Frets and strings
Control and indications
Algorithm of note detection
All elements in the scheme are using +5 V power, because the microcontroller is designed for this voltage and MIDI cable requires it too.
Since strings are connected to microcontroller pins directly, i.e. guitarist touches the scheme, the measures must be taken to protect guitarist from electrical hazard. Therefore 9V battery or accumulator is used as a source of power. I had temptation to use +5 V line of computer game port as variant of power supply, but it is not good idea by the reason stated above. As for another source of danger - counterpart MIDI device - it should use PCP on MIDI-IN if it is a standard MIDI-IN, and I relying on manufacturers' conscience.
Linear stabilizer LM2950 provides conversion of 9V or so to stable 5V power supply.
Total consumption of the scheme does not exceed 20 mA (in case of high amplitude of oscillation of all six strings), normally it is less than 10 mA.