Vibration alarm: a spring switch on pin 3, a buzzer on 13, on an Arduino Uno

// A vibration alarm: a shake fires an interrupt and the buzzer chirps.
//
//   SW-18010P spring switch: pin 3 -> switch -> GND   (INPUT_PULLUP is its pull-up)
//   pin 13 -> 1k -> an NPN transistor's base; the active buzzer from 5V
//   to its collector; emitter -> GND
//
// Pin 3 is one of the Uno's two interrupt pins. attachInterrupt() asks the
// chip to call shaken() the instant the pin changes, whatever loop() is
// doing, so even a short knock is never missed. shaken() only sets a flag;
// loop() sees it and sounds the buzzer.
//
// Press knock on the switch.

const int BUZZER = 13;
const int SENSOR = 3;

volatile bool knocked = false;

void shaken() {
  knocked = true;
}

void setup() {
  pinMode(BUZZER, OUTPUT);
  pinMode(SENSOR, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(SENSOR), shaken, CHANGE);
  Serial.begin(9600);
  Serial.println("Mokxi Uno: knock the sensor on pin 3");
}

void loop() {
  if (knocked) {
    knocked = false;
    Serial.println("vibration!");
    digitalWrite(BUZZER, HIGH);
    delay(50);
  } else {
    digitalWrite(BUZZER, LOW);
  }
}