#include<stdio.h>
const int TRIG_PIN = 6;
const int ECHO_PIN = 7; 
const int LED_PIN  = 3; 
const int DISTANCE_THRESHOLD = 150; 

float duration_us, distance_cm , readString , c;

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);

   while (Serial.available()) {
    char c = Serial.read();  
    readString += c; 
    delay(2);  //slow looping to allow buffer to fill with next character
  }


  duration_us = pulseIn(ECHO_PIN, HIGH);

  distance_cm = 0.017 * duration_us;

  if(distance_cm < DISTANCE_THRESHOLD)
    digitalWrite(LED_PIN, HIGH); 
  else
  printf("\nEnter a value");
    digitalWrite(LED_PIN, LOW); 
  Serial.print("distance: ");
  Serial.print(distance_cm);
  Serial.println(" cm");

  delay(500);
}