Expressiv MIDI guitar implementation
It may be a lot simpler than you think. Joe Brown has hacked a cheap guitar and is using resistance measurement to read the first point of contact on each string.

 

Figures 1 &amp; 2. A constant current is fed from the bridge to the top nut on each string. All the frets are grounded.







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 3. Basic layout.

My understanding is:


A constant current is driven through each string from the bridge to the top-nut. The strings are grounded at the nut (a zero-fret or metal nut would be simpler than the arrangement of Figure 1, I suspect) and all the frets are grounded.
A 4-wire measurement is performed to eliminate voltage drop along the current carrying wires. It's not clear how this is done at the nut.
A calibration scan is performed on power-up to measure the bridge voltage on each string.
The fretted position can now be read from the bridge voltage reading and a lookup or calculation performed to get the fret position.
I assume that bends can be accommodated as the resistance and length (and cross-section?) of the string will change with the stretch.


The amazing thing is that it appears to work. I think he has published the design and there is a lot of interest on the DIY stomp boxes forum.