New build idea: Dual Momentary Expression Pedal

[[WZ]]

Hi all! Long time lurker, first time starting a thread!
So here we are..

I had this idea for a new sort of Expression Pedal:
Using a Momentary DPDT stomp switch, you could switch between 2 adjustable CV levels (using simple voltage dividers).
This isn't too strange; it's basically any non-latching channel selector switch, but now you can set the voltage for ON and OFF individually.
Okay, now the fun starts if we add a Slew Rate limiter (basically a LPF) so we can adjust rise and fall time of the control voltage.
And the final rick: we add a toggle to bypass the Slew pot with a switching Diode in either direction.
(Think of it as a sort-of standalone 'Digitech Ricochet' but for CV/Exp.)

Here's the first attempt schematic:


On the left there are 2 pots as voltage dividers (both go from 'full on/toe down' to 'full off/heel down'), then a buffer circuit, with a 1M pot for slewing (gotta try what Cap works best), and an on-off-on toggle to select bypassing the rise/fall ramping.
Exp/CV is TRS: Input voltage comes from the RING (usually 5V or 3.3V), the output voltage goes to TIP, and SLEEVE is ground.

And here's a little theoretical example of the possibilities:


Oh, I forgot to mention; this should be pretty small so if we're doing this thing LETS MAKE IT A DOUBLE!!
There's only 3 knobs + 1 toggle for a single one, and the DPST switch has enough pins for it, so a Dual Momentary Expression Pedal it is!

Maybe I'm just weird, but I love the idea of being able to control 2 pedals simultaneously with a single burst!
You could do all kinds of cool stuff, I can already image the possibilities of controlling Feedback on a delay while ramping some Filter, or dive-bombing the Pitch of a Rainbow Machine (from Chorus to Madness!) while my Looper momentarily stutters this bit! 

So, what do you guys think?! Worth giving a shot?! any comments/concerns?
I'm gonna build it ASAP anyway


P.S. This is just a basic idea; you could swap to a latching switch (I just like the momentary option more), or you could swap out the Diodes and Slew pot for 2 Slew pots with diodes so you can set rise and fall time separately. Many things to try with this. I'll probably add some kind of indicator LED(s) near the output(s), so you can see what's going on with the control voltages.

[[WZ]]

Update:

the prototype works kinda, but I have 2 problems:

1) When the voltage level going into the buffer (and slew/drag/skew/lag thingy after that) approaches ground, the buffer craps out a pretty high voltage suddenly?
It's supposed to be a voltage follower, but at about 0.7V inpit, it jumps to a few volts at the output!
Maybe I'm slightly out of the OpAmp specs, since it's only powered from +9 to GND and my input is as low as GND itself (not gonna hassle with dual rail power supplies), but then still I'd expect it to just remain at a low-ish output...?

2) In the real world, it turns out a Momentary Stomp (or at least the ones I have) have a small moment in which the switch is 'in between states', so the input to the circuit is just floating around freely. This is causing the issue from 1) again, since a free (or grounded) input suddenly makes the output jump to a high voltage.

(Not sure if anyone is even interested, but for future generations maybe this is useful..)

[EBK]

Interesting.  I haven't studied your idea in depth, but wanted to ask what type of op amp you are using.  Have you tried a LM324 or similar chip designed to work from a single supply but allow inputs as low as ground?

Also, perhaps consider some transistor-based switching to combat your switch bounce issue?  (Just spitting this out without a huge amount of thought)

And, welcome into the non-lurker light! 

[Feral Feline]

Check out The Tone God's Switch Debouncing article.

I'm sure there's some debouncing threads in the ethers, as well, somewhere.

[[WZ]]

Thanks for the welcomes and the interest!

Interesting.  I haven't studied your idea in depth..

Yeah I know it's a bit hard to see the potential, but no worries, demo coming soon!*

For now, I've been playing around with the prototype a bit and the design is evolving further: I've added some simple, choppy square wave modulation, which can be smeared out into a triangle-ish LFO by the same Slew rate limiter that's already there..
So, to further distinguish this idea from the 'traditional' Expression pedal, now you can add some gentle vibrations or sickening wobbles to any CV-compatible pedal! (I'll update the schematic soon*..)

*soon = relative; I usually only have time for building in weekends.. But updates will follow!

..but wanted to ask what type of op amp you are using.  Have you tried a LM324 or similar chip designed to work from a single supply but allow inputs as low as ground?

Yeah, I managed to fix issue 1) and get 0V levels simply by changing to a TL072 OpAmp, now it's just 0V in = 0V out, as expected.
Still haven't figured out why the other one (4558 I think?) decides to jump to such a high voltage when it approaches 0V though..

Also, perhaps consider some transistor-based switching to combat your switch bounce issue?  (Just spitting this out without a huge amount of thought)

Check out The Tone God's Switch Debouncing article.

I'm sure there's some debouncing threads in the ethers, as well, somewhere.

Thanks, I'll check both out!
But for now, I got a quick-n-dirty fix:
-1M pulldown resistor at the common node of the DPDT stomp switch (so the "in-between-state" goes low instead of floating around freely..)
-Increase the fixed resistor in the Slew path (to ~10K) so that even at lowest slew-rate settings, the very brief connection loss doesn't really matter that much (but it's still fast enough to fully drop/increase when you want it too)

Thanks again, stay tuned for updates!

[duck_arse]

Yeah, I managed to fix issue 1) and get 0V levels simply by changing to a TL072 OpAmp, now it's just 0V in = 0V out, as expected.
Still haven't figured out why the other one (4558 I think?) decides to jump to such a high voltage when it approaches 0V though..

look for this [or similar] on your datasheet:
"No Latch-Up When the Common Mode Range is Exceeded"

input voltages outside the specified common mode range will produce indeterminate output level - unless the spec sheet says they won't. so, might flip high, might flip low, might do both at once.

[[WZ]]

look for this [or similar] on your datasheet:
"No Latch-Up When the Common Mode Range is Exceeded"

input voltages outside the specified common mode range will produce indeterminate output level - unless the spec sheet says they won't. so, might flip high, might flip low, might do both at once.

Cool.
Found this:

Some op-amps experience a phase reversal when the common mode range at the inputs is violated. For example, if you pull a non-inverting input below the negative supply voltage by more than a few hundred mV the output may snap to the positive rail. That can cause the output to latch in that state depending on the nature of the feedback... There's nothing in the datasheet that indicates that happens- but the makers don't always advertise it."

There we go. Exactly what was happening.
And indeed, Datasheet for TL072 says: LATCH UP FREE OPERATION!!!

Another lesson learned... Thanks man!

[PRR]

> Datasheet for TL072 says: LATCH UP FREE OPERATION!!!

It doesn't latch; but as long as you hold the input too far, the output stays "way wrong".

For Zero to Plus DC signals, the LM324 is a fine chip. Inputs go to zero (even a hair below), output does the right thing and will pull down to like 0.05V (about as good as it gets without a negative rail).

'324 is less-good as an audio chip, because it has crossover distortion, tamed by NFB but it starts to crap-up in the upper audio range.

[[WZ]]

> Datasheet for TL072 says: LATCH UP FREE OPERATION!!!
For Zero to Plus DC signals, the LM324 is a fine chip. Inputs go to zero (even a hair below), output does the right thing and will pull down to like 0.05V (about as good as it gets without a negative rail).

Cool, thanks, another lesson learned!
Ordered a 10-pack.
For now, TL072 gets the job done.