void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(21,OUTPUT);
  Serial.println("Hello, ESP32!");
}

void loop() {
  if(Serial.available()>0){
   char dato=Serial.read();
   if(dato=='a'){
    digitalWrite(21,HIGH);
   }
   else if(dato=='b'){
   digitalWrite(21,LOW);
   }
  }
  // put your main code here, to run repeatedly:
  delay(10); // this speeds up the simulation
}