Light theremin on an Arduino Uno

// Light theremin: a phototransistor reads the light and a piezo plays it as a
// pitch, higher for more light. Move your hand over the sensor and it sings.
//
//   phototransistor   C -> 5 V   E -> A0, and 10 k from A0 to GND
//   piezo             one leg -> pin 8, the other -> GND
//
// A phototransistor is a transistor whose base current is light: the
// collector current follows the light, and the 10 k resistor turns it into a
// voltage the analog pin can read. It is not a resistor like an LDR, so in a
// bright room it saturates and reads near the top, and the useful range is
// from dark to a dim room. Drag the light slider down to "cover" it.
//
// tone() plays the note from a timer, so the loop keeps reading while the note
// sounds. It is only called again when the pitch moves: every call restarts
// the timer, and calling it every 10 ms with the same note would cut a low
// note's cycles short and raise its pitch.

const int SENSOR_PIN = A0;
const int PIEZO_PIN = 8;
const int LOW_PITCH = 120;    // hertz, in the dark
const int HIGH_PITCH = 1500;  // hertz, at full scale

unsigned long printedAt = 0;
int playing = 0;

void setup() {
  Serial.begin(9600);
  Serial.println("Light theremin: move your hand over the sensor");
}

void loop() {
  int light = analogRead(SENSOR_PIN);
  int pitch = map(light, 0, 1023, LOW_PITCH, HIGH_PITCH);
  if (pitch != playing) {
    tone(PIEZO_PIN, pitch);
    playing = pitch;
  }

  if (millis() - printedAt >= 250) {
    printedAt = millis();
    Serial.print("light ");
    Serial.print(light);
    Serial.print(" -> ");
    Serial.print(pitch);
    Serial.println(" Hz");
  }
  delay(10);
}