Question about converting a CryBaby wah into an expression pedal.

[ianmgull]

I'd like to convert a standard cry baby into an expression pedal. Ultimately, I'd like it to control either a Strymon Mobius, or an EHX SuperEgo Plus. The Caveat is that I'm going to first run the expression pedal into a Boss ES-8 pedal switcher which I think outputs a standard 0 to 5v control voltage.

My question is related to how you ensure that the control voltage goes to zero in the toe up position, and the full +5v in the toe down position.

Since an expression pedal typically is just a voltage divider, it makes sense that a 5v input on the wiper would give the desired results when it is rotated either fully CW or CCW.

My uncertainty comes from the fact that a cry baby has a fairly small angular displacement of the pot between the toe up and toe down position. I haven't measured it, but I'm pretty sure it's not the full range of pot travel.

How do you get around this? Does it actually matter?

[ianmgull]

I'd like to convert a standard cry baby into an expression pedal. Ultimately, I'd like it to control either a Strymon Mobius, or an EHX SuperEgo Plus. The Caveat is that I'm going to first run the expression pedal into a Boss ES-8 pedal switcher which I think outputs a standard 0 to 5v control voltage.

My question is related to how you ensure that the control voltage goes to zero in the toe up position, and the full +5v in the toe down position.

Since an expression pedal typically is just a voltage divider, it makes sense that a 5v input on the wiper would give the desired results when it is rotated either fully CW or CCW.

My uncertainty comes from the fact that a cry baby has a fairly small angular displacement of the pot between the toe up and toe down position. I haven't measured it, but I'm pretty sure it's not the full range of pot travel.

How do you get around this? Does it actually matter?

[MikeA]

How about a tapped pot?  If you can find one with the value to the tap = your total max resistance needed, you reach that point with half the usual throw.  Bourns PTT111 series is one example (Mouser.)  You may have to reduce the treadle travel to suit.

[PRR]

Is the crybaby in your hand? Feed it the 5V and measure what comes out.

Foot-driven pots must generally gear-up to get full use of a 270 degree pot from a 20 degree ankle.

Also no controlled device should insist on flat-ZERO or full 5V to do the extreme things.

[ianmgull]

Thanks for the suggestions.

I don't actually have a crybaby yet, but that's a good idea. I was just going to buy a broken one I saw for sale.

[ElectricDruid]

My question is related to how you ensure that the control voltage goes to zero in the toe up position, and the full +5v in the toe down position.

Since an expression pedal typically is just a voltage divider, it makes sense that a 5v input on the wiper would give the desired results when it is rotated either fully CW or CCW.

My uncertainty comes from the fact that a cry baby has a fairly small angular displacement of the pot between the toe up and toe down position. I haven't measured it, but I'm pretty sure it's not the full range of pot travel.

How do you get around this? Does it actually matter?

You can't get around this without a good deal of messing about, either analog or digital. It would be possible to offset and scale the variation you get from the pot with an op-amp and get it into the range 0-5V, but it'd need some calibration. Alternatively, you'd use a cheap microcontroller to read the pot and then generate a 0-5V output depending on the position. Again, you'd have a little calibration routine so that it knew where "max" and "min" were.

But that's not that important since mostly that's not what people do. It doesn't matter that much. You get a voltage of (say) 1.5V to 3.5V out of your pedal and you can use that to control things. You've got 2V of variation. You might have more. You also have an offset (of 1.5V in this case), but there's usually some other control you can turn down to compensate. So no, it doesn't matter too much.

I have a blank wah shell that I use as an expression/CV pedal. I've had it connected up to my FilterFX pedal and it's pretty good. You have to dial it in a bit, but when don't you, really?

[ianmgull]

Thanks for the help everyone.

So I wired the crybaby up as an expression pedal and plugged it into my strymon mobius. It's working as expected, but there's a small issue:

Most of the action occours in a very small range of travel. The parameter I'm switching goes from 0 to 100 in only a few degrees of travel with very little change occouring as I approach both toe up and toe down positions (this is actually the opposite of what I expected). All of the change happens bunched up in the middle. So the quesiton now becomes, how do I spread the sweep out?

I'm thinking two resistors (or trimpots), each going from the wiper, to one of the external pot legs. If this is the case, what values would you recommend?

FWIW, I'm using the 100k pot that is standard in the crybaby. I know some manufacturers recommend 10k or greater for an expression pedal, I'm not sure if 100k is too high? I would think it doesn't matter that much since it's only acting as a variable voltage divider.

Thanks!