Siren on a passive buzzer (pin 9), on an Arduino Uno

// A siren: a passive buzzer on pin 9, swept up and down with tone().
//
//   pin 9 -> the buzzer's + leg; the other leg -> GND
//
// A passive buzzer has no oscillator of its own, so it makes whatever note
// the board drives it with. tone(pin, frequency) plays a square wave at that
// frequency until the next tone() or noTone(). Stepping the frequency up by
// 10 Hz every few milliseconds, then back down, gives the wail of a siren.

const int BUZZER = 9;

void setup() {
  Serial.begin(9600);
  Serial.println("Mokxi Uno: a siren on the passive buzzer on pin 9");
}

void loop() {
  for (int freq = 100; freq <= 1000; freq += 10) {
    tone(BUZZER, freq);
    delay(5);
  }
  for (int freq = 1000; freq >= 100; freq -= 10) {
    tone(BUZZER, freq);
    delay(5);
  }
}