#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); 
  }

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

  delay(500);
}