This is a blank MiFare Classic keychain fob - often used for electronic locks or customer identification but also found in other systems where a small and easy to carry tag is desired.

The tag contains a NXP S50 chip and an antenna, and is passively powered by the reader/writer when placed a couple inches away.

These can be read by almost any 13.56MHz RFID/NFC reader but make sure it can handle MiFare cards as there are a few other encoding standards (like FeLica). They are tested and work great with both our PN532 NFC/RFID breakout board and Adafruit NFC/RFID Shield for Arduino!

• These chips can be written to
• can store up to 1 KB of data in writable EEPROM divided into banks
• can handle over 100,000 re-writes.
• You can use our PN532 NFC/RFID breakout board or Adafruit NFC/RFID Shield for Arduino to read and write data to the EEPROM inside the tag.
• There is also a permanent 4-byte ID burned into the chip that you can use to identify one tag from another
• the ID number cannot be changed.

These use the S50 chipset, which used to be the 'classic' NFC chipset. In ~2014, the NFC forum decided not to support this chipset anymore, so newer phones do not support the MiFare classic. This only matters if you're trying to use this tag with a phone/tablet.

S50 chip specification:

• 1 KiloByte (8 KiloBit) non-volatile EEPROM storage
• Built in encryption engine with 48-bit key
• 4 Byte unique identifier burned into the chip
• 13.56 MHz frequency

Tag specification:

• 38.39mm / 1.51" x 38.39mm / 1.51" x 5.04mm / 0.2"
• 8.68g
• Works about 2" away from reader

NXP S50 datasheet