[sdiy] Hammond Aurora Keybeds

[Ben Bradley]

"Each key closes a switch to a common bus."

There's a couple things you need to make a "true analog" mono
keyboard. First is a constant current source to drive the resistor
divider, so that when more than one key is pressed the voltage remains
that of the lowest or highest key (unless you WANT a weird off-pitch
note effect when pressing more than one key). The other is  a separate
switch at every key that closes AFTER the key that gives the pitch
voltage as the key goes down, so that the proper voltage is sampled
and kept when the key is released. This is also the gate/trigger
output. Presuming there are several common busses, you need to bend
the contacts on each key to make sure these close in proper sequence.

I don't see a problem of being off-pitch with a resistive divider, as
1 percent resistors are cheap thesedays, whereas 1970s keyboards may
have used 5, 10 or even 20 percent tolerance resistors.

But still, I'd as soon make a digital circuit to scan/read all the
separate inputs, one for each key, scan it into a buffer with a
microcontroller and do the highest/lowest/latest keydown logic in
software and the usual DAC for control voltage output.

On Wed, 14 May 2025 at 22:54, drheqx via Synth-diy
<synth-diy at synth-diy.org> wrote:
>
> Hi all,
>
> I have the keybeds and draw bars etc. from a Hammond Aurora organ.
>
> These are not matrix type keybeds. Each key closes a switch to a common bus.
>
> Is this a good time for me to consider doing a volt
> /octave keyboard controller?
>
> I definitely don't want to midify these. That would be a waste.   I was considering building a voice per key synth because i love these key beds and it would be so unique, but I'm short on time like so many of us.
>
> Anyone know of a good kit to make a 1v/ oct controller, maybe with some added features.
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