The MS20 is a excellent beginner filter because it sounds awesome and there is tons of documentation available for those interested in how it works. Generally when I implement a circuit, I like to find a few different version of it. The MS20 has been cloned about a thousands times and so there are a variety of schematics in which you can compare and contrast component values. I primarily used Tim Stinchcombe's page but I printed and consulted tons of other MS20 filter schematics available online.
There are only a few things which need to be changed in order to switch the filter between lowpass and hi pass functions. I implemented a switch to move between the two functions. The lowpass filter output volume is much lower than the hi pass filter volume so I also incorporated a gain circuit which switches with the filter function.
In retrospect, my method for switching the gain circuit is pretty sloppy and wasn't really seeing the whole circuit when I designed it. The next version would switch the non-inverting buffer at the audio signal input to an inverting buffer, thus the gain amplifier (located at the end of the circuit) could also be inverting (returning it to the correct phase), then I would simply have it switch between two different "Rin" resistors to adjust the gain of the inverting amplifier. There is an audible bleep when switching between filter modes.
also, it is definitely NOT 1v/octave.
I got the boards made at OSH Park. This was one of my first projects in eagle cad. There are things I like about it better than photoshop and things I like less. Unfortunately, when a circuit board has repeating parts, it's next to impossible to copy and past PCB layouts.
Initially I was going to use a picture of Martin Sheen (MS pun) from Badlands on my panel but I couldn't find one that fit the panel layout. In the end, this picture of young Steve Martin (Still MS but backwards) won out. After all of the knobs and jacks are put in, it's hard to tell it Steve but it's still fun for me to know he's on there.
In my earlier modules, I've generally been pretty lazy and just done all my panel wiring in one color. My modules have become a lot complex and so I had to implement the multicolored "twist" method. Since doing so, I do enjoy the final "wire up" process a lot more. less stress.