#3
I think this was discussed a while ago on here. Some guys thought that there was a positive current going to each fret and a negative current going through the bridge. When the strings connected with the frets, this closed the "switch" and turned the LEDs on, or vice verca.

//EDIT:

Here you go matey, found it.

https://www.ultimate-guitar.com/forum/showthread.php?t=1085060&highlight=led+fretboard
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Last edited by boxie at Jul 17, 2009,