Checks the serial monitor for incoming values to signal whether to turn an LED on or off

p5_to_Arduino.INO

int LEDpin = 4;
int incomingByte;

void setup() {
  pinMode(LEDpin, OUTPUT);
  Serial.begin(9600);
}

void loop() {
  if (Serial.available() > 0) {
    incomingByte  = Serial.read();
    if (incomingByte == 'H') {
      digitalWrite(LEDpin, HIGH);
    } else {
      digitalWrite(LEDpin, LOW);
    }
  }
}