From Raspberry Pi Spy:
RC522 RFID modules are a simple add-on you can connect to a Raspberry Pi to read MIFARE tags and cards. This is potentially a great feature to include in a security system or any application where you need to identify an object or person without them pressing buttons, operating switches or other sensors. The contactless tags can be carried on a key-ring and the cards fit nicely in a wallet. Both of them can be hidden inside other objects to give them a unique ID that can be read by the Pi.
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