Motion alarm siren on an Arduino Uno

// Motion alarm siren: an HC-SR501 PIR on pin 2 trips a 12 V siren, which a
// D4184 MOSFET module on pin 8 switches from a 12 V adapter.
//
//   2       the PIR's OUT (its VCC on 5V, GND on GND)
//   8       the module's TRIG, its GND shared with the Uno
//   12 V    the adapter into the module's VIN+ and VIN-
//   siren   red wire on OUT+, black on OUT-
//
// The siren has its own oscillator: 12 V across it and it wails, at about
// 300 mA, so the pin only switches the MOSFETs' gates. Once tripped it sounds
// for ten seconds, then the alarm waits for the PIR to go quiet and re-arms.
//
// Press Run, then press the PIR's motion button (or drag someone in front of it).

const int PIR_PIN = 2;
const int SIREN_PIN = 8;
const unsigned long SOUND_MS = 10000;

bool sounding = false;
unsigned long since = 0;

void setup() {
  Serial.begin(115200);
  Serial.println("Mokxi Arduino Uno: motion alarm siren");
  pinMode(PIR_PIN, INPUT);
  pinMode(SIREN_PIN, OUTPUT);
  digitalWrite(SIREN_PIN, LOW);
  Serial.println("armed");
}

void loop() {
  unsigned long now = millis();
  if (!sounding && digitalRead(PIR_PIN) == HIGH) {
    sounding = true;
    since = now;
    digitalWrite(SIREN_PIN, HIGH);
    Serial.println("motion: siren on");
  }
  if (sounding && now - since >= SOUND_MS) {
    digitalWrite(SIREN_PIN, LOW);
    Serial.println("siren off, waiting for quiet");
    while (digitalRead(PIR_PIN) == HIGH) {
      delay(50);
    }
    sounding = false;
    Serial.println("armed");
  }
  delay(20);
}