#define TRIG 6
#define ECHO 7
#define BUZZER 3

void setup() {
  pinMode(TRIG, OUTPUT);
  pinMode(ECHO, INPUT);
  pinMode(BUZZER, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  digitalWrite(TRIG, LOW);
  delayMicroseconds(2);

  digitalWrite(TRIG, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG, LOW);

  long duration = pulseIn(ECHO, HIGH, 30000);

  if (duration == 0) {
    Serial.println("No echo!");
    noTone(BUZZER);
    delay(100);
    return;
  }

  float distance = duration * 0.0343 / 2;

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

  if (distance < 402) {
    tone(BUZZER, 1000);
  } else {
    noTone(BUZZER);
  }

  delay(100);
}