const int TRIG_PIN   = 6; 
const int ECHO_PIN   = 7; 
const int BUZZER_PIN = 3; 
const int DISTANCE_THRESHOLD = 50;

float duration_us, distance_cm;

void setup() {
  Serial.begin (9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(BUZZER_PIN, OUTPUT);
  
}

void loop() {
  
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  duration_us = pulseIn(ECHO_PIN, HIGH);
  
  distance_cm = 0.017 * duration_us;

  if(distance_cm < DISTANCE_THRESHOLD)
  {
    digitalWrite(BUZZER_PIN, HIGH); 
  }
  else
  {
    digitalWrite(BUZZER_PIN, LOW);  

  }
  Serial.print("distance: ");
  Serial.print(distance_cm);
  Serial.println(" cm");

  delay(500);
}