void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  Serial.println("Hello, ESP32!");
  pinMode(15, INPUT_PULLUP);
  pinMode(4, OUTPUT);
}

void loop() {
  bool a = digitalRead(15);
  if (a == 0) {
    digitalWrite(4, HIGH);
    delay(250);
    digitalWrite(4, LOW);
    delay(250);
  delay(10); // this speeds up the simulation
  }
}