This instructable details how to modify a wah-wah pedal for use as an expression pedal. It's actually fairly simple, but you need to know how to solder properly, and have basic mechanical ability. Total time: 1 hour. Total cost: $0-$100
Step 1: What You Need
Of course you'll need a wah-wah pedal. I used an original Vox wah from the 1970's. They make a reissue of this pedal that you can buy new for $100 or less. A Dunlop Cry Baby wah is basically the same housing so it will work just as well, and is even cheaper. I've seen them go for as llttle as $20 on eBay and in the pawn shops.
The potentiometer is the toughest item to find. I already had a suitable pot, but I imagine Radio Shack might be a source. You need a 10K ohm pot, such as used for volume/tone controls. The only problem you may have, is finding one with a long enough threaded shaft. It has to be long enough to span the mounting boss in the case with enough protruding to attach washers and nut. Use the stock pot as your guide.
You'll need one or two 1/4 inch stereo jacks, depending on which configuration you choose. Basically you can have just one input jack on one side of the pedal, or you can have an input on either side of the pedal utilizing the stock input/output jack locations. This makes more sense to me, and is how I did it, but you really don't need two jacks. Again, Radio Shack is your friend.
To wire it all up, we want to use shielded two conductor audio cable. You don't need much, probably a foot will be more than enough. I cut what I needed from an old mic cord. Of course you'll need a standard stereo 1/4 inch patch cable to plug the pedal into your controller.
Basic hand tools; screwdrivers, wrenches and/or sockets, safety glasses.
Soldering iron/gun and 60/40 solder.
Step 2: Disassembly
The first order of business is removing the electrical and mechanical components from the housing. That includes the circuit board, on/off switch, input/output jacks, and the potentiometer with the pinion gear attached to it. The rack gear can remain attached to the footpedal part of the housing. To remove the pinion gear, drive out the stake wire or loosen the set screw, whichever is holding it on the shaft, and pull the gear off. Save it, because we will need it again real soon, then store the rest of the parts together. The beauty of this mod is that it is highly reverseable, provided you remove it all as an assembly as shown in the photo.
Step 3: Pre-Assembly
Grab your pot and jack(s), a handful of wire, and get your iron hot. We are going to pre-wire the new assembly so we can just drop it in.
Measure a length(s) of the shielded wire long enough to easily reach from the pot mounting boss, to the jack hole(s) in the case, and carefully remove an inch or so of the outer jacket from both ends. Strip an eighth inch of insulation from both center conductors and twist the braided shield tightly. Tin the ends of all the wires and shields on both ends. Don't go bonkers. A little goes a long way. Next tin the lugs on the stereo jack(s).
Often, a volume pot has one leg soldered to the case and the case to ground, so that when you rotate the shaft, part of the signal is sent out this leg to ground to attenuate it. We don't want that to happen, so if there is a leg soldered to the case, unsolder it and bend it up even with the other two legs.
Clean up all three legs and add a fresh drop of solder to each. You could hook up the pot in parallel to two jacks mounted in the stock locations as I've done, enabling you to plug in from either side of the pedal, or just use one.
An easy way to hold everything in place while you solder, is to poke holes in a piece of cardboard, then stick the jack and pot shafts in them. Having said that, now solder the wires to the pot and jack as shown in the photo . Sleeve (shield) on the center lug, tip(red) on the left lug, and black (ring) on the right lug. Make sure you wire it the same on the jack, and it should look something like the photo.
Step 4: Assembly & Testing
Install the new pot assembly in the mounting boss first, then the jack. Rotate the pot shaft counter-clockwise all the way, then back it off a little and reattach the rack gear with the pedal down, adjusting the plastic tensioner to hold it firmly against the pinion gear, but not so tight it binds. Check the pedal for smooth operation, but don't close the unit up yet. You may have to readjust the pot range several times to get it just right.
Connect it with a stereo 1/4 inch patch cable to your pedalboard, and test and readjust to your liking, then put the cover back on. You could alternately test and adjust by "reading" the pot with an ohm meter connected between the tip and ring of a stereo patch cable plugged into the pedal. With most MIDI pedalboards, the actual values are not carved in stone. It's only looking at minimum and maximum values, and there is usually a provision to calibrate to a pedal.
Step 5: The Bottom Line
That wasn't so bad was it? Now you have an expression pedal that not only works good, but looks pretty spank too. This is one of the best and cheapest mods I've done to date. In fact, since I already had all of the parts including a wah-wah pedal I was willing to butcher, my total cost was $0, and about an hour of my time. If you bought everything off the shelf, you could easily duplicate this for well under $100. I've had a ton of people ask me how I get a wah-wah pedal to do so many different things, like control a pitch shifter or delay. I tell them, "it's magic!".
This instructable was condensed from the full article. Unfortunately, the site the article was on is gone now, but I will be republishing it soon and will add the link here