I am making a ton of Kosmo modules in quarantine, and I have been looking to build a nice design to create a simple expression pedal interface - I’m sure it couldn’t be terribly hard since expression pedals are basically just a potentiometer rigged to send and return on a TRS cable. In doing a little digging, I found this schematic from the late, great Ray Wilson of Music from Outer Space:
It’s for a joystick, but one could essentially replace the connections to the joystick potentiometer with the tip and sleeve connections on the expression pedal jack. Simple enough, and this schematic is for two axis, meaning you could have two expression pedal interfaces coming off the same TL072 op amp. Noice.
But here’s where my question comes in - I addition to modulating a DC voltage (which this module should do in it’s “normaled” configuration), I would also like the module to serve as an attenuator for an incoming control voltage signal, if one is plugged in. We a switching jack for the CV input, and when plugged in, the jack would need to break the connections between the + and - 12v rails and the pot, and instead instead send the incoming CV through the pot, to be attenuated and sent through the TL072. I haven’t fully worked out how this would work, but if anyone has ideas, I’d love to bounce something around, and come up with a stripboard layout to share. I think expression pedals are somewhat overlooked as a way to control parameters on a modular synth, but I bet we could come up with one that has a very minimal part count!