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Controller, Ribbon, Softpot, another.
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frijitz



Joined: May 04, 2007
Posts: 1712
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PostPosted: Mon Feb 16, 2009 9:08 pm    Post subject: Controller, Ribbon, Softpot, another.
Subject description: My entry into the Ribbon Controller Sweepstakes
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So ... after playing around with a 500 mm Softpot for a few weeks I've discovered that I'm probably not going to be able to develop any virtuostic technique with it. I did find, however, that I really enjoy using it with a breath controller to get all the slips and slides that are not possible with my more traditional wind controllers. So I'm going to emphasize working with a fairly bare-bones controller operating as a kind of sliding flute. But of course keeping in mind that the usual more advanced features can easily be added at any time.

The attached schematic shows the current version of the electronics. I've tweaked this up fairly carefully, so I don't think there will be any changes in the core circuitry. It works quite reliably, although the Softpot seems to take an awful lot of pressure to operate. This leads to a bit of pitch shift due to the "squish factor" on finger-up. But it's pretty easy to compensate for this with a bit of a roll of the finger as it comes off the ribbon. Of course this doesn't matter, if the CV hold on finger-up is not used, i.e., if the VCO gets gated off. (We see a very successful unit here using that approach.)

OK, here's the geeky stuff about how it works.

A1 and associated components form a standard current source that drives about 1/4 mA into the ribbon. The CV picked off from the ribbon's touch point is buffered by A2, then lagged, scaled for 1V / Oct (high-note priority) and presented to the input of the S/H circuit built around U1 and A5. The purpose of the S/H (actually operating as a track-and-hold) is to grab and maintain the last CV produced before finger-up. The S/H itself is a feedback design employing an OTA switched current source and a FET opamp buffer. Droop is about 2 mV / sec using a 0.022 uF polypropylene storage cap.

So that's the simple part. The rest of the core circuitry is for turning the S/H on and off, which is the trickier part. This is accomplished with two separate circuits.

The crucial consideration here is the method used to detect the finger up/down transitions. In previous work, the method used was to pull the ribbon's contact strip to a power supply rail with a large resistor. This resistor must be large (>2M) to avoid loading the signal from the ribbon. For the Softpot, this method is unsuitable. The reason is that the Softpot has a large capacitance (280 pF), which causes the finger up/down signal to be delayed by a significant time interval, making it imposssible to capture the last voltage prior to finger-up before it decays significantly.

The solution to this problem is simple -- just use the usual method of detecting current in a circuit, which is to add a series resistor in the current path and measure the voltage drop across it. In the schematic, this resistor is the 8.2 k unit drawn above the ribbon. The finger up/down transition causes a 2 V change in the voltage across this resistor. This change is rapid, because there is no large resistance in the circuit, rendering the ribbon's capacitance irrelevant. Additionally, having the ribbon at low impedance avoids potential trouble with noise pickup.

On finger-down, the voltage on the ribbon's pickoff point falls from about 7 V to < 5 V. Comparator A7 detects the 6.2 V crossing and feeds the 3.3M / .022 uF lag network. The lag delays the start of acquisition by ~30 ms, enough time for the finger to squish down. When the lagged signal crosses 0.6 V, U2 is turned on. U2 is an OTA used as a fast current switch. Its output turns on the S/H switch U1, and the U1/A5 combination begins tracking the changing ribbon voltage.

On finger-up, the lag circuit's capacitor is discharged quickly through the 1 k resistor and the diode (to the right of A7). This holds the OTA currents off. However, this process is too slow for accurate acquisition of the ribbon voltage at finger-up. A special, fast, auxilliary circuit provides a much more rapid initial acquisition. On finger-up, the voltage across the 8.2 k resistor in series with the ribbon goes from 0 V to 2 V. As soon as it starts to rise, comparator A6 switches and triggers a pulse from the 555 IC timer chip U3. This pulse drives U2's (-) input, shutting off the OTAs for a long enough time to allow discharge of the lag capacitor. The key to this operation is that A6, U3 and U2 are fast switches, and they are biased near their switching points for minimal switching delay.

Finally, A8 develops a gate signal, which is high on finger down. This can gate the VCO signal (through a VCA) to produce sound only when the finger is down. Like Mr. Flicker-Fingers here.

This circuit is being provided gratis with no string attached for experimenters. But no commercial use, please, and no copying of the schematic to other sites, blogs, etc. Thank you.

