#include <Servo.h>
#define SERVO_PIN 3
Servo servo;

int angle = 90;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(9600);
  Serial.print("\n");
  servo.attach(SERVO_PIN);
  servo.write(angle);
}

void loop() {
  // put your main code here, to run repeatedly:
  switch (Serial.read()) {
    case 'D':
    case 'd':
      if (angle > 0)
        angle -= 15;
      break;
    case 'A':
    case 'a':
      if (angle < 180)
        angle += 15;
      break;
  }
  servo.write(angle);
}