switch (esp_sleep_get_wakeup_cause()) {
    case ESP_SLEEP_WAKEUP_EXT0:
      Serial.println("Wakeup by EXT0");
      break;
    case ESP_SLEEP_WAKEUP_EXT1:
      Serial.println("Wakeup by EXT1");
      break;
    case ESP_SLEEP_WAKEUP_TIMER:
      Serial.println("Wakeup by RTC");
      break;
    case ESP_SLEEP_WAKEUP_TOUCHPAD:
      Serial.println("Wakeup by TouchPad");
      break;
}