Board and part

ESP32-C3 RC522 RFID Simulator: Wiring, Code and a Live Circuit

This is an RC522 (MFRC522) RFID reader wired to an ESP32-C3 DevKitM-1 over SPI, running live with the MFRC522 library: open it in the editor, lay a card on the reader, and the serial monitor prints its UID.

Run it in the editor Read the code

3.3 V logic
Library: MFRC522.h
SS on GPIO 7
3.3 V module

ESP32-C3 · RC522 RFIDlive0.000 s 0.00x

Click to open it in the editor

The circuit itself, running on the simulator with the sketch below. Press what can be pressed; click anything else to open it in the editor.

Connections

Wiring the RC522 RFID to the ESP32-C3

Seven wires: 3.3 V, GND, the four SPI lines to the ESP32-C3’s SPI pins, and RST to GPIO 10.

Every connection in the circuit above, with the board’s own pin names.

RC522 RFID pin	ESP32-C3 pin	Why
3.3V	3V3	The MFRC522 is a 3.3 V chip. Never 5 V on this pin.
GND	GND	Common ground.
SDA (SS)	GPIO 7	Chip select. Labeled SDA on the module, but this is SPI, not I2C.
SCK	GPIO 4	ESP32-C3’s SPI clock.
MOSI	GPIO 6	Data from the board to the reader.
MISO	GPIO 5	Data from the reader to the board.
RST	GPIO 10	Reset, held high while the reader runs.
IRQ	not connected	The library polls instead.

Power

The ESP32-C3 and the MFRC522 are both 3.3 V, so the SPI lines connect directly: the cleanest RC522 wiring there is.

Pins to leave alone on the ESP32-C3

GPIO 8 carries the onboard LED and GPIO 9 is the BOOT button, and both are also the default I2C pins. GPIO 2, 8 and 9 are strapping pins, and only GPIO 0 to 4 can read an analog voltage.

On a real bench

The ESP32-C3 DevKitM-1 is small enough to sit on one breadboard with room for the RC522 RFID beside it. Its USB port goes through a USB-to-serial chip; the cheaper SuperMini boards use the C3’s own USB, where Serial needs USB CDC on boot turned on in the IDE.

The sketch

The code, and what it does.

The sketch the circuit above runs. Open it in the editor, change a line and press Run: it compiles in your browser.

RC522 RFID on ESP32-C3

#include <SPI.h>
#include <MFRC522.h>

const int SS_PIN = 7;    // SDA on the module, GPIO 7
const int RST_PIN = 10;   // GPIO 10

MFRC522 rfid(SS_PIN, RST_PIN);

void setup() {
  Serial.begin(115200);
  SPI.begin();          // SCK 4, MISO 5, MOSI 6
  rfid.PCD_Init();
  Serial.println("Hold a card to the reader");
}

void loop() {
  if (!rfid.PICC_IsNewCardPresent() || !rfid.PICC_ReadCardSerial()) {
    delay(50);
    return;
  }
  Serial.print("Card UID:");
  for (byte i = 0; i < rfid.uid.size; i++) {
    Serial.print(rfid.uid.uidByte[i] < 0x10 ? " 0" : " ");
    Serial.print(rfid.uid.uidByte[i], HEX);
  }
  Serial.println();
  rfid.PICC_HaltA();    // done with this card until it is lifted
}

SPI.begin() starts the ESP32-C3’s SPI bus on its own pins (SCK 4, MISO 5, MOSI 6), and MFRC522 rfid(SS_PIN, RST_PIN) adds the chip select and reset. PCD_Init() resets the reader and turns its antenna on.

PICC_IsNewCardPresent() asks whether a card has entered the field and PICC_ReadCardSerial() reads its UID, four or seven bytes. The sketch prints them in hex with a leading zero where needed, then PICC_HaltA() puts the card to sleep so it is reported once.

Serial.begin(115200) opens the serial monitor. Serial is UART0, at 115200 baud in this sketch.

Pitfalls

Common mistakes with the RC522 RFID on the ESP32-C3

Powering it from 5V

The 3.3V pin means it: 5 V there damages the MFRC522. If the module gets warm or reports a version of 0x00 or 0xFF, check this first.

Treating SDA as I2C

The pin marked SDA is the SPI chip select. Wired to the ESP32-C3’s I2C data pin, the reader is never selected. It goes to SS_PIN, GPIO 7 here.

Expecting to read bank cards

The RC522 reads 13.56 MHz MIFARE tags and cards. Contactless bank cards answer, but only with a random ID each time, and 125 kHz key fobs not at all.

FAQ

Questions and answers

Can I simulate an RC522 RFID reader with an ESP32-C3 DevKitM-1?

Yes. The circuit at the top of this page is an ESP32-C3 DevKitM-1 with a RC522 RFID reader, running in your browser on a simulated ESP32-C3 (RISC-V). The simulated reader has four cards with fixed UIDs; click to lay one on the antenna. Press Run it in the editor to change the wiring or the code; it is free and needs no account.

Which pins does the RC522 use on the ESP32-C3?

SPI: SCK on GPIO 4, MISO on GPIO 5, MOSI on GPIO 6. SS and RST can be any digital pins; this page uses GPIO 7 and GPIO 10.

How do I make an access control lock?

Compare the UID bytes with a list of allowed cards, and drive a relay or a servo when one matches. Keep in mind that a UID alone is easy to copy, so it is a learning project, not a security one.

Can I write to the card?

The library has MIFARE_Write() for MIFARE Classic blocks after authentication. This simulated reader models reading UIDs only.

Build your own RC522 RFID project

Open this circuit in the editor, change the wiring or the code, and keep your version in a free account.

Start building free Open the editor