void setup(){
    pinMode(13,OUTPUT);
  }
  int degrees = 90;
  int value2;
  void loop(){
    int value1 = analogRead(0);
    int diff = value2 - value1;
    degrees += diff;
    degrees = constrain(degrees, 0, 180);
    servoMove(13,degrees);
    value2 = value1;
  }
  void servoMove(int pin, int angle){
    int pulseWidth = (angle*11)+500;
    digitalWrite(pin,HIGH);
    delayMicroseconds(pulseWidth);
    digitalWrite(pin,LOW);
    delay(20);
  }