const int TRIG_PIN = 6;
const int ECHO_PIN = 7;
const int LED_PIN = 13;
const int DISTANCE_THRESHOLD = 50;

float duration_us, distance_cm;

void setup() {
  Serial.begin (9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(LED_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(LED_PIN, HIGH);

  else
    digitalWrite(LED_PIN, LOW);

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

  delay(500);

}