const int motorPin = 13;

void setup() {
  Serial.begin(9600);
}

void loop() {
  if (Serial.available()) {
    char ch = Serial.read();

    if (isDigit(ch)) {
      int speed = map(ch, '0', '9', 0, 255);
      analogWrite(motorPin, speed);
      Serial.println(speed);
    } else {
      Serial.print("Unexpected character ");
      Serial.println(ch);
    }
  }
}