Motion-sensor security alarm

// Session 2 - Motion-sensor security alarm
int trigPin = 9;
int echoPin = 10;
int buzzerPin = 8;
int ledPin = 13;

int alarmDistance = 30;   // centimeters - anything closer sets off the alarm

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  pinMode(buzzerPin, OUTPUT);
  pinMode(ledPin, OUTPUT);
  Serial.begin(9600);     // lets us print to the Serial Monitor
}

// Send a ping and time the echo. Returns distance in centimeters.
long readDistanceCm() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);    // 10 microsecond "click"
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);

  long duration = pulseIn(echoPin, HIGH, 30000);  // microseconds until the echo comes back
  return duration * 0.034 / 2;    // sound moves 0.034 cm per microsecond, and it went there AND back
}

void loop() {
  long distance = readDistanceCm();

  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");

  if (distance > 0 && distance < alarmDistance) {
    // INTRUDER!
    digitalWrite(ledPin, HIGH);
    digitalWrite(buzzerPin, HIGH);
    delay(100);
    digitalWrite(buzzerPin, LOW);
    delay(100);
  } else {
    // all clear
    digitalWrite(ledPin, LOW);
    digitalWrite(buzzerPin, LOW);
    delay(100);
  }
}

Add the tilt ball switch: one leg → pin 2, other leg → GND. If someone picks up your alarm, it goes off. Add these pieces to the sketch above.

int tiltPin = 2;

// add to setup():
//   pinMode(tiltPin, INPUT_PULLUP);

// change the if line in loop() to:
//   if ((distance > 0 && distance < alarmDistance) || digitalRead(tiltPin) == HIGH) {

// The ball inside connects the legs when the switch is upright (LOW).
// Tilt it and the ball rolls away (HIGH). If yours is backwards, flip the switch over.