Building a 2x2 RFID board with RC522 + 74HC4067 + Raspberry Pi

June 4, 2026 · 8 min read

CS Researcher at UMR SENS, CIRAD

Goal: build a 2x2 board, with one RFID reader per cell, using a multiplexer already so the design can later scale to 4x5.

Components

4 × RC522 RFID modules
1 × 74HC4067 multiplexer
1 × Raspberry Pi 3B+
4 × RFID tags
4 × 10 kΩ resistors
Jumper wires
External 3.3 V power supply (recommended)

Cell Layout

Use this numbering:

+--------+--------+
| Cell 0 | Cell 1 |
| RC522  | RC522  |
|  #0    |  #1    |
+--------+--------+
| Cell 2 | Cell 3 |
| RC522  | RC522  |
|  #2    |  #3    |
+--------+--------+

The corresponding multiplexer channels are:

Cell 0 -> MUX C0 -> RC522 #0 SDA/SS
Cell 1 -> MUX C1 -> RC522 #1 SDA/SS
Cell 2 -> MUX C2 -> RC522 #2 SDA/SS
Cell 3 -> MUX C3 -> RC522 #3 SDA/SS

Raspberry Pi GPIO Pins Used

Use these Raspberry Pi pins:

Purpose	Raspberry Pi GPIO	Physical pin
SPI SCLK	GPIO11	Pin 23
SPI MOSI	GPIO10	Pin 19
SPI MISO	GPIO9	Pin 21
SPI CE0	GPIO8	Pin 24
RC522 reset	GPIO25	Pin 22
MUX S0	GPIO17	Pin 11
MUX S1	GPIO27	Pin 13
MUX S2	GPIO23	Pin 16
MUX S3	GPIO24	Pin 18
MUX enable	GPIO22	Pin 15
3.3 V	3.3 V	Pin 1 or 17
Ground	GND	Pin 6, 9, 14, 20, 25, 30, 34, or 39

Connect Power

Connect Ground First

All grounds must be connected together.

Connect:

From	To
Raspberry Pi GND	Breadboard ground rail
RC522 #0 GND	Breadboard ground rail
RC522 #1 GND	Breadboard ground rail
RC522 #2 GND	Breadboard ground rail
RC522 #3 GND	Breadboard ground rail
74HC4067 GND	Breadboard ground rail

If you use an external 3.3 V supply, also connect:

From	To
External power supply GND	Breadboard ground rail

Connect 3.3 V Power

For a 4-reader prototype, you can test using the Raspberry Pi 3.3 V pin.

Connect:

From	To
Raspberry Pi 3.3 V	Breadboard 3.3 V rail
RC522 #0 VCC / 3.3V	Breadboard 3.3 V rail
RC522 #1 VCC / 3.3V	Breadboard 3.3 V rail
RC522 #2 VCC / 3.3V	Breadboard 3.3 V rail
RC522 #3 VCC / 3.3V	Breadboard 3.3 V rail
74HC4067 VCC	Breadboard 3.3 V rail

For the future 4x5 board, use an external 3.3 V power supply instead of the Pi 3.3 V pin.

Do not connect RC522 VCC to 5 V.

Connect Shared SPI Wires

All four RC522 modules share the same SPI bus.

Connect these wires:

Raspberry Pi	RC522 #0	RC522 #1	RC522 #2	RC522 #3
GPIO11 / SCLK	SCK	SCK	SCK	SCK
GPIO10 / MOSI	MOSI	MOSI	MOSI	MOSI
GPIO9 / MISO	MISO	MISO	MISO	MISO

This means:

Pi GPIO11 goes to SCK on all 4 readers.
Pi GPIO10 goes to MOSI on all 4 readers.
Pi GPIO9 goes to MISO on all 4 readers.

Connect Reset

Connect one reset wire shared by all RC522 modules:

Raspberry Pi	RC522 #0	RC522 #1	RC522 #2	RC522 #3
GPIO25	RST	RST	RST	RST

This lets the Pi reset all RFID readers together.

Connect the 74HC4067 Control Pins

The 74HC4067 has address pins S0, S1, S2, and S3.

These pins tell the multiplexer which channel to connect.

Connect:

74HC4067	Raspberry Pi
S0	GPIO17
S1	GPIO27
S2	GPIO23
S3	GPIO24
EN	GPIO22

The EN pin is usually active-low:

EN = HIGH -> multiplexer disabled
EN = LOW  -> multiplexer enabled

During normal scanning, the script will:

Disable the multiplexer.
Choose a channel using S0-S3.
Enable the multiplexer.
Read the corresponding RC522.
Disable the multiplexer again.

