#include <Servo.h>
Servo servo1;

const int servoPin = 11;

const int echoPin = 12;
const int trigPin = 13;

float duration, distance;

volatile int pos = 90;

void setup() {

  Serial.begin(9600);

  pinMode(trigPin, OUTPUT);
  pinMode(echoPin, INPUT);

  servo1.attach(servoPin);
  servo1.write(pos);
  delay(1000);
}

void loop() {

  static unsigned long timer = millis();

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

  duration = pulseIn(echoPin, HIGH);
  distance = (duration * .0343) / 2;
  Serial.print("Distance: ");
  Serial.println(distance);
  delay(100);

  if (distance < 50.0) {
    while (pos < 180) {
      pos++;
      servo1.write(pos);
      delay(10);
    }
    timer = millis();
  }

  if (millis() - timer > 3000) {
    while (pos > 0) {
      pos--;
      servo1.write(pos);
      delay(10);
    }
  }
}