Serial Control of an Arduino from http://www.smolloy.com/2015/12/serial-control-of-an-arduino/

SerialControlArduino.ino

const int ledPin = 11;
const int photoPin = A0;

int ledState = LOW;
int oldState = LOW;

String input = ""; // This will capture the serial input commands

void setup() {
  pinMode(ledPin, OUTPUT);
  pinMode(photoPin, INPUT);
  Serial.begin(115200); // Make sure the Serial commands match this baud rate
}

void loop () {
  if (oldState != ledState) { // Only write to the LED when necessary
    analogWrite(ledPin, ledState);
    oldState = ledState;
  }
  while (Serial.available()>0){
    char lastRecvd = Serial.read();
    if (lastRecvd == '\n') { // The end of the command has arrived
      switch (input[0]) {
        case 'W': // A write command has come in
          input = input.substring(1,input.length());
          ledState = constrain(input.toInt(), 0, 255);
          input = "";
          break;
        case 'R': // A read command has come in
          Serial.print("R ");
          Serial.print(analogRead(photoPin)+'0');
          Serial.print("\n");
          input = "";
          break;
        default:
          break;
      }
    }
    else { // Input is still coming in
      input += lastRecvd;
    }
  }
}