8 knobs bank for midi control in eurorack format

HI!

I’m planning on building an Eurorack module for very basic midi control of other modules ( such as Es-9 and Hermod ), 8 knobs that send some cc messages over midi.

On very important concept of this idea is to make it as simple to build as possible ( that’s why I’m avoiding cv control, screen display for things, preset managemet etc… )

I have some initial ideas of how to do it but for sure I’m missing a big part of how to do it
Could someone point me out or help me? Would others be interested in something like that?

Thanks so much in advance!

Mockup

Format

  • Would like to be Eurorack ( so powered by normal eurorack power )
  • Size 8hp or 10hp

Features

  • 8 knobs to send cc’s via midi
  • 1 jack midi out trs type b
  • Would like to be able to easily configure which CC are mapped to every knob via usb front panel it’s good enough ( optional: web configurable )
  • (OPTIONAL) 1 button so i can switch between some programs
    ( every programa would map different cc’s to the previous knobs and buttons )

Connectivity:

  • MIDI OUT via back of module via pins ( so you can connect internally with other modules )
  • MIDI OUT on frontal ( trs type b ideally… ) ( so you can patch midi connection with other modules )
  • USB on frontal ( so you can configure cc’s mapped to knobs without unracking )

My main goal for this module

  • Control via midi some HERMOD params ( so from the type b to the midi in in hermod )
  • Control via midi ES-9 in standalone mode ( mainly for volume mixing duties via the back of the module ) , here some reference to ES-9 manual:
    Captura de pantalla 2022-05-03 a las 15.59.23

Inspiration

Not needing the CV out control, but MIdi wise that’s the inspiration

BOM / build parts

I would like to understand the needed building parts, some initial approach

  • The teensy

( via @nonverbalpoetry ) Teensy LC (Low Cost) Teensy LC should work

  • 8 pots ( which type? )

( via @nonverbalpoetry ) For pots, use these It doesn’t super matter what value for this application, but you want linear (B), single gang and I recommend 5 or 10k. There are lots of Arduino tutorials for how to wire a potentiometer to a development board about.

  • 8 knobs
  • Resistors?
  • A 8hp blank panel ( frontal )
  • A panel to mount the teensy on / a strip board will do
  • Some separators for the panel

( via @nonverbalpoetry ) You won’t need any seperators for the panel because the jack and all the knobs will connect the front panel to the PCB/strip board. This is how many eurorack panels work.

  • Something for regulating power coming from eurorack to teensy?

(via @nonverbalpoetry ) For power step down, use a 12v to 5v converter.
Also a good idea to use a couple of ceramic capacitors to smooth the power (this is what the i2c2midi does so see the schematic on the i2c2midi GitHub for how to wire that up)

  • Something to connect the jack on the frontal for the midi type be ( the connector and the thing holding the connector to the front panel )

( via @nonverbalpoetry ) For the midi jack socket, use one of these green Thonkiconn Stereo jacks. you will also need resistors between the jack and the teensy, see again the i2c2midi schematic for how this is wired.

  • System for Exposing USB connection on the front

( via @nonverbalpoetry ) The simplest way is to use a panel mount usb cable plugged into the teensy and fixed behind a hole in the front panel.

Warning regading using usb for midi while module is connected to power ( i2c2midi/BUILD-GUIDE.md at main · attowatt/i2c2midi · GitHub )

Coding references / Libraries / etc…

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You seem to have a fairly long, but fun, journey ahead of you (asking if you need resistors hints at where you are starting from): good luck! One comment from my side: if I were making such a device I’d probably put a display in it (maybe even 2 lines of text only) to allow you to configure it directly. Configuring via an app/browser seems to add a lot to the overall complexity.

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Someone more knowledgeable than me can maybe confirm, but you might be able to run this directly off of a 5V rail if your PSU supplies one

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That would be great, and it’s one of my main concerns.
If powering this from the rail it’s easy, i think its nearly half the battle.
Someone know how?

For sure it’d be usefull!
but in this case i want to keep it as simple as possible
I thught the teensy and the teensy midi libraries etc… can help you on making this remote configuration easier? If not at some point I’m ok with configuring over usb flashing whatever firmware, etc…

Take look at the schematic on the i2c2midi GitHub, it should get you started for implementing a midi out jack on a teensy, as well as powering a teensy via eurorack.
I would strongly suggest adding some CV ins too eventuly. All you’d need to do that is a voltage divider and some clamping diodes.

Edit to add some answers to your questions:

I suggest using a teensy LC, still readily available and plently powerful enough for this application.

You will need to decide wether you want to design a PCB and have it fabricated or if you will make this with stripboard (it’s simple enough that it’s definitely doable with stripboard)

For power step down, use a 12v to 5v converter.
Also a good idea to use a couple of ceramic capacitors to smooth the power (this is what the i2c2midi does so see the schematic on the i2c2midi GitHub for how to wire that up)

For pots, use these It doesn’t super matter what value for this application, but you want linear (B), single gang and I recommend 5 or 10k. There are lots of Arduino tutorials for how to wire a potentiometer to a development board about.

For the midi jack socket, use one of these green Thonkiconn Stereo jacks. you will also need resistors between the jack and the teensy, see again the i2c2midi schematic for how this is wired.

You won’t need any seperators for the panel because the jack and all the knobs will connect the front panel to the PCB/strip board. This is how many eurorack panels work.

You will need the usb exposed on the front panel if you want to be able to hook it up to a computer and co figure the ccs without any unracking. You will also want some kind of visual feedback to indicate what preset you are in, otherwise you are just pressing the preset change button without having a reference for what preset you are in. I suggest some kind of binary display with 4 LEDs like radio music, or an RGB led with each different colour relating to a different preset.

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You can get a voltage regulator that provides 5V from the 12V supply. The data sheet for the regulator typically includes an example schematic of how to use it. It’s usually very simple.

As for configuring the device: programming the microcontroller in circuit isn’t very convenient as a way to reconfigure unless you are absolutely sure you won’t need to change anything. If you want it to be configurable via a web browser you need to think about how to provide a network end point and a web server. This design has a lot more moving parts (meaning things that need to go right for it to work) than having a display. There are off the shelf solutions to do this, but it doesn’t seem like it makes the overall design easier. Moreover, you’ll need to figure out the details of making hardware either way.

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Befaco VCMC has 8 faders. And CC programing is easy to use. I made it in 16hp. You could use befaco’s schema without cv function. Better than from scratch, I think.

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I had been thinking of something like it’s a while back but once the 16n came around I kind of abandoned it.
Freeing up valuable HP and having something that speaks CV/Midi(usb+3.5mm)/i2c has been huge for performance and playability to me.

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Thats incredible soldering and assembling skills!
I will give it a thought though, I thought VCMC was much complex because the screen, the cv inputs controlling the faders, etc… but certaintly could be a good starting point.

Thanks! Updating the initial post with your references!
And also for the sake of mega simplicity will get rid of the “preset change” feature.

Regarding the “usb exposing” do you know how to do it?

The simplest way is to use a panel mount usb cable plugged into the teensy and fixed behind a hole in the front panel

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if you need more inspiration, check this work too ?

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