Four-key keyboard on an Arduino Uno

// Four-key keyboard: four buttons share one analog pin through a ladder of
// resistors, and a piezo plays a note for each.
//
//   every button   one side to 5V
//   button 1       its other side straight to A0
//   button 2       through one 10k resistor to A0
//   button 3       through two 10k resistors in a row to A0
//   button 4       through three 10k resistors in a row to A0
//   A0             also a 10k resistor to GND
//   pin 8          the piezo, its other leg to GND
//
// Each button makes a voltage divider with the 10k at the bottom, and the
// further along the ladder it is, the more resistance sits above A0 and the
// lower the reading:
//
//   button 1   5V straight in          about 1023
//   button 2   10k over 10k            about 511
//   button 3   20k over 10k            about 341
//   button 4   30k over 10k            about 256
//
// One pin, four keys. The sketch sorts the reading into a band and plays that
// key's note for as long as it is held.

const int PIEZO = 8;
const int NOTES[4] = {262, 294, 330, 349};  // C4, D4, E4, F4
const char *NAMES[4] = {"C", "D", "E", "F"};

int lastKey = -1;

int keyFor(int reading) {
  if (reading > 1000) return 0;
  if (reading >= 450 && reading <= 570) return 1;
  if (reading >= 300 && reading <= 400) return 2;
  if (reading >= 220 && reading <= 290) return 3;
  return -1;
}

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: four-key keyboard");
}

void loop() {
  int reading = analogRead(A0);
  int key = keyFor(reading);
  if (key != lastKey) {
    lastKey = key;
    if (key < 0) {
      noTone(PIEZO);
    } else {
      tone(PIEZO, NOTES[key]);
      Serial.print("key ");
      Serial.print(key + 1);
      Serial.print(": ");
      Serial.print(NAMES[key]);
      Serial.print(" (reading ");
      Serial.print(reading);
      Serial.println(")");
    }
  }
  delay(10);
}