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Drawbar controller

edited July 2013 in Hardware


in my Hammond MIDI controller I'll use a Drawbar set that behave like standard faders but its output is not linear. I just measured it and it looks more like an inverse logarithmic scale.
Is there a way to configure BrainJr so that it reads the values from the drawbars and outputs CC in a linear way or have I to alter the drawbars (I think installing some resistors) so that the output is more linear?


  • Great!

  • Hallo,

    I'm back with some issues on my drawbars controller...

    First of all: I'm using a Braing V.1 not Brain Jr nor Brain 2.
    Tested with some buttons and some "standard" potentiometers and all is functioning as axpected.


    The drawbar set I have have some sort of "resistors" built in (it looks like a path with the conductive lane narrowing from the start to the end, see the picture)

    If I hook up the drawbars as indicated in the following picture I would expect that the whole things behave like a potentiometer and so getting some values when I move the drawbar.


    On the contray all I get is a bunch of random CC value and suddenly the blue led on the Brain Board turn off. If I disconnect the ground or the power line I got another bunch of random CC values then the blue let oin the Brain turn on.

    What's wrong? 

    Could be that the "narrowing" path that I identified as a resistor isn't a resistor at all, but in this case I'm simply connecting the Ground with the Power... isn't this intended as simply "grounding" the analog input? why the Brain "turns off"?

    Thanks for your help
  • If you connect the voltage and ground then the brain will "ground out" and the blue light will go out. To properly ground a single analog channel you want to connect the signal to the ground.

  • Thank you Mark for the explanation. it's then evident that I have to put resistors between each bussbar so it can pass from 5v to 0 in the 9 steps..

    Can you suggest a suitable value for those resistors? I have some 10k ohm, could it be ok?

  • Im not exactly clear on what you are trying to do. Can you share an explanation and a schematic?

  • edited June 2013

    Sure Mark, I'll wrote some schematics tonight and I'll share with you!

    Other than a schematics about what I would like to achieve, do you need an explanation about what a standard drawbars set is and how it functions or can I skip it?

  • An explanation would be helpful :)

  • Ok Mark, so here we are,

    My Plan:
    What I want to do is to MIDI-ize an Hammond Drawbar set, so when I push or pull a drawbar, it sends CC (via the Brain) to my VST

    What are Hammond Drawbars?
    Hammond drawbars are used to balance the harmonic content of the organ sound: the more you pull a drawbar out, the more the corrisponding harmonic sound is loud.
    They are not continuous they have only 8 step each.
    (see just few seconds of this video and it should be clear:

    How does Hammond Drawbars are made and how they functions?
    First of all, please look at the following pictures. Here you see a Drabar set from the bottom:

    As you can see there are 9 "bussbar", when you push or pull the Drawbar at each step it makes contact with a different bussbar (note the green line i drew where shold be the drawbar "signal" wire).
    Since each bussbar have a different voltage, at each step the Drawbar "sends" it via the green line I drew.

    A very clear and concise explanation (wrote in proper English, not mine ;-) ) with a nice animation is here:

    Mark... Mark... are you still here? :-D 

    What I would like to do
    What I would like to do was already done by others using different devices than the Brain, but I think the principle is valid.

    The goal is to turn the drawbar set in a sort of potentiometer, connecting the 1st bussbar to the voltage, the last bussbar to the groung and soldering beetween each bussbar a resistor so that at each step (each bussbar) we have a decreasing voltage.The drawbars then are connected (the green line in the picture) to the analog Input 3rd, 4th ... pin like it were the wiper pin of a potentiometer.

    The schematics is the following:

    it's borrowed from the MIDI gadget user manual.


    What is the value of the resistors I have to put between the bussbar?
    (that presumes another question: do you thik it can work?)

    Thank you for your patience and help!
    Hope to hear you soon

  • Mark... Mark... are you still here? :-D

    Thank you for the clear explanation. Im with you now!

    I think this will work.

    I would have a hard time recommending an exact resistor value without some research.

    I think 10k is a good place to start. I would connect the resistors, then take a multimeter to it.

    Very cool project, keep us posted :)

  • Thank you! In the WE I'll do some test then I'll share my results ;-)

    Thank you again!


  • edited July 2013

    During the Weekend I conducted some tests.

    It looks like I've got some good results, thanks to your input!

    Before soldering the resistors I re-created something similar on the Breadboard:
    Basically the vertical breadboard lanes where the resistors legs are simulates the drawbar bussbar, and the Yellow cable behave as it would be the drowbar in itslef. When I move the yellow clip from a resistor to the following I can simulate the drawbar push or pull.

