[sdiy] Drawbar and pedalboard

[brianw]

I designed a drawbar controller for a friend who plays various Hammond organs. I sourced physical drawbar switches from folks who salvage and service old organs. The components that I found were greasy, oily, and foul-smelling, but they worked after a little cleanup.

As others have pointed out, these are not potentiometers, per se. However, they're also not a simple array of switches that could be read like a standard keyboard of independent switches. I started the project thinking that it would be that simple, but found out differently. It turns out that each drawbar has a common pin that basically is like the wiper on a shared fader, but it steps through a fixed resistor array that's logarithmic rather than linear.

>From my notes: "Bus bar are connected by a tapered series of resistors, so the Hammond output is effectively the mix of 9 discrete faders. No modification of the drawbar hardware is needed if A/D inputs on the MPU are used instead of a button switch matrix, and the resistor divider is fed by the ADC reference voltage."

I ended up designing around the PIC18F24K50, a 28-pin chip with USB, serial (MIDI), and ADC. This chip has at least 8 ADC input pins, using an internal analog multiplexer, allowing a set of drawbars to be connected.

The firmware was fairly simple to implement: USB-MIDI plus classic MIDI on serial, along with scanning all drawbar inputs for their current voltage. Since the resistor array is not precise, I had to allow large dead zones between each value, to avoid jittery output that would flood the MIDI stream. In this case, I implemented the Hammond standard drawbar MIDI CC messages, which combine all drawbars into only 1 MIDI CC 'control' - since each individual drawbar only uses 9 values (0 through 8). Hammond decided to stack all individual drawbar controls into a single CC, with non-overlapping value ranges for each drawbar. Weird, but it works with the modern Hammond MIDI controller / synths.

Brian Willoughby
Sound Consulting

p.s. I assume that a custom-designed 13-note pedalboard would be much simpler. It would only have 13 switches for non-velocity, and 26 switches for velocity (which would require some sort of timing reference and measurement).


On Mar 26, 2025, at 6:06 AM, francesco mulassano wrote:
> Hi!
> Do you know where to buy potentiometers suitable for building drawbars?
> While we're at it, do you also know any OEM manufacturers of 13-note pedalboards and custom volume/expression pedals?
> 
> Thanks!
>