Tilt switch on an Arduino Uno

// Tilt switch: a ball in a tube that closes a contact while it stands up.
//
//   pin 2   one leg of the tilt switch, the other leg to GND
//   pin 13  the LED, through 220 ohm to GND
//
// Standing upright, the ball rests on the two contacts and joins them, so pin
// 2 is pulled to ground and reads LOW. Tip it over and the ball rolls away,
// the contact opens, and the internal pull-up brings the pin back HIGH. The
// LED lights while the switch is tipped.
//
// A rolling ball bounces on the contacts for a few milliseconds, so the sketch
// only believes a new state once it has held for 50 ms.

const int TILT = 2;
const int LED = 13;
const unsigned long SETTLE_MS = 50;

int stable = LOW;
int last = LOW;
unsigned long changedAt = 0;

void setup() {
  pinMode(TILT, INPUT_PULLUP);
  pinMode(LED, OUTPUT);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: tilt switch");
  stable = digitalRead(TILT);
  last = stable;
}

void loop() {
  int now = digitalRead(TILT);
  if (now != last) {
    last = now;
    changedAt = millis();
  }
  if (now != stable && millis() - changedAt >= SETTLE_MS) {
    stable = now;
    digitalWrite(LED, stable == HIGH ? HIGH : LOW);
    Serial.println(stable == HIGH ? "tipped over" : "upright");
  }
}