#include <Servo.h>
//https://www.instructables.com/Distance-Measurement-Using-HC-SR04-Via-NodeMCU/

const int trigPin = 2;  
const int echoPin = 3;

void setup() {
  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);
  
  Serial.begin(9600);
}

void loop() {
  digitalWrite(trigPin, LOW);
  delayMicroseconds(2);
  digitalWrite(trigPin, HIGH);
  delayMicroseconds(10);
  digitalWrite(trigPin, LOW);
  
  long time = pulseIn(echoPin, HIGH);
  //D = 1/2 × T × C --> D is distance, T is time between the echo time, and C is the sonic speed of HC-sr04
  int distance = time * 0.034 / 2; //sonic speed is is 340 m/s = cm/µs. It is 0.034 cm/μs.

  //code to show distance on the screen. only used for testing.
  Serial.print("Distance: ");
  Serial.print(distance);
  Serial.println(" cm");
  Serial.print("time: ");
  Serial.print(time);
  Serial.println(" µs");
  delay(100);
}