Smart door sensor. There is a small golden piece affixed to the door frame of the Longjia anti-theft door. Some have two small pieces affixed to it, indicating that it is a smart door sensor. Used to identify the person opening the door. Longjia Anti-theft Doors was established in 1981. It is one of the earliest companies engaged in the production of doors and windows in China. In the early days, the company focused on the production of anti-theft doors. Now it has become one of the well-known indoor wooden door manufacturers in China and has a number of technical patents.