# Prototyping DIY Expression Pedal interface for Kosmo
**URL:** https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649
**Category:** KOSMO MODULES
**Created:** [April 17, 2020, 1:18am UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649 "2020-04-17T01:18:51Z")
**Posts on this page:** 20
**Page:** 2
### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [May 22, 2020, 8:23pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/24 "2020-05-22T20:23:10Z")
Wow, this is awesome! A clean looking build, too. I would love to see it in action. If you make a sick jam with it, please share! I am trying to finish off a couple of half done Music From Outer Space modules and then I plan on building one myself.
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 3, 2020, 6:27pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/25 "2020-06-03T18:27:20Z")
Panel’s done!
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### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [June 3, 2020, 8:36pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/26 "2020-06-03T20:36:00Z")
Beautiful. Does the final version still use the same schematic that you posted on 5/3?
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 3, 2020, 8:38pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/27 "2020-06-03T20:38:15Z")
yes, it does…
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### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [June 6, 2020, 7:06am UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/28 "2020-06-06T07:06:08Z")
I just breadboarded it - I might need to nudge the values around because I have a different pedal but it seems to work great. Well done - can’t wait to get it soldered and racked up.
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 6, 2020, 11:38am UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/29 "2020-06-06T11:38:37Z")
I made a Github repository:
> **[holmesrichards/exped](https://github.com/holmesrichards/exped)**
>
> Expression pedal interface synth module. Contribute to holmesrichards/exped development by creating an account on GitHub.
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### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [June 6, 2020, 5:06pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/30 "2020-06-06T17:06:11Z")
Nice. I put together a stripboard layout last night. I’ll solder it up this weekend and post once it’s confirmed working.
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 6, 2020, 5:44pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/31 "2020-06-06T17:44:44Z")
Video
[ ](https://www.youtube.com/watch?v=kQo8faIRnnQ)
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### Author:  [@devicex](https://lookmumnocomputer.discourse.group/u/devicex)
#### Post date: [June 6, 2020, 6:45pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/32 "2020-06-06T18:45:29Z")
it was interesting getting the back story on the build .
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### Author:  [@CTorp](https://lookmumnocomputer.discourse.group/u/CTorp)
#### Post date: [June 6, 2020, 9:38pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/33 "2020-06-06T21:38:50Z")
5 seconds in and I spit my beer
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 7, 2020, 11:03am UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/34 "2020-06-07T11:03:03Z")
> [@DIY Power Supply Thread - Questions and Help](https://lookmumnocomputer.discourse.group/t/diy-power-supply-thread-questions-and-help/117/39):
>
> The TL072 rails are backwards, right? (unless you’re using red for −12 V in case your color coding is backwards, and your caps too 😀).
Yes, red must be +12V since that’s what’s connected to the tip switches. But then the power connections on the TL072s are reversed.
(I’m surprised you didn’t use a TL074, but I suppose you had reasons.)
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### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [June 7, 2020, 9:07pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/35 "2020-06-07T21:07:31Z")
Yep, I threw out the old board and started a new layout, this time using a TL074, which made for a cleaner build and works much better. I want to post my layout, but I wanted to seek opinions on one small modification:
When there is no expression pedal plugged in, the input to one of the op-amp stages is floating, which seems to cause the corresponding output to behave erratically. I am considering taking the expression pedal jack’s switching ring pin and connecting it to the sleeve (ground) so that when no expression pedal is plugged in, the op-amp input will be grounded and the output will be locked in at 0 volts. My hope here is to prevent stray control voltages from going around if no expression pedal is present, but a cable is connected to the output. Does that seems sensible, or am I off in the weeds? I hardly trust myself any more.
I’m also finding that for some reason I still have about 0.3 volts output when my pedal is rolled all the way back, which I’m hard pressed to account for.
I used a 15k resistor on the LED to make it dimmer and to make (in my case) a smoother gradient from off to on as my pedal rolls forward. I also went with a 100k resistor from 12v to the CV input because I wanted to decrease the overall range. Your mileage may vary.
Once I am 100% verified on this layout I will post the TL074 dual pedal version.
Best,
-Wes
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 7, 2020, 9:14pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/36 "2020-06-07T21:14:09Z")
Good catch on the floating op amp input. Your fix sounds reasonable to me but I’m far less qualified to say than some others around here.
> [@WesleyV](#):
>
> I’m also finding that for some reason I still have about 0.3 volts output when my pedal is rolled all the way back, which I’m hard pressed to account for.
I was puzzled by the same kind of thing until I remembered my pedal has a low limit pot and indeed it wasn’t at minimum. Having turned that down I see about 6 mv on the output with the pedal back, consistent I guess with the TL072’s offset voltage. If yours doesn’t have a low limit pot or if it does but you have it turned all the way down and you’re seeing 0.3V, that seems wonky to me.
