the mk2 pcb features 8 breakout pins on the left side of the board, for easy access to do your own modifications.
the pins give you access to ground, 3.5 vcc, 4 analog pins (a6,a7,a8,a9) and 2 digital pins located on the teensy. due to my mistake the two digital pins are not as labeled on the pcb but the pins 12 and 13 instead of 11 and 12.
through those you can add your own components or hook up the expansion board.
the expansion board adds 6 more rotary potentiometers on board and a single push button, with the same layout as the main board. also through an cd4067 analog multiplexer chip it breaks out 10 more analog inputs our outputs you could use for any extensions of your own.
the sequencer expansion adds a 4 character 7 segment display and two more buttons. it allows easy access to patterns voices and editing without changing pages. also shows note, velocity values, bpm, chords etc. its based on two 74hc595n shift register chips, the ph qdsp-6064 bubble display, four 4.7 ohm resistors and three 12mm tactile buttons.
i made a small mistake in the wiring of the display and you have to cut three traces, and add a jumper wire like in the picture below (right click view image for full size). also note that the display has a white dot on the underneath side, it should be facing the dip on the pcb layout.
here's a simple diagram that shows how to add more photometers.