have a Source Audio Dual Expression Pedal that receives voltage on the Ring & returns it on the Tip (otherwise known as a TRS configuration), whereas the M-Audio receives voltage on the Tip & returns it on the Ring (RTS configuration). If anyone else has a TRS expression pedal, all you need to do is swap the Ring & Tip connections and you’re golden.
I also subbed in 100k trimpots for the 100k resistors in order to have the option to dial in my maximum voltage. Being that the Source Audio expression pedal uses a 50k pot, that was definitely helpful.
One cool thing about the dual version of the module is that I can run both sides separately from the dual outputs on my expression pedal. The second Source Audio expression output has its own integrated Range control that works a bit differently from the module’s Range control. With the expression pedal range control turned fully CCW, the minimum (heel down) output voltage will be half of what the module’s Range control is set for. For example, if the max expression pedal (toe down) yields +10V, then minimum (heel down) output voltage is +5V. Makes for quite an interesting foil running in tandem with the other standard channel.
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Hey y’all, this is wonderful. I have a question however: I’m an electronics noob kinda (I can solder just fine, but I have a hard time wrapping my brain about creating circuits): I have 2HP in my 1U eurorack row left, which is just enough for 2 jacks. I was thinking of squeezing a simple expression pedal to voltage output in there for some additional control.
2HP is super slim, not much room for an op amp (unless I go SMD, and with 0 knowledge of PCB design I’d rather not), could I just wire 12V to the expression pedal and to the output? Maybe with a simple voltage divider to get it down to 10V? I don’t need an input, just expression pedal to voltage 
What would be the downsides of this? Should I avoid that or can I just go ahead with the idea? I’ve attached a rough sketch:
You have the whole depth and height of your case to fill with electronics if you mount a PCB at a 90 degree angle to the front panel …
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Yes, but in the palette, right above the power and midi headers, there’s not really any room left. A small manufactured pcb with SMD parts would probably fit, a stripboard won’t.
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Hi finnglink,
There is a video online where someone does it that way, and it seems to work decently well. There is a more detailed discussion of it earlier in the thread. I believe the main takeaways were that it is do-able, but uses up a bit more current than would otherwise be necessary, and the voltage might sag depending on the impedance of what you plug it into. I would want to be very certain that you are not momentarily shorting power directly to ground at any point when inserting and removing the plug.
Stoked to see this thread still comes up sometimes.
Yo that’s a sick looking build.
Definitely add a resistor, maybe 1k, in series with the +12 V to guard against shorting to ground.
I would think impedance would not normally be a problem, that would only be if the downstream module’s input impedance were much smaller than the pedal’s and that’s pretty unlikely.
(Can’t believe I’m encouraging someone to put something in a 1U by 2 HP space. I must be going soft.)
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Thanks man, it ended up being a pretty easy verboard build. The panel is defo ugly (mounting holes needed some massaging…), but it’s just laser-cut from 2-ply acrylic, so it’s an easy fix if I want to run it again.
If you know anyone with a laser cutter (even a cheap, low power one) laser engraving materials like Gravoply or Lasermax are without a doubt the most accessible options for fast & cheap panels. I work at a industrial control panel fabricator, so it’s easy enough to throw some scraps on the bed & knock a few out.