Add an Expression Pedal to Any Effect
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Paul C. writes:

I am relatively new at effects building, but I was wondering if I could create an expression pedal for my Dunlop tremolo using a dead wah or volume pedal's pot and the stereo jack input/output as can be seen on the LERA diagram? This(or something like it) seems to me like it work, but I AM still new/ignorant about certain things. Is this idea total crap?

Not total crap - in fact, it's a good idea. Let's look at the general case, modding pedals with a remote rocker pedal control for any one knob. 

Let's do practicalities first. If this is going to get used, we have to have it easy to paste into a pedal and remove, and have it not mess up the function of the basic effect, nor be a pain to set up. So what we'd like is to have the rocker control connect to the pedal by a plug-in cable, and have the pedal work normally without the rocker cable plugged in. A stereo jack offers three contacts (ground and two ground-isolated signal contacts) and a closed circuit stereo jack offers the ability to short the internal  speed pot to the contacts. There's enough switching and contacts there to make it work if the speed control is either three terminal with one terminal to ground, or wired as a variable resistor and no terminals to ground. This will be the case for most effects pedals - the controls usually are series variable resistors, or three-terminal pots that have one connection to ground.

There is a third possible hookup for a speed control in the pedal that uses all three pot terminals but the third terminal does not go to ground. That would be more difficult, and we'd have to use the "advanced" method of controlling the effect - an LED/LDR variable.

Another practicality is that you want the cable that connects the pedal being controlled to the rocker doing the controlling to be different from your normal guitar cords so they won't get confused. This is a good reason to use 1/8" stereo jacks instead of 1/4" stereo jacks - the 1/8" plugs are not likely to be mistaken for your usual cords!

So we know the basics of what we need - a rocker pedal assembly, two 1/8" stereo jacks and matching plugs, a shielded, twisted pair cable long enough to go from where the pedal will be to where the rocker will be, and then some tools and time.

We will also need a pot to be rocked. Before making any other changes, open up the pedal and find the control pot that you want to work remotely; then using an ohmmeter, figure out whether it's linear, audio, or reverse audio taper, and what the total pot resistance value is. You then buy a pot for the  remote pedal that mechanically fits the rocker pedal mechanics and that either has the correct resistance taper, or can be modified by tapering resistors (See "The Secret Life of Pots") to have the right taper. Then you rig up the mechanics so that rocking. It is by no means certain that you will be able to find the resistance you need in a mechanical size that fits your rocker pedal, so figure out the pot needed and actually obtain it - yes, have it in your hand! - before drilling holes into your pedal. 

Once you have your pot and rocker assembly ready, start the mods. Install a 1/8" closed circuit stereo jack in your pedal. Take the wires that went to the pot loose, and connect them to the *signal lugs* on the 1/8" jack. Take two new wires from the switched contact on the stereo jack, and connect these to the in-pedal pot's non-grounded contacts. Set up like this, when there is no plug in the jack, the pot in the pedal is still connected as it used to be, and works normally. When the plug is inserted into the jack, the contacts open and the pot in the rocker pedal is connected instead. 

At the rocker, note from the pedal which signal contact (tip or ring) connects to which pot contact, and wire the rocker up so that the pot in the rocker is connected the same way that the pot in the pedal is. Now make a cable with a stereo plug on each end that matches the jacks you've used. As I said,  I recommend the 1/8" size so it won't get mixed up with normal guitar  cords. Verify that the pedal works correctly without the cord plugged in, then test  out the pedal with cord and pedal. You might have to mess with the rotation direction in the rocker to get the right control sense on the rocker. Oughta work just fine!!