#include <Servo.h>

Servo sg90;

int led = 6;
int echo = 5;
int trig = 4;
int dist = 0;
int zmn = 0;

void setup() {

  pinMode (led, OUTPUT);
  pinMode (trig, OUTPUT);
  pinMode (echo, INPUT);

  sg90.attach (3);
}

void loop() {

  for (int i = 0; i <= 180; i++)
  {
    sg90.attach (3);
    digitalWrite (led, LOW);
    sg90.write (i);
    delay (15);

    digitalWrite (trig, HIGH);
    delay(1);
    digitalWrite (trig, LOW);
    zmn = pulseIn (echo, HIGH);
    dist = (zmn / 2) / 28.5;

    if (dist < 50)
    {
      digitalWrite (led, HIGH);
      sg90.detach();
    }
  }
  for (int i = 180; i >= 0; i--)
  {
    sg90.attach (3);
    digitalWrite (led, LOW);
    sg90.write (i);
    delay (15);

    digitalWrite (trig, HIGH);
    delay(1);
    digitalWrite (trig, LOW);
    zmn = pulseIn (echo, HIGH);
    dist = (zmn / 2) / 28.5;

    if (dist < 50)
    {
      digitalWrite (led, HIGH);
      sg90.detach ();
    }
  }
}