String dato;
int led = 8;
void setup() {
  // put your setup code here, to run once:
pinMode(led, OUTPUT);
Serial.begin(9600);
Serial.println("Escribe 'Encender un led' para encender el LED o 'Apagar un led' para apagarlo");
}

void loop() {
  // put your main code here, to run repeatedly:

if(Serial.available()>0){
  dato=Serial.readString();
  dato.trim();
  if(dato == "Encender un led"){
    digitalWrite(led, HIGH);
    Serial.println("led encendido");
  } else if(dato == "Apagar un led"){
    digitalWrite(led, LOW);
    Serial.println("led apagado");
  }else{
    Serial.println("No se conoce comando");
  }
}
}