const int TRIG_PIN = 22;
const int ECHO_PIN = 23;
const int LED_RED = 18;
const int LED_YELLOW = 19;
const int LED_GREEN = 21;

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

void loop() {
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);
  long duration_us = pulseIn(ECHO_PIN, HIGH);
  float distance_cm = 0.017 * duration_us;
  Serial.print("distance: ");
  Serial.print(distance_cm);
  Serial.println(" cm");
  delay(500);

  if (distance_cm <= 15) {
    digitalWrite(LED_RED, HIGH);
    digitalWrite(LED_YELLOW, LOW);
    digitalWrite(LED_GREEN, LOW);
  } else if (distance_cm <= 30) {
    digitalWrite(LED_RED, LOW);
    digitalWrite(LED_YELLOW, HIGH);
    digitalWrite(LED_GREEN, LOW);
  } else {
    digitalWrite(LED_RED, LOW);
    digitalWrite(LED_YELLOW, LOW);
    digitalWrite(LED_GREEN, HIGH);
  }
}