warm Reminder: Please note that this product should be used with a doorbell emitter. It is useless to buy this Ding Dong machine separately. It must be used with a transmitter, this Ding Dong is inserted indoors, and the transmitter is installed outside the door. After the two products are connected, press the Ding Dong inside the transmitter, prompting you to have guests coming, which is suitable for families and companies.