Smile

Ian


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Last edited by frijitz on Sun Mar 01, 2009 3:40 pm; edited 2 times in total
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RF



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PostPosted: Mon Feb 16, 2009 9:24 pm    Post subject: Reply with quote  Mark this post and the followings unread

Awesome - Thanks Ian!
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Dragon's Lair



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PostPosted: Mon Feb 16, 2009 9:34 pm    Post subject: Reply with quote  Mark this post and the followings unread

I have been playing around with the 500mm Softpots also, but mine take very little pressure to activate, do you have more then one softpot to try with? you shouldn't have to "squish" your finger to activate it.
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frijitz



Joined: May 04, 2007
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PostPosted: Mon Feb 16, 2009 10:33 pm    Post subject: Reply with quote  Mark this post and the followings unread

Dragon's Lair wrote:
I have been playing around with the 500mm Softpots also, but mine take very little pressure to activate, do you have more then one softpot to try with? you shouldn't have to "squish" your finger to activate it.

Oh, well, you're one of those big lumberjacks, right. Laughing

The force is large enough that it is difficult to slide along the ribbon because of the friction. Stites sees the same thing. Yes, I have two of them, and they seem the same. But of course it is all relative. My reference point is a ribbon I built out of an antistatic bag, which has a nice feel, but is small and doesn't make very good contact.

Was yours shipped bent into a U, also. I have trouble with some waviness because of that. If it is a problem for you too, I suggest talking to those Spark-whatever-it-is folks. I'm afraid they won't ship them properly unless everyone complains.

Very Happy

Ian
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Dragon's Lair



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PostPosted: Tue Feb 17, 2009 12:38 am    Post subject: Reply with quote  Mark this post and the followings unread

I got 10 of them directly from Spectra Symbol, they shipped them in a tube, so they are straight and flat.
Do you have them mounted on a base? I have a 200mm and a 500mm mounted to a piece of melamine, along with a power supply and some knobs, for testing purposes. I found that if you don't stick them down to a solid surface, they can be a bit hard to use.
With this setup, I use about the same force on them as I use on the touch plate of my laptop, and have no problem sliding my finger up and down it.
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frijitz



Joined: May 04, 2007
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PostPosted: Tue Feb 17, 2009 8:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Yes, I have them mounted on bases. (Photo on Ondes Martinot thread). They certainly require much more force than my laptop touchplate. Hmmm ... Maybe we should try to figure out a way to measure the contact force so we can compare? I'm going to be very angry if it turns out that Sparkfun damaged them.


EDIT: I put a small square of balsa wood on the ribbon and stacked on coins until it switched. I takes 9 quarters. Maybe other folks could try the same experiment?


Very Happy

Ian
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RF



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PostPosted: Tue Feb 17, 2009 9:59 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Ian -

I placed a Digi-Key order just a day before you posted this - Sad

Just how critical is the OPA134 to the circuit? I'm curious as to why you used that particular opamp - and if there might be a reasonable sub for this circuit?

Thanks for the insight...

bruce

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Dragon's Lair



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PostPosted: Tue Feb 17, 2009 10:09 pm    Post subject: Reply with quote  Mark this post and the followings unread

There is definitely a difference in the softpots, mine only took 6 quarters to activate.
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frijitz



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PostPosted: Wed Feb 18, 2009 7:53 am    Post subject: Reply with quote  Mark this post and the followings unread

RF wrote:
Just how critical is the OPA134 to the circuit? I'm curious as to why you used that particular opamp - and if there might be a reasonable sub for this circuit?

I use the 134/2134 when I want better performance that the 071/072. For A6 you need high speed. I will probably eventually try something even faster than the 134, maybe the LM318. You can try the TL071, LF351, etc., at least to start, but I have no idea how much this will affect the acquisition speed.

A7 is probably not critical, after adding A6 for the fast initial acquisition. So TL071, etc., would probably be OK.

A5 needs to be low leakage for low droop. I will eventually put a CA3140 in there. For initial testing, again you can try any decent FET opamp. But you will get more droop.

Just use sockets, see how it works, then upgrade if you want to. You may hate the whole thing anyway. Laughing

Very Happy

Ian
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frijitz



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PostPosted: Wed Feb 18, 2009 8:01 am    Post subject: Reply with quote  Mark this post and the followings unread

Dragon's Lair wrote:
There is definitely a difference in the softpots, mine only took 6 quarters to activate.

