The Mux (Multiplexer) Shield adds the capacity for up to 48 inputs or outputs on the Arduino and Arduino Mega.
not really my project, but, seen this on Mayhew Labs web site and it looked like a good test for the Fritzing program.
didn't bother with straightening the schematic out or making the beard board pretty, goal was to test the PCB program side and keep it close to the same foot print as the Arduino.
due to limits the file does show errors, and I used more VIA's than I wanted.
Info and code can be found on their web site.
if you try to put this together pay close attention to the bus bars over the Arduino USB connector as each pin is cut flush with the bottom of the PCB to allow for clearance.
I plan on reworking the PCB to be functional but at a cost of a larger footprint(bigger PCB then the Arduino)and bigger than Mayhew Labs MUX.