Tilt alarm on an Arduino Uno

// Tilt alarm: knock it over and it shouts until you press the big button.
//
//   pin 2  the tilt switch, other leg to GND, internal pull-up
//   pin 3  the arcade button, other leg to GND, internal pull-up
//   pin 8  the buzzer, other leg to GND
//   pin 9  the lamp, through 220 ohm to GND
//
// A tilt switch is a metal ball in a can. Upright, the ball rests on both
// contacts and the pin reads LOW; tipped past about fifteen degrees, the ball
// rolls off and the pull-up takes the pin HIGH. That is the whole sensor, and
// the ball rattles for a good few milliseconds as it lands, which is why the
// sketch waits for the reading to hold before it believes it.
//
// # Latching
//
// The alarm does not stop when the box is stood back up. It has seen a tip,
// and it keeps sounding until somebody presses the button, which is what an
// alarm is for: the point is that it happened, not that it is still
// happening. Click the switch on the canvas to tip it, then press the button
// to reset.

const int TILT = 2;
const int BUTTON = 3;
const int BUZZER = 8;
const int LAMP = 9;

const unsigned long SETTLE_MS = 50;
const unsigned long BEEP_MS = 150;

bool alarm = false;
int lastRaw = LOW;
unsigned long changedAt = 0;
int tilt = LOW;

void setup() {
  pinMode(TILT, INPUT_PULLUP);
  pinMode(BUTTON, INPUT_PULLUP);
  pinMode(BUZZER, OUTPUT);
  pinMode(LAMP, OUTPUT);
  digitalWrite(BUZZER, LOW);
  digitalWrite(LAMP, LOW);
  Serial.begin(115200);
  Serial.println("Mokxi Uno: tilt alarm armed. Tip the switch.");
}

void loop() {
  // Debounce: a reading counts once it has held for SETTLE_MS.
  int raw = digitalRead(TILT);
  if (raw != lastRaw) {
    lastRaw = raw;
    changedAt = millis();
  } else if (raw != tilt && millis() - changedAt >= SETTLE_MS) {
    tilt = raw;
    if (tilt == HIGH && !alarm) {
      alarm = true;
      Serial.println("TILT! alarm on");
    }
  }

  if (digitalRead(BUTTON) == LOW && alarm) {
    alarm = false;
    digitalWrite(BUZZER, LOW);
    digitalWrite(LAMP, LOW);
    Serial.println("reset");
    while (digitalRead(BUTTON) == LOW) {
      delay(10);
    }
  }

  if (alarm) {
    int on = (millis() / BEEP_MS) % 2 == 0 ? HIGH : LOW;
    digitalWrite(BUZZER, on);
    digitalWrite(LAMP, on);
  }
  delay(5);
}