[sdiy] Two parameter mechanical controller thoughts....

Pete Hartman pete.hartman at gmail.com
Fri Jun 7 08:13:40 CEST 2019

At the risk of depriving myself of a market advantage....

If you haven't seen videos of the Collidoscope, go to youtube and watch now.

The control mechanism -- encoder sliding in a slot on some kind of carriage
that presses down on a softpot underneath -- strikes me as a really
appealing and expressive means of controlling two parameters
simultaneously. However, they don't provide a lot of mechanical detail at
the micro level (that I've been able to find... they call out the encoders
and the wiper for the softpot etc, but not the carriage mechanism).

I've been thinking about how this could be accomplished as just a CV
generator -- one CV that reflects the position in the slot, and one that
reflects the position of the encoder. Would be good to use an LED shaft
encoder as well, so you could light the LED with a brightness that reflects
the level of the encoder, so you can look at a glance and see what both CV
values are, roughly.

The tough problem to solve here, of course, is the mechanical carriage. I
need 4 signals and ground to deal with the encoder and an LED. Maybe only 3
signals, but I'm going to go with 4 cos it's symmetrical. Don't want to use
a set of wires rolling back and forth for the signals because they will
wear out and break off.

My first thought is something to somewhat mimic a slide pot. PCB that holds
the encoder, has a hole for the wiper screw. Glue a nut to the PCB on the
bottom side, screw the wiper through it and adjust for the height to the
soft pot on a PCB underneath.

PCB has 4 spring contacts (two on each side of the encoder) that ride on
bare traces on the PCB that comprises the panel These are on the "top" of
the carriage PCB, pressing up against the panel. These are the 4 signals.

PCB rides on a pair of rails that are made to be soldered into the panel
PCB, tied to ground, and the PCB has a pair of bare traces that connect
into its ground. So the rails carry the ground to the carriage PCB.

SO here is where my ignorance of what's out there takes over... is there
anything like this rail "out there" that I could buy off the shelf? What
about the spring contacts? I'm concerned about the rigidity of the rails --
pressing down on the encoder while it's in the middle of travel shouldn't
be able to deform the rails. What about twisting of the carrier PCB? I
don't want it to rotate off the rails. Presumably if the encoder has a
reasonably loose rotational resistance it won't be a problem, but I don't
think that's a good thing to assume. I have a bunch of copper wire of
various thicknesses, and my initial thoughts about this were I could
prototype using that as springs and rails, but I definitely don't think
copper has the necessary rigidity for a 100mm long throw in the slot, which
is what I'd want to have.

Is there some other approach that would work better/more reliably? Are
there pre-made slide pots that have encoders built into their shafts? Am I
missing some other prior art on how this could be accomplished?

Obviously I'm inspired by the Collidoscope; it's not a particularly novel
mechanism. But it's not something that "exists" in a form generally usable
for synths either.....  At least not that I've seen.



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