There are schematics on the web for various commercial wah wahs so why did I design my own? I don't know.
Once one knows what a wah wah does, it is not hard to design one. ... has a decent analysis of the Clyde McCoy wah wah on his web page. Trusting his analysis, I set about to create a wah wah using an op-amp. A second-order low pass filter with a quality factor around 1 or 2 and an adjustable break frequency is all there really is to it. The pedal changes the break frequency, and slighty changes the quality factor.