Wah pedal with linkage instead of rack&pinion

[Seljer]

This might be just food for thought

I've been messing around with linkages in MATLAB for the past two days and have come to this:

a mechanism for interfacing the rocker to the potentiometer with a four bar linkage


Lots of tweaking is probably still needed if one were to bring it into the real world, but as visible, it's not that hard to get more than 180° of rotation on the pot. If you mess with the ratios of the links you can make the sweep bunch up at one end quite noticeably. I haven't calculated the angles but I figure you could get a nice truely smooth semi-audio type taper from just a linear pot.

Its also probably easier to DIY than the standard rack and pinion mechanism if anyone is feeling adventerous. Also easier to make (and maybe more durable) than the plastic cam based design on the Colorsound wahs.

[haveyouseenhim]

Have you thought of adding a linkage from the heel that engages when the front maxes out? I probably couldn't design it on computer but I'll take this idea out to the shop at some point.

[Ronan]

This DIY pedal uses a straight linkage like you suggest. The bracket connected to the treadle is like swiss cheese, to get some adjustment. It works fine. Depends also on the circuit and the pot taper. The one in the pic is using a Colorsound clone circuit. I measured just over 180 degrees on a standard wah pedal recently, 180 is enough most of the time.

[kaycee]

Most of the time for a wah application at least you don't really need the full sweep of the pot, there's a sweet spot you tend to hang around and the extremes tend not to be musically useful. If its something like a tremolo or expression pedal the I'd give it more mileage. As you hint, being able to mess with the taper using the linkage might well be more useful. I've never been sure if the colour sound cam linkage gives a linear rotation or not? BTW if you have a CS wah with the linkage is worn or broken you can get replacements from the Macari's shop/online business.

To make this a goer you'd need to figure a way of mounting using current rack or cam mounting positions, otherwise you'll need to make the whole shell. Perhaps more useful is a different way of varying the resistance, RG's method of using an LDR run from the pot is a step forward as it stops the problem of the scratchy pot. The really clever ones are those that actuate when you move the treadle eliminating the need for both the pot and pesky latching switch.

[joegagan]

hotpotz1 and 2 from dunlop have same taper.

same as a 72 - 80s crybaby pot, regardless of AB, claro, centralab.

they concentrate 100% of the R sweep into around 30 - 35% of the rotation. i am guessing this was done for ease of adjustment rather than tonal or expressiveness reasons. when the hot spot is in such a narrow range, you can get the entire wah action sorta wherever you set the gear!

[R O Tiree]

There was a thread on here in April 09 about the Colorsound Wah cam gearing. An awful lot of the pics in that thread have been moved/deleted/unobtainable as people have moved ISPs etc. Certainly mine are defunct from that thread, but I still have them in my DropBox:

This is what I hacked from Paul Marossy's AutoCad drawing:



And this is the Treadle Angle vs Pot Resistance plot that I got from it:



As you can see, it's the geometry of the cam gearing that gives us this S-shaped curve. It is obviously cheaper to source a linear pot than an S-taper one, and the cost of cam/lever vs rack/pinion will be more or less the same. What is also different is the range of resistance - As you'll see from that other thread, the CryBaby pot goes from 100 ohms all the way up to 98k whereas the ColorSound only goes from 50k up to 91k (there are small changes in the rest of the circuit to cater for this reduction in "throw")

As to that 3-lever arrangement at the top of this thread, the last 15-20 degrees of "toe-down" will not be achievable... the lever arms cannot be infinitely thin and neither can the pot shaft, so you're going to get interference. If you offset the hinges, so there is no interference, then the torque required to get the shaft to move from its close-to-"TDC" point will ultimately bend and/or break the hinges.

[PRR]

> messing around with linkages in MATLAB

http://archive.org/details/mechanicaltextbo00rankrich

BTW, your plan looks like a 3-bar linkage. And will bind somewhere barely-beyond 180 degrees of pot.

[R O Tiree]

> BTW, your plan looks like a 3-bar linkage. And will bind somewhere barely-beyond 180 degrees of pot.

...there's an echo in this thread somewhere...

[Ronan]

Here's some pics of the linkage in the pedal above. The top and bottom pics need to be rotated 90 degrees. The linkage is a large paper clip, bent to shape, with a dog-leg kink at each end to keep it located in the brackets. It is much smoother than any rack and pinion setup I've owned. Total rotation appears to be around 120 degrees, but could go more, it appears I only needed the 120 degrees. I built it about 14 years ago, and it still works except the pot is knackered.

[pjwhite]

Here's how I made my volume pedal.  A slide pot, a piece of string and two pulleys