By the power of eBay, I have acquired this 13 key bass pedal, just what I needed for a bit of midi hacking. As it comes, this pedal is simply a set of thirteen switches. My plan is to  add a midi output to the back of the box.

Inside, each key is switched so that as the pedal is pressed it connects to horizontal common bus running across the full length of the pedal. ( you can see it running across the botton of the brass coloured section below.)

Each switch then has its own colour coded wire running in the bundle, top left of the picture above.

From the side, you can see the spring wire bending and touching on the bus rod as the pedal, to the left of the picture, is pressed.

Okay, so that’s the mechanics sorted out, next step, take the thirteen switches and turn them into a midi output. The Arduino, my choice for microprocessor comes with 14 digital inputs, one for each key and one for the output. Alternatively I might use an I/O expander chip such as the PCF8574AN then I’d have more inputs available for other switches  and cool stuff.