const int  TRIG_PIN = 15;
const int  ECHO_PIN = 2;
int  LED_RED    = 27;
int  LED_BLUE   = 26;
int  LED_YELLOW = 5;
int  LED_GREEN  = 18;

long duration;
float distanceCm;

void setup() {
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(LED_RED, OUTPUT);
  pinMode(LED_BLUE, OUTPUT);
  pinMode(LED_YELLOW, OUTPUT);
  pinMode(LED_GREEN, OUTPUT);
}

void loop() {
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  duration = pulseIn(ECHO_PIN, HIGH);
  distanceCm = duration * 0.034005/2;
  Serial.print("Distance (cm):  ");
  Serial.println(distanceCm);

  if (distanceCm < 50) {
    digitalWrite(LED_RED, LOW);
    digitalWrite(LED_BLUE, LOW);
    digitalWrite(LED_YELLOW, LOW);
    digitalWrite(LED_GREEN, LOW);  
  } else
  if (50 < distanceCm && distanceCm < 100){
    digitalWrite(LED_RED, HIGH);
    digitalWrite(LED_BLUE, LOW);
    digitalWrite(LED_YELLOW, LOW);
    digitalWrite(LED_GREEN, LOW);
  } else if (100 < distanceCm && distanceCm < 200){
    digitalWrite(LED_RED, HIGH);
    digitalWrite(LED_BLUE, HIGH);
    digitalWrite(LED_YELLOW, LOW);
    digitalWrite(LED_GREEN, LOW);
  } else if (200 < distanceCm && distanceCm < 300){
    digitalWrite(LED_RED, HIGH);
    digitalWrite(LED_BLUE, HIGH);
    digitalWrite(LED_YELLOW, HIGH);
    digitalWrite(LED_GREEN, LOW);
  } else if (distanceCm > 300) {
    digitalWrite(LED_RED, HIGH);
    digitalWrite(LED_BLUE, HIGH);
    digitalWrite(LED_YELLOW, HIGH);
    digitalWrite(LED_GREEN, HIGH);
  }
}