Systech Envelope Follower - Contains analysis of a VCVS bandpass filter auto wah circuit.
If you can take a filter and somehow electronically control it, then you've potentially got an "auto wah." You'll also need to take the incoming signal and convert the amplitude into the controlling voltage. This ac-to-dc conversion step is the same for auto wah as it is for compressors or "envelope" anything.
The Mu-Tron III is the original auto wah, first built in 1972. Many models of auto wah were produced in the 1970s. One nice thing about looking at auto wah and envelope filter schematics is that they are quite varied, and some very interesting circuits are used.
A growing list of popular auto wahs and what type of filter they used (and the control element):
Mu-Tron III - State Variable Filter (Optocoupler)
MXR Envelope Filter - Switched Capacitor Filter (uses a supersonic clock and analog switches)
Systech Envelope Follower - VCVS Bandpass Filter (JFETs)
Seamoon Funk Machine - Multiple Feedback Bandpass Filter (BJT)
Electro Harmonix Dr. Q - Multiple Feedback Bandpass Filter (BJT)
Moog Moogerfooger Low Pass Filter MF-101 - Unknown, possibly the famous Moog transistor ladder filter
Surely someone has a "Twin T" based auto wah. Just can't remember who at the moment.
The State Variable Filter allows for switching between the classic bandpass (like a regular wah) or low pass and high pass modes for more variety.
Sophisticated control circuits allow for the filter sweep to go up or down. There is usually a control to set the decay time of the follower circuit.