char* buildPayload() {
  const size_t capacity = JSON_ARRAY_SIZE(1) + JSON_OBJECT_SIZE(4);
  DynamicJsonDocument doc(capacity);
  JsonObject doc_0 = doc.createNestedObject();
  doc_0["mac_address"] = "3c71bfffa9c4";
  doc_0["co2"] = co2;
  doc_0["tvoc"] = tvoc;
  doc_0["present"] = presence;
  // verificar se pode tirar isso ou nao
  char buffer[100];
  serializeJson(doc, buffer);
  // validar se ta retornando o payload
  return buffer;
 }