Connect the Multiplexer Signal Pin

Connect the common signal pin of the multiplexer to the Raspberry Pi SPI chip-select pin.

Depending on your 74HC4067 board, this pin may be called:

SIG
COM
Z

Connect:

74HC4067	Raspberry Pi
SIG / COM / Z	GPIO8 / CE0

This means that the Raspberry Pi chip-select signal goes into the multiplexer.

The multiplexer then sends that signal to one selected RC522 reader.

Connect Each RC522 SDA/SS Pin to the Multiplexer

The RC522 module usually labels its chip-select pin as:

SDA
sometimes SS
sometimes SDA/SS

This is not I2C SDA in this setup. On RC522 SPI modules, this pin is used as SPI chip select.

Connect:

Cell	RC522 module	RC522 pin	74HC4067 channel
0	RC522 #0	SDA/SS	C0
1	RC522 #1	SDA/SS	C1
2	RC522 #2	SDA/SS	C2
3	RC522 #3	SDA/SS	C3

So:

RC522 #0 SDA/SS -> 74HC4067 C0
RC522 #1 SDA/SS -> 74HC4067 C1
RC522 #2 SDA/SS -> 74HC4067 C2
RC522 #3 SDA/SS -> 74HC4067 C3

Do not connect the RC522 SDA/SS pins directly to the Raspberry Pi.

Do not connect the four SDA/SS pins together.

Add Pull-Up Resistors on SDA/SS

Each RC522 SDA/SS pin must be pulled up to 3.3 V with a 10 kΩ resistor.

Connect:

RC522 pin	Resistor	Power
RC522 #0 SDA/SS	10 kΩ	3.3 V
RC522 #1 SDA/SS	10 kΩ	3.3 V
RC522 #2 SDA/SS	10 kΩ	3.3 V
RC522 #3 SDA/SS	10 kΩ	3.3 V

In practice:

RC522 SDA/SS pin -> one side of 10 kΩ resistor
other side of resistor -> 3.3 V rail

Why this is needed:

When a reader is not connected through the multiplexer, its SDA/SS pin would otherwise float. The pull-up keeps it HIGH, which means deselected.

Complete Connection Checklist

Raspberry Pi to RC522 Readers

Raspberry Pi	Connects to
GPIO11 / SCLK	SCK on all RC522
GPIO10 / MOSI	MOSI on all RC522
GPIO9 / MISO	MISO on all RC522
GPIO25	RST on all RC522
3.3 V	VCC on all RC522
GND	GND on all RC522

Raspberry Pi to 74HC4067

Raspberry Pi	74HC4067
GPIO8 / CE0	SIG / COM / Z
GPIO17	S0
GPIO27	S1
GPIO23	S2
GPIO24	S3
GPIO22	EN
3.3 V	VCC
GND	GND

74HC4067 to RC522 Readers

74HC4067	RC522
C0	RC522 #0 SDA/SS
C1	RC522 #1 SDA/SS
C2	RC522 #2 SDA/SS
C3	RC522 #3 SDA/SS

Enable SPI on Raspberry Pi

Run:

sudo raspi-config

Then:

Interface Options -> SPI -> Enable

Reboot:

sudo reboot

After reboot, check that SPI exists:

ls /dev/spidev*

You should see something like:

/dev/spidev0.0
/dev/spidev0.1

Install Python Dependencies

Install required packages:

sudo apt update
sudo apt install -y python3-pip python3-gpiozero python3-spidev python3-rpi.gpio
pip3 install mfrc522

If pip3 install mfrc522 fails because of system package restrictions, use a virtual environment:

python3 -m venv ~/rfid-env
source ~/rfid-env/bin/activate
pip install mfrc522 spidev RPi.GPIO

Python Code: Scan the 2x2 Board

import time
import RPi.GPIO as GPIO
from mfrc522 import SimpleMFRC522

# 74HC4067 address pins
MUX_S0 = 17
MUX_S1 = 27
MUX_S2 = 23
MUX_S3 = 24

# 74HC4067 enable pin
# Usually active-low:
# HIGH = disabled
# LOW = enabled
MUX_EN = 22

# Shared RC522 reset pin
RC522_RST = 25

# Number of cells in the 2x2 board
CELL_COUNT = 4

CELL_NAMES = {
    0: "Cell 0: top-left",
    1: "Cell 1: top-right",
    2: "Cell 2: bottom-left",
    3: "Cell 3: bottom-right",
}


def setup_gpio():
    GPIO.setmode(GPIO.BCM)
    GPIO.setwarnings(False)

