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);
}