I remember reading a great explanation of how a guitar tuner works in the Electronics for Guitarists book by Denton J. Dailey.
There are different approaches to do it with arduino:
- The most simple could be applying a big gain to the signal to make it square type and then measuring the time the signal stays high and then change to low.
- I have also read
some people talking about doing FFT
so you can measure the main frequency of the signal.
Nobody did a pedal like this before but I reckon that is a great idea, if you star doing a quick search in google "guitar tuner arduino due" you can get some background and ideas to start with.