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Expression pedal for modular synth
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agustindead



Joined: Jul 22, 2015
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PostPosted: Tue May 24, 2016 4:49 pm    Post subject: Expression pedal for modular synth Reply with quote  Mark this post and the followings unread

I'm thinking of building a module for my eurorack to connect an expression pedal and generate a CV output related to its position. I never designed anything and my knowledge in electronics is pretty limited, so could anyone check if this is too wrong?

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Sorry for the ugly schematic but this was my first time using Eagle as well.

Maybe I should omit the potentiometer and just use a lower value for R2 so the max output is not that hot?
But my main question is: does this design make any sense? Should I put the expression pedal controlling the op-amp gain, in series with R1 or R2? Is the op-amp necessary at all or would a resistor divider be enough to attenuate the 12v from the power supply? Is there any safety measures I'm missing?

Thanks!
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AlanP



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PostPosted: Tue May 24, 2016 7:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

Instead of a pos/neg switch, perhaps a pot going from +12V to -12V, with a buffer/voltage follower of some sort, then to the expression pedal? And a buffer between the pedal's ring terminal, and the output CV?
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agustindead



Joined: Jul 22, 2015
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PostPosted: Tue May 24, 2016 9:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

AlanP wrote:
Instead of a pos/neg switch, perhaps a pot going from +12V to -12V, with a buffer/voltage follower of some sort, then to the expression pedal? And a buffer between the pedal's ring terminal, and the output CV?

Hey thanks! I really like the idea of the port working as a sort of attenuverter. I don't really know how to wire it as to not interfere with the power supply though... would this circuit create an unnecesary load or whatever?

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Is something like this what you suggested?
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mcop



Joined: Apr 27, 2012
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PostPosted: Tue May 24, 2016 10:51 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi there, I designed an expression pedal module last year. Here's the schematic if it's of any use to you: https://drive.google.com/open?id=0B1-ujBH3ItowdUp4WktTUk9xeUU

And here's the user guide to give you some idea what it does: https://drive.google.com/open?id=0B1-ujBH3ItowSWNQSWlnY3M5T1U
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Grumble



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PostPosted: Tue May 24, 2016 11:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

@agustindead:

You could wire up IC1A the same way as IC1B.
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agustindead



Joined: Jul 22, 2015
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PostPosted: Wed May 25, 2016 11:48 am    Post subject: Reply with quote  Mark this post and the followings unread

mcop wrote:
Hi there, I designed an expression pedal module last year. Here's the schematic if it's of any use to you: https://drive.google.com/open?id=0B1-ujBH3ItowdUp4WktTUk9xeUU

And here's the user guide to give you some idea what it does: https://drive.google.com/open?id=0B1-ujBH3ItowSWNQSWlnY3M5T1U


Hey, thanks a lot for sharing! Is it much trouble explaining quickly how U1.4 and U3.2 are working? What is R21 doing? Is U3.2 just a low pass to filter any "jitter" in the pedal sweep? I like the idea of using a voltage regulator, maybe this is the tidiest way to get 9/10v out of the power supply for the pot feeding the voltage follower. Also the LED is a great implementation.

Grumble wrote:
@agustindead:

You could wire up IC1A the same way as IC1B.


I put it that way to attenuate the CV out a bit and not get too close to 12v, since that seems a bit too hot for a CV out. Should I proceed differently?
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mcop



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PostPosted: Wed May 25, 2016 2:30 pm    Post subject: Reply with quote  Mark this post and the followings unread

It's been a while since I had this going round my brain but here goes Smile

U1.4 acts as a mixer mixing the signal from the expression pedal with an offset to make the voltage swing from -5v to +5v. As the signal is now inverted U3.2 re-inverts it and mixes it with another offset signal from P2. The offset from P2 alows you to sweep the output from U3.2 between +/-5v and 0-10v.
The voltage regulator was used as different Expression pedals have different resistances and it allows you to calibrate the circuit to the pedal you use.

U3.1, U3.3 & U3.4 offer variations of the output and U1.1 & U1.2 are comparators giving gate and trigger outputs.
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mcop



Joined: Apr 27, 2012
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PostPosted: Wed May 25, 2016 2:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thinking about your mention of using the expression pedal as an attenuverter - you could possibly do this if the pedal uses the full sweep of the pot inside it. Then you could just replace the pot in an attenuverter circuit with the pedal and feed it from a jack normalised to say a 5volt reference. This would allow you to either attenuvert a signal plugged in or sweep the outpet between +/- 5 volts with nothing plugged in.
I hasten to add I haven't tried this, this is straight off the top of my head!
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agustindead



Joined: Jul 22, 2015
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PostPosted: Fri May 27, 2016 11:43 am    Post subject: Reply with quote  Mark this post and the followings unread

mcop wrote:
Thinking about your mention of using the expression pedal as an attenuverter - you could possibly do this if the pedal uses the full sweep of the pot inside it. Then you could just replace the pot in an attenuverter circuit with the pedal and feed it from a jack normalised to say a 5volt reference. This would allow you to either attenuvert a signal plugged in or sweep the outpet between +/- 5 volts with nothing plugged in.
I hasten to add I haven't tried this, this is straight off the top of my head!

Thanks for the explanation and the new ideas. I'm having a hard time wrapping my head around all this information... but it's really exciting, I can at least grasp some of it and "designing" something feels so nice...!
The idea of a CV input is pretty good! I have made some changes based on that, using a voltage regulator (I don't know if this is a bit overkill though) and correcting the ring/tip of the pedal input that I had mixed up.

Posted Image, might have been reduced in size. Click Image to view fullscreen.

This design should work as:
-a constant output, ranging from -10v to 10v, with nothing plugged into its inputs.
-an attenuverter of a CV input (with no pedal in).
-an attenuverter of a CV input with further attenuation controlled by the expression pedal.
-a CV output ranging from -10v to 10v (determined by the pot) controlled by the expression pedal.

am I right??
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