Thanks for doing that, Dave. Looks like mine are messed up, either from the shipping method or from my attempts to flatten them.

Another issue with the softpot is how narrow the strip is. I have to wonder if Vongole could do what he does on such a narrow strip?

I may try to make a ribbon that works better for me. The ESD material here looks very interesting:
http://www.lessemf.com/catalog2/55.pdf
(Scroll way down.)

A 1" x 20" strip would be 20 kOhm, same as the softpot.

Very Happy

Ian
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RF



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PostPosted: Wed Feb 18, 2009 8:57 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Ian -
I appreciate the response. The OPA134 isn't something I have in the parts box here - so I'll try some others. I'll likely order some OPA134's next time I order parts and then try those, too.

Thanks again

bruce

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Dragon's Lair



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PostPosted: Wed Feb 18, 2009 11:09 am    Post subject: Reply with quote  Mark this post and the followings unread

That does look interesting, it would work well with this -

http://www.wiseguysynth.com/larry/ribbon/ribbon.htm

Damn! another project to put on the pile Shocked
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Scott Stites
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PostPosted: Thu Feb 19, 2009 7:29 am    Post subject: Reply with quote  Mark this post and the followings unread

What an elegant design, Ian!

Maybe you did get a bum SP. I haven't tried the quarter test on mine, but I'd say I use pretty much the same amount of pressure I use on my computer touchpad. That's not to say it's as sensitive to the touch, but there's not any bearing down, either. Finger-weight is where I'd put it.

I liked your cotton gloves idea. I had a mental image of a man in tuxedo standing before a ribbon controller as the conductor raises his baton....

Cheerios,
Scott

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frijitz



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PostPosted: Thu Feb 19, 2009 8:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks, Scott. This is still pretty experimental. Time will tell if it is adequate, or if a bullet-proof design like yours is needed.

I started from the J. Simonton design, and was surprised to see that it didn't even come close to working. It took some head-scratching to realize that his wire contact has negligible capacitance vs a quite high value for the SP, and that that was a serious problem for acquiring the finger-up voltage. Switching over to the series-resistor sensing approach fixed most of the problem. (It might tighten up the timing in your system also.)

Adding the side-chain speedup blaster got the acquisition time down to a pretty small value. The main voltage still needs a bit of lag, which may bother some people. I think, though, that the glide time is comparable to the transition time between notes in acoustic instruments.

Actually, I'm enjoying using the controller in the Vangole mode, gating the VCO off on finger-up.

As far as the SP bouncing, I think I have a better understanding for what is happening. Since the blaster strobes the system hold for a finite time after the first finger-up signal is detected, there cannot be bouncing on the signal in the usual sense of a switch bouncing between on and off. But there is still some jumping around of the pitch if the finger is pulled off slowly. So I now think that remaining flakeyness is due to an intermittent change in contact resistance. This can be mostly avoided with clean fingering, I think (hope).

As far as the SP response, I guess it's not so much that it takes a huge force, more that it is hard to maintain the pressure while sliding around. I'm using the paper tape over the stripe now, and that helps a lot. Moving the fretboard back a bit and playing with flatter fingers is also helping. So maybe just need more practicing.

Ian
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Minimalist



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PostPosted: Sat Feb 28, 2009 3:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thanks for posting this Ian

I'm thinking of having a go at building this and I've a few questions ( I'm a electronic novice so bear with me ) . What would you recommend for the PNP transistor next to A1, something like a 2n3906 ?. What value is the trimmer between A3 and the 33k resistor ? and is this a +15/-15v circuit ? . So thanks again for posting this and any help is appreciated.

Marcus
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frijitz



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PostPosted: Sun Mar 01, 2009 3:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

Mini:

Yes, 3906 is fine for the PNP -- it's not critical.

The trimmer can be 2k or 5k -- just a divider, so doesn't matter too much.

I use +/-12V supplies, but +/-15V should be fine.

All:

I've modified the circuit slightly and replaced the diagram in the first post.

I noticed that my 3.6V zeners don't seem to be very accurate. The 12k resistor in the I source may need to be selected to give the correct current. Just adjust it until there is about 5V at the top of the ribbon.

Also, I replaced the 5.6V zener at the input of U2 with a 4.7V unit, because of the 5V maximum input to the OTA.

