#define TRIG_PIN  18 // ESP32 pin connected to Ultrasonic Sensor's TRIG pin
#define ECHO_PIN  19 // ESP32 pin connected to Ultrasonic Sensor's ECHO pin
#define LED_PIN   21 // ESP32 pin connected to LED's pin
#define DISTANCE_THRESHOLD 50 // centimeters

// variables will change:
float duration_us, distance_cm;

void setup() {
  Serial.begin (9600);       // initialize serial port
  pinMode(TRIG_PIN, OUTPUT); // set ESP32 pin to output mode
  pinMode(ECHO_PIN, INPUT);  // set ESP32 pin to input mode
  pinMode(LED_PIN, OUTPUT);  // set ESP32 pin to output mode
}

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)
    digitalWrite(LED_PIN, HIGH); // turn on LED
  else
    digitalWrite(LED_PIN, LOW);  // turn off LED

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

  delay(500);
}