#include <Servo.h>

// Constants won't change
const int TRIG_PIN = D2; // Arduino pin connected to Ultrasonic Sensor's TRIG pin
const int ECHO_PIN = D1; // Arduino pin connected to Ultrasonic Sensor's ECHO pin
const int SERVO_PIN = D13; // Arduino pin connected to Servo Motor's pin
const int DISTANCE_THRESHOLD = 50; // centimeters

Servo servo; // Create servo object to control a servo
float duration_us, distance_cm; // Variables will change

void setup() {
  Serial.begin(9600); // Initialize serial port
  pinMode(TRIG_PIN, OUTPUT); // Set Arduino pin to output mode
  pinMode(ECHO_PIN, INPUT); // Set Arduino pin to input mode
  servo.attach(SERVO_PIN); // Attach the servo on pin D13 to the servo object
  servo.write(0); // Initialize servo position to 0 degrees (closed)
}

void loop() {
  // Generate 10-microsecond pulse to TRIG pin
  digitalWrite(TRIG_PIN, HIGH);
  delayMicroseconds(10);
  digitalWrite(TRIG_PIN, LOW);

  // Measure duration of pulse from ECHO pin
  duration_us = pulseIn(ECHO_PIN, HIGH);

  // Calculate the distance
  distance_cm = 0.017 * duration_us;

  if (distance_cm < DISTANCE_THRESHOLD) {
    // Rotate servo motor to 90 degrees (open)
    servo.write(90);
  } else {
    // Rotate servo motor to 0 degrees (closed)
    servo.write(0);
  }

  // Print the value to Serial Monitor
  Serial.print("Distance: ");
  Serial.print(distance_cm);
  Serial.println(" cm");

  delay(500);
}