This simple analog electronic circuit can detect the presence of 13.56 MHz RFID tags that used by all plastic cards like access badges, bank cards, library cards, loyalty cards. Also many other objects like books, toys, or clothing that may carry RFID tags without you knowing it.
Carrying tagged objects with you can show your identity or whereabouts to anyone equipped with the appropriate tools to read RFID tags. RFID sniffer helps you to find out whether an object is tagged or not. The circuit is easy to build. No MCU is needed, just op-amp, transistor and passive components.
RFID sniffer is available as open-source. If you want to build it, you can download schematic (eagle) and gerber file here