Arduino RFID access control

Finished 2024

A card reader that lights a green LED for the right card and sounds a buzzer for the wrong one, built on an Arduino Uno with an RC522 module, plus a few smaller sensor labs.

An Arduino Uno wired to an RC522 RFID module and a breadboard with a green LED, a red LED and a buzzer

The build

An Elegoo Uno R3, an RC522 RFID module on SPI, a green LED, a red LED and a buzzer. The pins are reset on 9, chip select on 10, green on 5, red on 4 and the buzzer on 6, and the MFRC522 library does the talking.

Pencil wiring sketch of the Arduino, the RFID module, a buzzer, two resistors and two LEDs — My wiring sketch

The first program just prints card UIDs over serial at 9600 baud. That’s the library’s example. The second one compares the card’s 4-byte UID with one stored in the code, byte by byte. A match prints “Access Granted” and turns the green LED on for 2 seconds. A mismatch prints “Access Denied” and turns the red LED and the buzzer on for 1 second, then halts the card. The sketch is 5316 bytes, 16% of the flash. It isn’t secure: matching a 4-byte UID is easy to clone, and the allowed UID sits in the sketch.

The serial monitor below shows what a run looks like: a few denied scans, then four granted ones with the right card, then one more denied.

A hand holding a keyring up to the RFID reader while the green LED glows — A good card, green LED

Serial monitor text showing several Access Denied lines, then four Access Granted lines, then one more Access Denied — The serial monitor

The smaller labs

These were lab-partner labs, and the code and wiring sketches are mine.

Humidity and temperature: a DHT11 on pin 7 and a 16x2 LCD, updating once a second.
Distance: an HC-SR04-style sensor, with distance in cm as the pulse time times 0.017015, printed twice a second.
Clock: a DS1307 real-time clock and an LCD. I used hand-written offsets in the code to fix the time display, which is a hack and can print wrong values.
Gyro: an MPU6050 on I2C, printing acceleration and rotation every 500 ms at 115200 baud. I don’t trust that one: the sketch skipped mpu.begin() and the readout showed 132 m/s^2, more than the sensor’s range.