Tilt and vibration alert

/*
  13 — Tilt and vibration alert
  Board: NodeMCU V3 ESP8266 development board.  Parts: SW-520D tilt switch, buzzer module, tactile switch (acknowledge).

  The SW-520D is a ball in a sealed tube: upright, the leads connect; tilted,
  they part. It is a bare two-lead part with no onboard circuitry, so YOU
  supply the pull-up (the board's internal one) and read it like a button.
  Check for the SW-520D marking — it looks almost identical to a 100 uF capacitor.

  Wiring:
    Tilt switch: one lead -> D5, other lead -> GND   (INPUT_PULLUP: upright = LOW, tilted = HIGH)
    Buzzer:      VCC -> 3V3, GND -> GND, IO -> D0
    Button:      one leg -> D3, other -> GND          (press to acknowledge and silence)

  Watch for the CHANGE rather than the level, debounce it, and LATCH the alert
  on until someone acknowledges it — instead of beeping once and going quiet.
*/

const int TILT_PIN   = D5;
const int BUZZER_PIN = D0;
const int ACK_PIN    = D3;

const unsigned long DEBOUNCE_MS = 50;
int  stableTilt = LOW;
int  lastReading = LOW;
unsigned long lastChange = 0;
bool alert = false;

void setup() {
  Serial.begin(115200);
  pinMode(TILT_PIN, INPUT_PULLUP);
  pinMode(ACK_PIN, INPUT_PULLUP);
  pinMode(BUZZER_PIN, OUTPUT);
  stableTilt = lastReading = digitalRead(TILT_PIN);
  Serial.println("Armed. Tilt to trigger, press the button to acknowledge.");
}

void loop() {
  int reading = digitalRead(TILT_PIN);
  if (reading != lastReading) { lastChange = millis(); lastReading = reading; }
  if (millis() - lastChange > DEBOUNCE_MS && reading != stableTilt) {
    stableTilt = reading;                       // a real change, not ball rattle
    if (!alert) { alert = true; Serial.println("ALERT: moved"); }
  }

  if (alert && digitalRead(ACK_PIN) == LOW) {   // acknowledged
    alert = false;
    Serial.println("acknowledged, re-armed");
    delay(300);
  }

  // Latched alert: pulse the buzzer until acknowledged.
  if (alert) { digitalWrite(BUZZER_PIN, (millis() / 250) % 2); }
  else       { digitalWrite(BUZZER_PIN, LOW); }
}