I've made a little circuitboard to attach to the analog input sockets on the Bela cape, which contains the circuit for all 7 piezos.
I have cut some side panels for the on/off buttons, audio in/out, etc.
I still need to solder some cables to the master volume potentiometer and the rotary encoders. (I'm testing them with a breadboard first today)
The idea for having detachable side panels is to allow for every user to customise the amount of control and the type of control they have over the sound of the instrument. One panel could have 4 knobs to control the envelope, another panel could have a soft potentiometer to control effects, maybe even more exotic controls like an LSR or an ultrasound proximity sensor. At the moment, I only have 1 analog input left (which I am using for the master gain), but some of the digital pins can be used. When I receive my multiplexer capelet in June, things are going to get even more interesting.
At the moment, this is what the stripboard with the four MPR121 boards looks like. I haven't designed a way to mount this to the base yet, but all of those heavy ribbon cables keep it from moving.
Here is the small circuitboard attached to the analog input sockets. The positive leads from the piezos are in between a resistor that goes to 3.3V and a larger value resistor that goes to ground.
Finally, here's what it looks upside down (look at those rubber feet). I can't wait to start playing with this!