DIY MIDI Expression Pedal





Introduction: DIY MIDI Expression Pedal

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.
Common Sense

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
Happy stompin'.




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    Hi Sam,

    I send you a PM but I'm not sure if you received it. I tried your solution to make work a handmade pedal with my PMC-10 but no success. I used a 100k pot and tried everything from programming to your instructable.
    I want to use my pedal to send CC messages to a Tc Fireworx or an Axon guitar synth but I can't find a way to do it with the pedal plugged to the continuous controller jacks on the midi board.
    When I read your instructable for the 1st time I thought EUREKA!! but I'm now totally dissapointed. The pedal works ok when plugged directly to the multifx unit  but no clue bout how to make it work with the PMC-10.

    Which pedals do you use? only EV-5? could it be just a problem of pot values?

    Any ideas/suggestions/whatever are allowed.


    hey, just put it together. trying to use it with my ps-5 . it kind of works but instead of doing big pitch sweeps it kind of just warbles. then returns to guitar pitch. is there somthing i can try? not sure how the boss exp works but obviously not keen to spend! plus i dont have any other pedals that would utilise an external exp pedal. thanks for your help! great simple project by the way!

    2 replies

    You could try different value pots. Also check the output of your pedal with an ohm-meter. The Boss expression pedal is basically the same as the Roland pedal that I used to determine the pot value. Both of those pedals actually have two pots- the one the pedal moves and one on the side of the pedal to fine tune the resistance value. Unfortunately, I can't tell you the values of those individual pots, or how they were wired, since I no longer own the Roland pedal. I seem to remember the range of adjustment was between 10k and 100k, but I wouldn't swear to it. When I was testing the Roland pedal, I got some of the effect you describe when the pot value was too high. I hope this helps, and sorry I couldn't give you more specifics. Let me know if you solve the prob, and I will update the instructable.

    ok thanks, prob not wise to head into my local music store with a multi meter and screw driver! :) wouldnt know the first thing about adding a second pot. do you think theres a chance i could damage the ps-5 by trying random pot values? thanks again. happy new year!

    nice job man! but your website seems defunct....???

    1 reply

    Thanks Mark, glad you liked it. Hopefully I will get the website back up soon. The domain expired and it slipped my mind to remove the link.

    Two questions. Will this work with a Boss PS-5? And if you don't use the stereo jack and wire it to both 1/4" jacks could you use it as a volume pedal?

    2 replies

    I don't have a Boss PS-5 so I can't say for sure, but it probably will work. Yes, you could do that, but you'd probably want to use a different value pot more like those in a volume pedal, e.g. a 250k or 500k pot.

    Actually the Roland expression pedal uses a 10k pot. I built one using your instructable and it works great. Thank you.

    What a waste of a Vox pedal man. You could have sold this one and just purchased an expression pedal. :[

    2 replies

    I don't see it as a waste at all. I have a couple of expression pedals, they just don't look as cool as the Vox. Since it will control the wah effect in my processor (which sounds way better than the wah circuit) what have I lost? This mod is also easily reverseable in 5 minutes in case I ever decide to sell it (which I'd never do, since it is an original V846 model from the late '60s).

    If I hooked this up to my computer + guitar, would it work with software such as guitar rig?

    1 reply

    thanks for the input. Sorry about the frames, I know they gotta go. gmoon is correct that it is an expression pedal and it sends no midi data on it's own. My particular application uses the pedal for the continuous controllers in my Digitech PMC10 programmable controller pedalboard. It plugs into one of two CC ports on the PMC10 and can be assigned to control multiple parameters of just about any midi string including note on/off, on any channel not just channel 11 (expression pedal channel). My rack mounted preamp/effects processor (Rocktron Chameleon) allows controlling up to eight parameters in real time. By mapping controller assignments between the PMC10 and the Chameleon, I can for example, use any channel on the PMC10 that sends min/max or on/off messages. So yea gmoon, the "controller" is really my pedalboard, the pedal is just an expression pedal. I stand corrected!

    4 replies

    cool, i'm actually in need of a couple of expression pedals. i could use a pair for my boss rc-50 and yet another for my alesis qs6 keyboard. i'm under the impression, looking for malfunctioning wah pedals is a good place to start since we're basically scrapping all the guts. fair assessment?

    You are indeed correct sir. Beer soaked wah pedals are cheap ;o) All you need to salvage besides the housing ,is the rack n pinion gear.

    i just watched a broken dunlop go for $20 plus shipping.....?.....? i've seen working ones go for that.

    oh, and do you really mean "threaded post" or do you mean a pot like this...

    radio shack