Now I have a bit more of a clear game plan it's time to assemble a box to test with.
I had an old black enclosure I'd bought for my Pi powered Guitar effects pedal that didn't end up getting used, so I've drilled in some holes for jack sockets and a hole for a power cable.
There are 4 jacks; 3 x stereo pad inputs and 1 x stereo output.
Inside the case I've added an Arduino Micro, mini breadboard and a breadboard power supply.
The input jacks will be having some wires soldered to them and attached to the arduino's analog pins and 5v rail via the breadboard; using a breadboard in this instance allows for the inputs to be free configured for different input devices easily.
The output jack will have a 3.5mm audio cable chopped and soldered onto it and plugged into a Pi 3, which will be mounted in the case very soon; quality doesn't need to be great for testing, but I have a justBoom amp/DAC HAT which would vastly improve the quality of the ouptut.
For the software I'm going to load Patchbox OS onto the Pi, as it is preconfigured for low latency audo performance, and can run Pure Data well.
Coding for the Arduino will be fairly simple; loop through analog pins and check the value. If it is >0 and greater than the previously checked value then a hit has been detected, so send a corresponding serial message for the pure data patch to then route and play the correct sample.