What is it?

This board acts as an interface between MFRC522 RFID card readers and 5V devices like most Arduinos.

The MFRC522 is a very popular RFID card reader used in many Arduino projects. Unfortunately it runs on 3.3 volts, meaning that you need interfacing circuitry to use it. Signals from your Arduino (for example) need reducing to 3.3 volts and signals from the MFRC522 need stepping up to 5V to be reliably read.

This board takes care of this for you, as well providing a 3.3V power supply for the MFRC522.

In addition, the board allows connection of up to 8 shifters on the same SPI bus.

How does it work?

The board is very simple. 5V to 3.3V conversion is done using simple voltage dividers. 3.3V to 5V conversion is done using a transistor buffer.

If using more than one device, there is a 74HC138 3-to-8 line decoder that allows selection of which CS line to enable.

For a connection to a single MFRC522, only the two SPI bus connections and 5V power are required.