This paper presents a development of 125 KHz ubiquitous RFID tag signal detector system that can be used as a door lock for home security. The RFID tag signal detector or also known as RFID reader consists of three elements which are the RFID reader, RF antenna and transponder (tag). The reader is a device that generates the digital pulse code required to activate the transponder and also designates the operating frequencies for the system. The main component for this system design is AVEA KL042, an RFID transceiver module. Since the frequency used for this project is low, that means that the power associated with the wave is low and the reading range will be short.