Abstract- This paper discuss about how to use smart card as room security by giving entering access only for people allowed. Some aspects written are about hardware, smart card data form and work tool algorithm. First analysis is about how hardware works. And then for read mifare smart card used ACM120-SM Contacless Small Module. Second analysis is about the interface used for communicating between the devices. To communicating host and smart card reader used UART interface. As for controlling magnetic lock from host used GPIO port. Last, how to build security system based on that tools.