I'm working on a college project building analog guitar effects. I want to build an auto wah. It's essentially a variable resonant frequency band pass filter. Traditional auto wah's either use a low frequency oscillator for a constant 'sweep' across the filter's frequency band or use an envelop follower to gauge the amplitude of the input signal and produce a proportional sweep across the frequency range of the filter. The problem is that I cant figure out how to create the frequency sweep. I know I could look at the already existing circuit diagrams and just reproduce the effect, but that defeates the purpose of the exercise. Plus, something like this shouldnt be too difficult to do. What I have so far is this: As this effects is a variable frequency band pass filter, if I just change the resonant frequency of the filter, I should, theoretically, get the effect I'm after. This much I can do. The part that I cant figure out is how to automatically change teh resonant frequency depending on the input signal. One way I thought of doing it was to change the inductance or capacitance of an LC filter to change the resonant peak. What I need is a way to change the capacitance of the capacitive element based on the input voltage or current. So a current or voltage controlled capacitor. What device should I use? I'll be using TL 084 op amps and 7809 IC's for the power supply. Also, I'll be using a gyrator circuit for the inductive elements, which is why I need to use a voltage or current controlled capacitor. I hope this makes sense.