Maker Pro

Unlock Your PC With an Arduino Using a Phone, RFID Card or RFID Tag

June 03, 2019 by Petr Lukáš

How to make simple USB device which allows you to unlock your computer without keyboard by RFID card/chip.

In this short tutorial, I'd like to explain how to make a simple USB device which allows you to very quickly unlock your computer without a keyboard. 

You will need only the Pro Micro microcontroller with ATmega32U4 (or Arduino Leonardo with this chip), RFID reader together with card and breadboard.

For RC522 reader USE ONLY 3.3V version of Pro Micro. VCC is the voltage supplied to the on-board ATmega32U4. This voltage will depend on whether you’re using a 3.3V/8MHz Pro Micro or a 5V/16MHz version, it’ll be either 3.3V or 5V respectively (but for this project you need 3.3V version!) This voltage is regulated by the voltage applied to the RAW pin. If the board is powered through the ‘RAW’ pin (or USB), this pin can be used as an output to supply other devices.

Arduino RFID Library

Arduino RFID Library for MFRC522:

Required Hardware

* --------------------------------------------------------------------------------------------------------------------
* Example sketch/program showing how to read data from a PICC to serial.
* --------------------------------------------------------------------------------------------------------------------
* This is a MFRC522 library example; for further details and other examples see:
* Example sketch/program showing how to read data from a PICC (that is: a RFID Tag or Card) using a MFRC522 based RFID
* Reader on the Arduino SPI interface.
* When the Arduino and the MFRC522 module are connected (see the pin layout below), load this sketch into Arduino IDE
* then verify/compile and upload it. To see the output: use Tools, Serial Monitor of the IDE (hit Ctrl+Shft+M). When
* you present a PICC (that is: a RFID Tag or Card) at reading distance of the MFRC522 Reader/PCD, the serial output
* will show the ID/UID, type and any data blocks it can read. Note: you may see "Timeout in communication" messages
* when removing the PICC from reading distance too early.
* If your reader supports it, this sketch/program will read all the PICCs presented (that is: multiple tag reading).
* So if you stack two or more PICCs on top of each other and present them to the reader, it will first output all
* details of the first and then the next PICC. Note that this may take some time as all data blocks are dumped, so
* keep the PICCs at reading distance until complete.
* @license Released into the public domain.
* Typical pin layout used:
* -----------------------------------------------------------------------------------------
* MFRC522 Arduino Arduino Arduino Arduino Arduino
* Reader/PCD Uno/101 Mega Nano v3 Leonardo/Micro Pro Micro
* Signal Pin Pin Pin Pin Pin Pin
* -----------------------------------------------------------------------------------------
* SPI SS SDA(SS) 10 53 D10 10 10
* SPI MOSI MOSI 11 / ICSP-4 51 D11 ICSP-4 16
* SPI MISO MISO 12 / ICSP-1 50 D12 ICSP-1 14
* SPI SCK SCK 13 / ICSP-3 52 D13 ICSP-3 15
#include <SPI.h>
#include <MFRC522.h>
#include <Keyboard.h>
constexpr uint8_t RST_PIN = 9; // Configurable, see typical pin layout above
constexpr uint8_t SS_PIN = 10; // Configurable, see typical pin layout above
MFRC522 mfrc522(SS_PIN, RST_PIN); // Create MFRC522 instance
int RXLED = 17;
void setup() {
//while (!Serial); // Do nothing if no serial port is opened (added for Arduinos based on ATMEGA32U4)
SPI.begin(); // Init SPI bus
mfrc522.PCD_Init(); // Init MFRC522
mfrc522.PCD_DumpVersionToSerial(); // Show details of PCD - MFRC522 Card Reader details
void readHex(byte *buffer, byte bufferSize) {
for (byte i = 0; i < bufferSize; i++) {
Serial.print(buffer[i] < 0x10 ? " 0" : " ");
Serial.print(buffer[i], HEX);
void loop() {
// Look for new cards
if ( ! mfrc522.PICC_IsNewCardPresent()) {
digitalWrite(RXLED, HIGH);
// Select one of the cards
if ( ! mfrc522.PICC_ReadCardSerial()) {
digitalWrite(RXLED, LOW);
Serial.print("RFID UID: ");
readHex(mfrc522.uid.uidByte, mfrc522.uid.size);
// SET YOUR UID HERE (you should see it in your serial monitor)
if (mfrc522.uid.uidByte[0] == 0x00 &&
mfrc522.uid.uidByte[1] == 0x00 &&
mfrc522.uid.uidByte[2] == 0x00 &&
mfrc522.uid.uidByte[3] == 0x00) {
Serial.println("Correct UID");
else {
Serial.println("Incorrect UID");
digitalWrite(RXLED, HIGH);
Serial.println("Printing password...");
Keyboard.print("<your secret password>");
view raw MFRC522.ino hosted with ❤ by GitHub


Petr Lukáš

Application Software Developer, Performance and capacity reporting for IBM zSeries team, experimenting with home automation, sensors, Arduino and ESP

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