int distanceThreshold = 0;
int cm = 0;
int inches = 0;

long readUltrasonicDistance (int triggerPin, int echoPin)
{
  pinMode (triggerPin, OUTPUT);// Clear the trigger
  digitalWrite (triggerPin, LOW);
  delayMicroseconds(2);

  // Sets the trigger pin to HIGH state for 10 microseconds
  digitalWrite (triggerPin, HIGH);
  delayMicroseconds(10);
  digitalWrite (triggerPin, LOW);
  pinMode (echoPin, INPUT);
  // Reads the echo pin, and returns the sound wave travel time in microseconds
  return pulseIn (echoPin, HIGH);
}
void setup()
{
  Serial.begin(9600);
  pinMode(3, OUTPUT);
  pinMode(4, OUTPUT);
  pinMode(5, OUTPUT);
}
void loop()
{

  // set a threshold distance to activate LEDs
  //considering the features of ultrasonic sensor
  distanceThreshold = 60;

  // measure the ping time in cm, 340m/s=0.034cm/µs, therefore 0.034/2=0.017 mainly as the signal is working as echo.
  cm = 0.017 * readUltrasonicDistance(7, 6);

  inches = (cm / 2.54);
  Serial.print(cm);
  Serial.print("cm, ");
  Serial.print(inches);
  Serial.println("in");

  if (cm > distanceThreshold) {
    digitalWrite(5, LOW);
    digitalWrite(4, LOW);
    digitalWrite(3, LOW);
  }
  else if (cm < distanceThreshold && cm > distanceThreshold - 20 ) {
    digitalWrite(5, HIGH);
    digitalWrite(4, LOW);
    digitalWrite(3, LOW);
  }
  else if (cm < distanceThreshold - 20 && cm > distanceThreshold - 30 ) {
    digitalWrite(5, HIGH);
    digitalWrite(4, HIGH);
    digitalWrite(3, LOW);
  }
  else if (cm < distanceThreshold - 30 && cm > distanceThreshold - 50 ) {
    digitalWrite(5, HIGH);
    digitalWrite(4, HIGH);
    digitalWrite(3, HIGH);
  }
  delay(100);
}