MalcolmV8:
OK so I drew out what I think we're talking about here. The diagram on the left is all you're saying I need to do? Simply place a 47K resistor in line of the input like that and I'm protected? And this does not affect the 0 - 5 volts I'm reading? The Arduino is still going to see the same voltage as before?
Pretty much. Each input.
MalcolmV8:
Now if I was to add the Zener (which it sounds like you guys are saying is not required at all) it would be added like the diagram on top right or bottom right? I'm thinking top right.
Indeed. The resistor prevents too much current getting through to the Arduino either way, whereas your "bottom right" wiring would provide no protection whatsoever. The Zener is merely supplementing - by paralleling - the two diodes already in the Arduino.