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### Author:  [@antoine.pasde2](https://lookmumnocomputer.discourse.group/u/antoine.pasde2)
#### Post date: [June 7, 2020, 9:42pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/37 "2020-06-07T21:42:34Z")
> [@WesleyV](#):
>
> I am considering taking the expression pedal jack’s switching ring pin and connecting it to the sleeve (ground) so that when no expression pedal is plugged in, the op-amp input will be grounded and the output will be locked in at 0 volts.
That should work, but if you connect the ring switched connection (RN) to the output of the first opamp, you could use this module as a 0 to 12V voltage source when nothing is connected to its inputs.
> [@WesleyV](#):
>
> I’m also finding that for some reason I still have about 0.3 volts output when my pedal is rolled all the way back, which I’m hard pressed to account for.
Do expression pedals really use the full range of their potentiometer? Does it go to 0 ohm between Ring and Sleeve?
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 7, 2020, 9:47pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/38 "2020-06-07T21:47:43Z")
> [@antoine.pasde2](#):
>
> That should work, but if you connect the ring switched connection (RN) to the output of the first opamp, you could use this module as a 0 to 12V voltage source when nothing is connected to its inputs.
👍
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### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [June 7, 2020, 9:57pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/39 "2020-06-07T21:57:12Z")
Antoine,
I think it most cases it does not, which I suppose accounts for the bit of extra voltage at the output.
Good suggestion on turning the module into a source of DC voltage. Since the output of the first op amp is already connected to the tip of the Expression jack, I suppose you can just wire together the switching tip and switching ring pins and bam - a handy little DC voltage adjustable from the range pot. I’ll give it a shot.
Edit: Was able to get rid of the extra 0.3 volts by fiddling with the switches and trimmers on my pedal.
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### Author:  [@fredrik](https://lookmumnocomputer.discourse.group/u/fredrik)
#### Post date: [June 7, 2020, 10:16pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/40 "2020-06-07T22:16:43Z")
> [@WesleyV](#):
>
> I also went with a 100k resistor from 12v to the CV input
My comment in the other thread was about using one 100k for both CVs, which makes them depend on each other in more or less subtle ways.
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### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [June 7, 2020, 10:30pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/41 "2020-06-07T22:30:14Z")
Fredrik - I was concerned that might happen, but I thought I’d save a resistor. My inexperience is showing on that one.
On a lighter note, I revamped my layout and confirmed that it works beautifully, and Antoine’s suggestion works like a charm. Tames the floating output and adds extra utility to the module. I love it. Here is the layout for a dual version, which I have working happily on my bench:
Edit: Here is a link to the 3D printable cosmo panel. The description has a link to this thread and Analog Output’s github page (it’s showing an old draft of the text at the moment, it takes the site a while to show updates):
> **[Dual Expression Pedal Interface for Kosmo Format Modular Synth by wvanderveen](https://www.thingiverse.com/thing:4436055)**
>
> This is a panel for a dual expression pedal interface for an analog modular synth.
> Note: the knob that I labeled as "bias" would be more accurately labeled as "range." You can decorate your panel any way you like.
> This panel is in the Kosmo format,...
It didn’t occur to me before but the mod Antoine suggested also allows the module to serve as a good ole fashioned attenuator for incoming CV when the expression pedal is absent. This thing has a lot of utility. I’d be shocked if more people don’t build this.
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### Author:  [@analogoutput](https://lookmumnocomputer.discourse.group/u/analogoutput)
#### Post date: [June 7, 2020, 11:06pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/42 "2020-06-07T23:06:56Z")
Yep, I just tested it myself. It’s a pedal interface, it’s a voltage source, it’s a CV attenuator, it’s a dessert topping. Thanks for pointing out the problem and thanks to @antoine.pasde2 for a fix that was more than a fix! I’ll go update the GitHub repository now (and I’ll throw in a link to the above dual pedal stripboard layout).
One note on that layout: I don’t see where the values of the pots are shown. 100k? Seems like you should get less than 6V max out if so…
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### Author:  [@WesleyV](https://lookmumnocomputer.discourse.group/u/WesleyV)
#### Post date: [June 7, 2020, 11:09pm UTC](https://lookmumnocomputer.discourse.group/t/prototyping-diy-expression-pedal-interface-for-kosmo/649/43 "2020-06-07T23:09:31Z")
This seems like a total win. It’s simple as hell and it does so much. I can’t wait to see if it catches on (and I can’t wait for my second pedal to arrive). Analog Output, do you mind if I upload my own demo to youtube? I’ll of course give credit for the schematic and link to your github.
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