#define BUZZER_PIN 3

long _ms;

void setup() {
  pinMode(LED_BUILTIN, OUTPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  
  _ms = millis();
}


void loop() {
  digitalWrite(BUZZER_PIN, HIGH);
  delay(1);
  digitalWrite(BUZZER_PIN, LOW);
  delay(1);

  if (millis() - _ms > 3000) {
    digitalWrite(LED_BUILTIN, !digitalRead(LED_BUILTIN));
    _ms = millis();
  }
}