analog

Passive RFID tags are basically just little antennas with an extremely low power integrated circuit. When the IC is activated by power induced by the antenna, it generates a signal to transmit a small amount of data, such as an ID number.

This RC522 RFID module can read RFID tags and write to RFID tags that support writing. The RFID tag has a 4-byte (or 32-bit) UID, which means there are about 4.3 billion unique possibilities. Although that sounds like a lot, there are some situations where it's not considered enough to prevent collisions or brute-force guessing.

Here we will explore encoding data with analog signals or with digital signals that serve a purpose similar to analog signals.

Reading analog voltages

Each Arduino board has a row of pins capable of reading analog input. By "analog" we mean that the input data is determined by the voltage applied to the pin. Although in theory there are infinitely many levels between any two voltages, the actual number of distinct values is limited by noise and the type of hardware reading the voltages.