Best path forward if I want to use expression control for... well, everything

[kat]

Hi everyone,

I keep finding myself with a great-sounding circuit that I'm ready to box up but then I don't, because my ideal final version would involve expression-pedal control of one or more of the parameters.  My problem is an obvious and basic one:  basically what I want is to be able to swap in an expression pedal for all kinds of random pots that are playing various roles, and rarely do they match what's in a typical expression pedal.  The only expression pedal I have sitting around has a 10K pot wired as a variable resistor, and that's almost never what the circuit calls for as-is.

So on the one hand, I could make circuit modifications so that a 10K variable resistor would work in place of whatever is in the circuit... but I haven't fully worked out in my head whether this is always possible (or easy). Anybody know of any simple schematics to show in a generic way how to do this?   On the other hand, I could order some stuff from Smallbear to build one or more stereo-input expression pedals with different pot values.  That seems expensive but in some ways easier.... then it's just about extra jacks and wiring up the switching correctly.

I can see this issue coming up again and again so I guess I am looking for a generic "system" - if you're doing a lot of DIY pedals and want to be able to adapt for expression pedal control of one or two selected parameters for each, what's the best approach to take?  Or is this a question that needs to be answered individually for every type of circuit (I don't see why but might be missing something...)...

Thanks as always for the wealth of expertise!

[Mark Hammer]

The simplest strategy, though not always the most universal, would be to use an expression pedal to control one or more homebrew vactrols/optoisolators.  The pedal gets used to adjust the current-limiting resistance between a power source (battery of your choice) and a suitable LED.  The LED then adjusts the resistance of an LDR, and the LDR then gets used as either a parallel resistance, or substitute resistance, for something in the pedal.  The expression pedal pot can be easily reconfigured to make the LED shine brighter for toe-down or heel-down.

This would require that you:
a) be able to identify parameters that are controllable via a single variable resistance;
b) have LDRs that you know can mimic the required range of resistance;
c) be able to simply stick a plug into a jack on the pedal to either supplement or replace an existing resistance.

This leaves out any parameters that require use of a voltage divider (i.e., one resistance goes up while another goes down)

[kat]

Thanks Mark for the suggestions.  I have considered LED/LDR schemes but that's new to me so I haven't yet tried it out.  I guess as you say, it's a matter of being able to mimic the necessary ranges of resistances.  I do want to work this out for some other long-term projects that I think will need it as a means of interfacing with other methods of expression control (not a rocker pedal). 

But the frustrating thing is wanting to use expression control for voltage-divider applications as well as variable-resistance applications.  That's why I was thinking that maybe a stereo cable to an appropriately-sized pot in the expression pedal might actually be the easier way to go, although expensive if I want a few options.

If I give up on that...I guess it's not totally crazy to imagine, in the LED/LDR variable resistor scheme, building a box with a few different switchable options for resistances (and tapers, if multiple options are possible) and setting it up between the expression pedal and the pedal-to-be-controlled. 

Anybody have other ideas, and/or suggestions about whether such a switchable LED/LDR expression pedal interface "box" is possible and/or a good idea?

[Mark Hammer]

Yeah, it's not exactly a universally applicable approach, but it'll work...to the extent that it'll work.

In the modular synth world, this sort of thing is far more standardized, with voltage control, and all circuits adapted for voltage control such that you can pretty much plug anything into anything else.  That was sort of Bob Moog's contribution to civilization and the state of the art: voltage control.

In the world of pedals, expression control is not quite as standardized  I got one of those Source Audio Hot Hand units recently, and it provides two kinds of control outputs, simply because there are different standards for expression control.  One standard, used by Line 6 and others, has a variable resistance to ground (Line 6 uses 0-10k).  The other outputs are 0-3.3V control voltages.  I had to build a breakout box to adapt those to other devices that might want 0-5V or 0-10V.

The other route is to score yourself either a vintage EHX Hotfoot, or a Tone in Progress Third Hand ( http://www.musiciansfriend.com/amplifiers-effects/tone-in-progress-third-hand-expression-pedal ), where the electronics are not an issue.  The expression is via direct mechanical rotation of the pot.

[kat]

Aaaghh... the notion of a device that mechanically turns knobs for me (at $125) is partly hilarious but also somehow just rubs me the wrong way.  Maybe that is the most practical solution, but since I'm building pedals myself, I should be smarter than that!  Well, I'm probably not, but still fantasizing...

Yes, I would love for pedals to have something like voltage control (or better).  It does make me wonder... about going digital for the imagined "variable resistor box" ... I don't know if you could have a digital or digitally-controlled pot that would have a smooth taper or if it would run into a lot of basic conflicts trying to get such a thing to play nice with old fashioned analog circuits. 

Hmm....

[nocentelli]

Anybody have other ideas, and/or suggestions about whether such a switchable LED/LDR expression pedal interface "box" is possible and/or a good idea?

If you only have one expression pedal, you could build the Rock N' Control 3 pole into a box and make the trimmers and R1 and 2 external controls so you could adjust the response to suit different pots. You could then use a stereo  switching jack (or two.. or more) on each new pedal you build, wired up to whatever pot(s) you might want to manipulate.

http://tech.thetonegod.com/rockncontrol/rock.html

[kat]

Ooo.... thank you nocentelli... that looks kind of like what I was wondering about and it's filling in the gaps... studying studying...

[PRR]

> ready to box up but then I don't

Box it up.

The 99-cent pot is a miracle device. Circuit changer several ways, also user interface and memory.

A remote-control version is far more involved, and often there is a different better way to implement the circuit in a way that simplifies the remote system. A volume-pot should be changed to a VCA. Tone pot may be a more complicated network and chips. Worth it unless you really fancy servo-motors turning your knobs. (Works for Neil Young, but he doesn't dare tamper his precious amplifier.)