There are several other changes in R and C values. The most important is the RC of the slewing filter, which I increased to 3 ms. You can experiment with this.

Very Happy

Ian
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Minimalist



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PostPosted: Tue Mar 03, 2009 2:22 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Ian

thanks for the generously supplied clarification, one last amateur type question, I'm a little confused by the Extra arrow, (is that biasing?) on U1 & U2 the LM13700, could you add the leg numbers for the LM13700?

thanks again
marcus
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frijitz



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PostPosted: Tue Mar 03, 2009 10:24 am    Post subject: Reply with quote  Mark this post and the followings unread

Minimalist wrote:
I'm a little confused by the Extra arrow, (is that biasing?)

The arrows indicate the control current inputs (pins 1 and 16).

Very Happy

Ian
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tooheys



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PostPosted: Thu Dec 10, 2009 8:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hey Ian, this designs is impressive - & I'm really keen to try it out with a home-brew ribbon controller meets ondes martenot design.


My hotpot is hurtling to me in the post - now I'm trying to layout the parts on a pcb in EagleCad. Only the 5 ICs are creating havok when trying to arrange them in a single-sided way for DIY etching. So I was just wondering if u had a recommended layout position to minimise crazy amounts of jumper wires?

Or just a pic of your circuit in action

Also - for the caps - any non-polar caps work? - i.e. just ceramics or should some/all of them be film caps?


Cheers mate! I just thought I'd give this design ago rather than the more indepth appendage - as I'm really only after a simple ribbon part


now to work out how to make an optical button controller like the ondes
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frijitz



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PostPosted: Sun Oct 21, 2012 9:35 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm resurecting this thread, since I have been working on my ribbon controller ideas again. Here is an improved version of the original circuit. The main change is that one of the comparators was redundant (A7), so I took it out. There are also some changes in component values.

Ian


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Benjamin AM



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PostPosted: Wed Nov 07, 2012 11:16 am    Post subject: Reply with quote  Mark this post and the followings unread

If one was to use a 100k ribbon would he want to run a resistor in parallel ??25k??
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frijitz



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PostPosted: Wed Nov 07, 2012 5:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

Benjamin AM wrote:
If one was to use a 100k ribbon would he want to run a resistor in parallel ??25k??

Benjamin --

You would want to run 5V / 100k = 0.05 mA through the ribbon to get 5 V at the top. For this, change the Iset trimmer to 100k. Also increase the series resistor from 8.2k to 39k. Just set the trimmer to give 5 V across the ribbon and you're set.

What is your ribbon?

Ian
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Benjamin AM



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PostPosted: Wed Nov 07, 2012 7:38 pm    Post subject: Reply with quote  Mark this post and the followings unread

I'm experimenting with ESD ribbon taken from foot ground straps. THey seem durable and were cheap on Ebay.
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frijitz



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PostPosted: Wed Nov 07, 2012 8:47 pm    Post subject: Reply with quote  Mark this post and the followings unread

Benjamin AM wrote:
I'm experimenting with ESD ribbon taken from foot ground straps. THey seem durable and were cheap on Ebay.

I'd be interested in hearing how those work out for you. I tried this Desco strap (you can barely see it in the photo).
http://www.hmcelectronics.com/product/Desco/17250
Wasn't able to get reliable contact with it.

Ian
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Benjamin AM



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PostPosted: Sun Nov 18, 2012 1:51 am    Post subject: Reply with quote  Mark this post and the followings unread

Hi Ian,
I finally got around to getting this circuit on a breadboard. I eventually ended up using a softpot instead of using ESD ribbon. I noticed that the CV output ranged from 0V to 4V. To change this I adjusted the value of the 120k resistor attached to A4's negative input. Doubling the value(changing it to 240k) dropped the output to a range of -2V to 2V as the schematics indicates. I was curious if this sounds right.
Also, I was curious if other ribbon controllers designs that use the softpot exhibit the phenomenon that has been deemed as the "squish factor", that is, pitch bending on finger pull off. Not that this is a problem, I found that the effect to be controllable with said roll off technique, and even at times(for the few hours I've spent playing the controller) desirable.
Anyhow, thank you so much for contributing this design, I look forward to the spending many hours playing this thing.
Edit: I forgot to mention that I used CA3080s in place of the LM13700. Seems to be no problem.
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