Touch piano on an Arduino Uno

// Touch piano: twelve foil keys on an MPR121 and a piezo.
//
//   SDA, SCL  the MPR121 (address 0x5A), VIN and GND from the rails
//   pin 8     the piezo; its other leg to ground
//
// Each electrode is a key, an octave of semitones from middle C. The
// MPR121 does the touch sensing itself and hands back twelve bits; the
// sketch plays the lowest key held, and stops when no key is.
//
// The note frequencies are the equal-tempered ones rounded to the hertz,
// from 262 for C4 to 494 for B4.

#include <Wire.h>
#include "Adafruit_MPR121.h"

const int PIEZO = 8;
const unsigned int NOTES[12] = {262, 277, 294, 311, 330, 349, 370, 392, 415, 440, 466, 494};
const char *NAMES[12] = {"C", "C#", "D", "D#", "E", "F", "F#", "G", "G#", "A", "A#", "B"};

Adafruit_MPR121 cap = Adafruit_MPR121();
int playing = -1;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: touch piano");
  if (!cap.begin(0x5A)) {
    Serial.println("no MPR121: check SDA, SCL and power");
    while (1) delay(10);
  }
  Serial.println("ready: touch a key");
}

void loop() {
  uint16_t touched = cap.touched();
  int key = -1;
  for (int i = 0; i < 12; i++) {
    if (touched & (1 << i)) {
      key = i;
      break;
    }
  }
  if (key != playing) {
    playing = key;
    if (key < 0) {
      noTone(PIEZO);
      Serial.println("quiet");
    } else {
      tone(PIEZO, NOTES[key]);
      Serial.print("key ");
      Serial.print(key);
      Serial.print(": ");
      Serial.print(NAMES[key]);
      Serial.print("4, ");
      Serial.print(NOTES[key]);
      Serial.println(" Hz");
    }
  }
  delay(10);
}