// Referenced by https://arduinogetstarted.com/tutorials/arduino-ultrasonic-sensor

#include <Servo.h>

const int TRIG_PIN = 0;
const int ECHO_PIN = 1;
const int SERVO_PIN = 9;

const int DISTANCE_THRESHOLD = 20;

Servo servo;

float duration_us, distance_cm;

void setup()
{
  Serial.begin(9600);
  pinMode(TRIG_PIN, OUTPUT);
  pinMode(ECHO_PIN, INPUT);
  servo.attach(SERVO_PIN);
  servo.write(0);
}

void loop()
{

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

  duration_us = pulseIn(ECHO_PIN, HIGH);

  distance_cm = 0.017 * duration_us;

  if (distance_cm < DISTANCE_THRESHOLD)
  {

    servo.write(180);
  }
  else
  {
    servo.write(0);
  }

  delay(500);
}