    Initially I connected the flat cable from the Brain with crocodile clips but although the Brain reads the correct values on the resistors legs, it keep getting a stream of values "around" the correct one. 

    So I thought there was too much "noise", due maybe to the weak connections by the clips etc so I switched to a more direct wiring:

    The violet cable would be the "leg" of the drawbar and now when I plug it on the lane of the various resistors legs (would be the Drawbar set's bussbar) I get the correct and stable values!


    As expected I get spikes of values when I disconnect the violet cable to move it to another position, but as soon I re-connect it in the new position the value just returns to be ok.

    Now I'll move the whole things to the first drawbars set hoping to get the same steady values... but i fear that the noises will be much higher.

    I'll let you know.

    Thank you!
  • Great progress! I am excited to see how this turns out :)

  • Hallo Mark,

    here I am again ;-)

    I had great progress but still need your help ...

    I soldered the resistors to the drawbar set and connected it.
    It almost work, it sends the CC I mapped in the Brain Configure, but I had a lot of spikes in the values.

    I tried adding a 0.1uF capacitor between the Voltage and the Ground and the results were decent: when I connect only 2 or 3 Drawbar the valueas are pretty stable, but some "dance" restart when I start connecting 5, 6.. 8 drawbars.
    It's not unusable but not as stable as I would.

    So I'm wondering (and asking) if there is something I can do to improve my wiring:
    - As I said I had a 0.1uF capacitor between the Ground and the Voltage. Is it OK?
    - If is ok the former question, should I change the value of the capacitor?
    - Elsewere on this forum I read the suggestion of putting the capacitor between the Ground and the wiper (in my case the wiper is the drawbar in itself)... could be a solution?
    - What other suggestion do you have?

    Thanks as always for your great help!


    I'm sorry that this thread, that's all about Drwabars MIDI controller, is under a very unrelated title. My Fault!!
    Is there a way to change the title so that is more explicit for other people?
  • I think if you edit your first post, you can change the title. Thats how I did it.

    You want to use a 10uF capacitor between ground and voltage.

    You also want a .01uF capacitor between each ground and signal.

  • Thank you Mark! I'll try your suggestions and report the results!

    Btw, this is one of the best support forum I ever seen!


  • :))) Thank you!

  • Mark,

    sorry to bother you
    I'm not shure I'm following you...
    are you suggesting a circuit like the following? :

    - I only found 01uF polyester capacitor, not polarized; Are they ok?
    - I only found 10uF electrolitic capacitor, polarized. Are they ok?


  • In your circuit drawing, C4 looks good.

    The other connections i'm not too sure about. What do you mean by having the capacitor going into the resistor?

    I think you should just have the .01uF caps going between each signal and ground.

    These are the .01uF caps that we use:

  • Oh no Mark, probably I used a wrong symbol. The symbol of the resistor with the arrow pointing to it would be a potentiometer, with the arrow being the wipe, the signal.

    So a more direct question could be: for "signal" do you mean the wipe contact in a pot? (would be the single drawbar in my case) ?

  • So a more direct question could be: for "signal" do you mean the wipe contact in a pot? (would be the single drawbar in my case) ?

    Yes, exactly.

    I also noticed that you signal lines seem to intersect and connect with the GND. That would not be good.

  • Mark your help is invaluable!

    I think I understood... 
  • I can't seem to get that link to work. Send me another and i'll take a look!

  • Looks like the link was just missed typed, so when clicking on it you don't get to the page, try copying and pasting it.

    P.S. I'm just following this interesting project a bit

  • edited July 2013

    Ops, sorry for the link!

    Do look ok to you my drawing?

    I'll be away for the next few days (hiking on the Alps!) so you too can relax a bit ;-)


  • Yes the drawing looks good now!

    Wow enjoy the Alps! Post a snapshot on here when you get back :)

  • Hallo Mark!

    I'm back from the Alps and.... I've got my Drawbars working!!!
    It's a little step for the mankind but a GIANT step for me !! LOL

    To have a nice reading of the analog values I had to put capacitors as you suggested but now it pretty ok.

    Here you can see a short video with the drawbar set attached to the Brain and controlling the VB3 vst:
    (the background music is not mine)

    I've some more pictures that I'll post in the next few days.

    Mark thank you very very much for your invaluable help. I really wouldn't be able to make this without your suggestions! Thank you again!


    BTW There's still a lot of work to do (potentiometers, switches, the organ "case" etc) so... don't relax yourself too much: I'll be here with new questions in a few hours ;-)

  • LOL keep em coming!

    The video you linked to is set to private FYI.

  • now the video it's avaiable ;-)

  • Very coool!

  • wow that looks awesome!

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