const int TRIG_PIN = 8;  
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()  
{ 
  // generate 10-microsecond pulse to TRIG pin 
  digitalWrite(TRIG_PIN, HIGH); 
  delayMicroseconds(10); 
  digitalWrite(TRIG_PIN, LOW); 

  // measure duration of pulse from ECHO pin 
  duration_us = pulseIn(ECHO_PIN, HIGH); 
  

  distance_cm = 0.017 * duration_us; 
  
  if(distance_cm < DISTANCE_THRESHOLD) 
    digitalWrite(LED_PIN, HIGH); // turn on LED 
  else 
    digitalWrite(LED_PIN, LOW);  // turn off LED 
  // print the value to Serial Monitor 
  Serial.print("distance: "); 
  Serial.print(distance_cm); 

 
  Serial.println(" cm"); 
  delay(500); 
}