    GPIO.setup(MUX_S0, GPIO.OUT)
    GPIO.setup(MUX_S1, GPIO.OUT)
    GPIO.setup(MUX_S2, GPIO.OUT)
    GPIO.setup(MUX_S3, GPIO.OUT)
    GPIO.setup(MUX_EN, GPIO.OUT)

    GPIO.setup(RC522_RST, GPIO.OUT)

    # Disable mux at startup.
    GPIO.output(MUX_EN, GPIO.HIGH)

    # Keep RC522 readers out of reset.
    GPIO.output(RC522_RST, GPIO.HIGH)


def disable_mux():
    GPIO.output(MUX_EN, GPIO.HIGH)


def enable_mux():
    GPIO.output(MUX_EN, GPIO.LOW)


def select_mux_channel(channel):
    # Channel number is binary encoded on S0-S3.
    GPIO.output(MUX_S0, GPIO.HIGH if (channel & 0b0001) else GPIO.LOW)
    GPIO.output(MUX_S1, GPIO.HIGH if (channel & 0b0010) else GPIO.LOW)
    GPIO.output(MUX_S2, GPIO.HIGH if (channel & 0b0100) else GPIO.LOW)
    GPIO.output(MUX_S3, GPIO.HIGH if (channel & 0b1000) else GPIO.LOW)


def select_cell(cell_index):
    # Connect Raspberry Pi CE0 to the SDA/SS pin of one RC522 reader.
    disable_mux()
    select_mux_channel(cell_index)

    # Small settling delay after switching channel.
    time.sleep(0.002)

    enable_mux()

    # Small delay before using SPI.
    time.sleep(0.002)


def main():
    setup_gpio()

    # SimpleMFRC522 uses /dev/spidev0.0, which corresponds to CE0.
    reader = SimpleMFRC522()

    last_seen = {}

    print("Scanning 2x2 RFID board.")
    print("Press Ctrl+C to stop.")
    print()

    try:
        while True:
            for cell in range(CELL_COUNT):
                select_cell(cell)

                try:
                    tag_id, text = reader.read_no_block()
                except Exception as error:
                    tag_id = None
                    text = None
                    print(f"{CELL_NAMES[cell]} -> read error: {error}")

                if tag_id:
                    value = str(tag_id)

                    if last_seen.get(cell) != value:
                        last_seen[cell] = value
                        print(f"{CELL_NAMES[cell]} -> tag {value}")
                else:
                    if last_seen.get(cell) is not None:
                        last_seen[cell] = None
                        print(f"{CELL_NAMES[cell]} -> empty")

                disable_mux()

                # Avoid scanning too aggressively.
                time.sleep(0.05)

    except KeyboardInterrupt:
        print()
        print("Stopping.")

    finally:
        disable_mux()
        GPIO.cleanup()


if __name__ == "__main__":
    main()

Expected Output

When a tag is placed on Cell 0:

Cell 0: top-left -> tag 123456789

When a tag is removed:

Cell 0: top-left -> empty

When several tags are placed:

Cell 0: top-left -> tag 123456789
Cell 1: top-right -> tag 987654321
Cell 2: bottom-left -> tag 555555555
Cell 3: bottom-right -> tag 222222222

Components​

Cell Layout​

Raspberry Pi GPIO Pins Used​

Connect Power​

Connect Ground First​

Connect 3.3 V Power​

Connect Shared SPI Wires​

Connect Reset​

Connect the 74HC4067 Control Pins​

Connect the Multiplexer Signal Pin​

Connect Each RC522 SDA/SS Pin to the Multiplexer​

Add Pull-Up Resistors on SDA/SS​

Complete Connection Checklist​

Raspberry Pi to RC522 Readers​

Raspberry Pi to 74HC4067​

74HC4067 to RC522 Readers​

Enable SPI on Raspberry Pi​

Install Python Dependencies​

Python Code: Scan the 2x2 Board​

Expected Output​

Components

Cell Layout

Raspberry Pi GPIO Pins Used

Connect Power

Connect Ground First

Connect 3.3 V Power

Connect Shared SPI Wires

Connect Reset

Connect the 74HC4067 Control Pins

Connect the Multiplexer Signal Pin

Connect Each RC522 SDA/SS Pin to the Multiplexer

Add Pull-Up Resistors on SDA/SS

Complete Connection Checklist

Raspberry Pi to RC522 Readers

Raspberry Pi to 74HC4067

74HC4067 to RC522 Readers

Enable SPI on Raspberry Pi

Install Python Dependencies

Python Code: Scan the 2x2 Board

Expected Output