You may think you are lucky when you have multiple solutions to a problem and you can pick, but you are more lucky when there is a solution to many problems! This week I stumbled upon an old solution in a new place. The project is to build a gorgeous old MIDI guitar, by Tryndelka [Aleksandr Goltsov]. And the old solution? Change the matrix diode.
You see [Aleksandr] Creating an electric guitar where the strings are pulled up to a certain voltage and then contacted with a metal freight. Each fret is cut into six pieces, so that the strings can be read separately, and the microcontroller scans each string one by one to see if it is pressed. Done, isn’t it? Wrong! The problem arises when two or more strings are pressed at once – the electrical path from the string you want will travel through the off switch of a string that you are not scanning. The solution is a ton of diodes.
I learned this problem from the hard way of wiring the MAME cabinet, about 3 o’clock the night before we brought it to Schmuck. We’ve finally got the whole USB / button code working, so we’ve played some celebration rounds on Street Fighter. We’ve finally noticed that hitting one button, or even moving the joystick in a certain direction, will block some other buttons from working, or completely change their effectiveness. Quickly search the internet later, and we were soldering 64 diodes by hand until dawn. Good time!
But the need for a switch matrix diode, and exactly why, is burning in my brain forever. It’s fun to see pop ups in all sorts of contexts, from DIY keyboards to MIDI guitars to charlieplexing. (It’s “D” on the LED!) It’s one of the classics – the solution to many problems.