#define TRIG_PIN 5
#define ECHO_PIN 18
#define LED_PIN 2

long duration;
int distance;

void setup() {
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);

  Serial.begin(115200);
}

void loop() {
  // Send pulse
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);

  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Read echo
  duration = pulseIn(ECHO_PIN, HIGH);

  // Convert to cm
  distance = duration * 0.034 / 2;

  Serial.print("Distance: ");
  Serial.println(distance);

  // Condition
  if (distance < 21) {
    digitalWrite(LED_PIN, HIGH); // Light ON
  } else {
    digitalWrite(LED_PIN, LOW);  // Light OFF
  }

  delay(200);
}