#define TRIG_PIN  5      // Pin TRIG HC-SR04
#define ECHO_PIN  18     // Pin ECHO HC-SR04 (melalui divider resistor)
#define RELAY_PIN 23     // Pin Relay IN

long duration;
float distance;

void setup() {
  Serial.begin(115200);

  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  pinMode(RELAY_PIN, OUTPUT);

  digitalWrite(RELAY_PIN, HIGH); // Relay OFF (aktif LOW biasanya)
}

void loop() {
  // Hantar pulse TRIG 10µs
  digitalWrite(TRIG_PIN, LOW);
  delayMicroseconds(2);
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Baca pulse ECHO
  duration = pulseIn(ECHO_PIN, HIGH);
  // Kira jarak (cm) = masa * kelajuan bunyi / 2
  distance = duration * 0.0343 / 2;

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

  // Kawal relay ikut jarak
  if (distance < 20.0 && distance > 2.0) { 
    digitalWrite(RELAY_PIN, LOW);  // Relay ON
    Serial.println("Relay ON");
  } else {
    digitalWrite(RELAY_PIN, HIGH); // Relay OFF
    Serial.println("Relay OFF");
  }

  delay(500); // Baca setiap 0.5s
}