I'm building a MIDI guitar, using an Arduino, sound synthesising and wood.
To properly know which note I'm pressing, I built a voltage divider with 12 equal resistors (10K). One side is connected at +5V and the other end at GND, so each of the 6 strings connects the right tap to the ADC. I had just calculated the expected value w/ a bit of math considering I also needed a pullDown resistor(1M) and capacitor for every analog channel to keep the readings stable when no key is pressed.
When measuring with my multimeter the readings are stable, and there are variations of less than 0.02V, and since the difference from tap to tap is about 0.4V that's quite good.
The problem is when reading the 6 channels together: channels supposed to be pulled down to gnd read voltages caused by nearby channels, readings are shaky and similar errors.
This mess disappears on one channel when I connect the multimeter to that one. Instead of buying 6 multimeters, I figured I could ask you if someone has an idea on why this happens, my guess is on the pulldown resistor, but I'm not sure about Arduino's ADC; I'm also worried about having 6 wires half of a meter each so close, so any other idea on finding the keys is welcome.
@EdgarBonet was right, I used 12x220ohm resistors, and at first the results were absolute garbage, then I made it so the code would read each channel about 10 times, discard the readings, wait a millisecond and finally read and store the value. Now it is perfect. Thanks again.