Police lights and siren on an Arduino Uno

// Police lights and siren: two red LEDs, two blue LEDs, a piezo and one
// button on an Arduino Uno.
//
//   red LEDs    pins 12 and 11, each through 220 ohms to GND
//   blue LEDs   pins 10 and 9, each through 220 ohms to GND
//   button      pin 2 and GND (INPUT_PULLUP: a press reads LOW)
//   piezo       pin 7 and GND
//
// Press the button to step through four modes:
//
//   wail    the siren rises from 650 to 1500 Hz and falls back, about every
//           four seconds, while the lights flash
//   yelp    the same sweep, but fast: eight times a second
//   lights  the lights flash and the siren is quiet
//   off     everything off
//
// # Two things at once, without delay()
//
// The lights and the siren each keep their own clock with millis(). Every
// time loop() goes round it asks "is it time for the next flash?" and "is it
// time to move the siren's pitch?", does whichever is due and moves on. With
// delay() the board could only do one of them at a time, and the button would
// be ignored while it waited.

const uint8_t RED[2] = {12, 11};
const uint8_t BLUE[2] = {10, 9};
const uint8_t BUTTON = 2;
const uint8_t PIEZO = 7;

enum Mode { WAIL, YELP, LIGHTS, OFF };
const char *const MODE_NAME[4] = {"wail", "yelp", "lights only", "off"};
Mode mode = WAIL;

const unsigned int LOW_HZ = 650;
const unsigned int HIGH_HZ = 1500;

// The flash pattern: red, red, red, then blue, blue, blue, 60 ms each flash.
// Even steps are on, odd steps are off.
uint8_t flashStep = 0;
unsigned long flashAt = 0;

// The siren: a pitch that climbs and falls, moved every 10 ms.
unsigned long sirenAt = 0;
unsigned long sirenStarted = 0;

bool lastButton = HIGH;
unsigned long buttonAt = 0;

void setLights(bool red, bool blue) {
  for (uint8_t i = 0; i < 2; i++) {
    digitalWrite(RED[i], red ? HIGH : LOW);
    digitalWrite(BLUE[i], blue ? HIGH : LOW);
  }
}

void updateLights(unsigned long now) {
  if (mode == OFF) {
    setLights(false, false);
    return;
  }
  if (now - flashAt < 60) return;
  flashAt = now;
  flashStep = (flashStep + 1) % 12;
  bool on = flashStep % 2 == 0;
  bool redTurn = flashStep < 6;
  setLights(on && redTurn, on && !redTurn);
}

void updateSiren(unsigned long now) {
  if (mode != WAIL && mode != YELP) {
    noTone(PIEZO);
    return;
  }
  if (now - sirenAt < 10) return;
  sirenAt = now;
  // One cycle up and back down: 4000 ms for the wail, 125 ms for the yelp.
  unsigned long cycle = mode == WAIL ? 4000 : 125;
  unsigned long t = (now - sirenStarted) % cycle;
  unsigned long half = cycle / 2;
  // 0 at the bottom of the sweep, `half` at the top.
  unsigned long rise = t < half ? t : cycle - t;
  unsigned int hz = LOW_HZ + (unsigned long)(HIGH_HZ - LOW_HZ) * rise / half;
  tone(PIEZO, hz);
}

void checkButton(unsigned long now) {
  bool reading = digitalRead(BUTTON);
  // A press is a fall from HIGH to LOW; 50 ms of quiet after one ignores the
  // bounce of the contacts.
  if (reading == LOW && lastButton == HIGH && now - buttonAt > 50) {
    mode = (Mode)((mode + 1) % 4);
    sirenStarted = now;
    Serial.print("mode: ");
    Serial.println(MODE_NAME[mode]);
  }
  if (reading != lastButton) buttonAt = now;
  lastButton = reading;
}

void setup() {
  Serial.begin(115200);
  for (uint8_t i = 0; i < 2; i++) {
    pinMode(RED[i], OUTPUT);
    pinMode(BLUE[i], OUTPUT);
  }
  pinMode(BUTTON, INPUT_PULLUP);
  pinMode(PIEZO, OUTPUT);
  Serial.println("Police lights and siren. Press the button to change mode.");
  Serial.print("mode: ");
  Serial.println(MODE_NAME[mode]);
}

void loop() {
  unsigned long now = millis();
  checkButton(now);
  updateLights(now);
  updateSiren(now);
}