#include <Servo.h>
//define our pins
#define SERVO_PIN 9
#define SERVO_MAX 180
#define SERVO_MIN 0
Servo servo;
//inital position
int ServoPos = SERVO_MAX;
//desired psition
int DesiredPos = SERVO_MIN;
//timer vars
unsigned long lastStep = 0;
int servoSpeed = 10;


void setup() {
  //attach servo to pin
  servo.attach(SERVO_PIN);
  //move to inital position
  servo.write(ServoPos);
  //set our new desired position..
  DesiredPos = ServoPos - SERVO_MAX;//back to 0
}

void loop() {
  //step to our new postion..
  //step returns true desired pos reached..
  if (ServoStep()) {
    //switch pos..
    if (DesiredPos == SERVO_MIN)
    {
      DesiredPos = SERVO_MAX;
    }
    else {
      DesiredPos = SERVO_MIN;
    }
  }
}


//step servo 1 setp at a time till we reach
//our final position, returns true if done..
bool ServoStep() {
  bool fin = false;
  //timer check
  if (millis() - lastStep >= servoSpeed) {
    lastStep = millis();
    //if we're not there yet..
    if (ServoPos != DesiredPos) {
      servo.write(ServoPos);//step
      if (DesiredPos < ServoPos)
        ServoPos--;//subtract 1
      else
        ServoPos++;
    } else {
      fin = true;
    }
  }
  return fin;
}