In her build nikoala3 shows you how you can take an AC signal and offset it using a voltage divider to bring it into the 0-5V range that the Arduinos Analog to Digital Convert (ADC) uses. The Arduino has a 10bit resolution but the project only uses the high 8 bits giving you a range of 0-255 counts. The sketch detects the frequency at which the signal crosses a given threshold. Different strings produce different amplitudes so you have to tweak the threshold to where it works with all the strings on your guitar.
A few LEDs and one cool enclosure with a laser cut top really made this project pop. There is something really refreshing about finding well documented builds like this. Check out more build photos after the break.