Softpot Ribbon Controller

[Scott Stites]

[Luka]

i was going to get a stribe kit a while back but the price knocked me out. i built the klee instead and had a couple of hundred left ontop for other things.

it would be great if they can make it so you dont need maxmsp/reaktor to run it.

[Scott Stites]

Well, if my ribbon controller idea pans out, it wouldn't be such a leap to put a bunch of them on a unit and have something that could control a number of different voices, though that leads back to "how many modules you got?". In fact, Germaniac mentioned something exactly along those lines a few days ago. My cart is before the horse, though.

[numbernone]

Oh man this is good news!!!

I have dicked around with ribbon controllers in my mind and less so in plastic reality for the entire 2 years DIYing that I have so far indulged in. The position memory being key,and probably more complicated than the sample and hold simplicity I have always imagined.

[Scott Stites]

Well, it probably is exactly the same S&H simplicity. The Moog ribbon controller was spec'ed on the schematic to have a droop of better than 10 mV/minute, which worked well enough for Keith Emerson, I guess. In any event, it's going to have to be a very good S&H.

There are three approaches to try, but they all depend on the physical properties of the Softpot material - if there's any signficant contact/decontact time and/or hysterisis.

Also I'd like to avoid using an extra plate to generate the Gates/Triggers (such as on the Moog ribbon controller). I'd prefer the contact on the ribbon itself to generate those signals. Two of the approaches would work in that regard. The third (Dr. Moog's approach) I haven't figured on yet in that regard.

If the transistion from "touched" resistance to infinite resistance is fast enough, then a constantly clocked Sample that locks to hold when infinite resistance is sensed should work. If the clock is fast enough (and infinite resistance comes in quickly enough), there would be slim to no chance of a sample being taken in the transition.

The third method would be to create an imperceptible lag on the CV (IE, fast enough to be non-apparent to the performer in the form of slew from one extreme of the ribbon to the other) that is yet significantly longer than the transition to infinite resistance when the pressure is removed, and is the point at which the S&H would hold the lagged CV, which (hopefully for all intents and purposes) hasn't had a chance to drop any noticeable amount.

[Scott Stites]

And, of course, the ignition system for the flamethrower on the end of the chassis must be considered.

[jnuaury]

awhile back i was debating getting the PCB for the MFOS keyboard and using the soft pot for the voltage divider instead of the keys... i didnt test it out but i didnt see any problems with it other than the PCB was way too big for that project

http://musicfromouterspace.com/analogsynth/MFOSMINICONTROLLER/MFOSMINICONTROLLER.php



one thing im sure youve run into is that you need a pull down (or up) resistor if you do want that "infinite" resistence you mentioned

i set something up for an arduino project months ago where i had a switch that would switch in a pull up, a pull down, or neither

the neither is great because you get a fun randomly fluctuating voltage

[Scott Stites]

Right, the "wiper" of the softpot disconnects with no pressure applied, going to (for all practical purposes) infinite resistance. I figure a pull down on the wiper would work for detecting that condition.

I believe the Moog circuit lets the input to a sample cap, buffered by a FET, float when no pressure is applied, providing the memory.

I've got two smaller strips that I plan to configure as the modulation controllers typical of a MiniMoog. Center zero, with no positional memory, and 0 to + with positional memory.

[Coriolis]

Scott, the DIY community (well, me at least) waits with baited breath for a device like you're talking about...
I've already got one of those Spectrasymbol things, that I've been pondering what to do with for a while...

C

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[Scott Stites]

Yep, that's exactly it, Bill - the comparator signal would control S&H and the generation of gates and triggers.

I was thinking of an LM398 for S&H - they're extremely good and easy to work with as well.

[State Machine]

[frijitz]

[Luka]

would it be possible to sit it ontop of led bars as well using the lm3914 to power the bars?

[Scott Stites]

Thanks, guys!

That app note is killer, Ian

Seems to me (I haven't looked at it in years) the datasheet had a low droop rate app, too(?). IIRC, it employed two 398's. Seems to me someone mentioned years ago it was the technique used by MOTM for their S&H.

I applied voltage to the SoftPot tonight. Oh, yes, this is going to be a very fun project, indeed.

The action is very smooth, and quite linear (I think it's spec'ed for 3% linearity). Quite sensitive, but not too sensitive - just about perfect, actually. With higher voltage range, I felt like Morton Subotnick on a Silver Apples flashback rolling my finger around on it . I hooked the output up to my EG (which uses a comparator to gate) and had a lot of fun controlling a VCO and filter cutoff with the control voltage and filter cutoff and VCA amplitude with the EG.

It's a bit awkward to use still because I don't have it glued to anything. I notice there's a bit of bounce on it, which surprised me a little. That'll have to be dealt with, but it doesn't worry me. Near as I can tell, the fall time is around 200 uS, though I could be wrong about that.

Luka - I s'pose one could use 3914's to monitor the voltage if one wanted.

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[frijitz]

[Scott Stites]

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[Scott Stites]

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[Scott Stites]

[Scott Stites]