Build an RFID Inventory Tracker
The Story
Inventory systems work because every scan becomes a reliable event. RC522 and SD share the SPI bus, while GPIO5 and GPIO4 keep separate chip-select signals so the ESP32 can talk to one module at a time. The component list, wiring table, GPIO map, code constants, expected output, troubleshooting, and FAQ now describe the same bench build so a learner can verify each step instead of guessing.
Explain Like I'm 12
An RFID tag has a small ID number. The reader asks for that number, the ESP32 checks the known list, writes a line to the memory card, and flashes the matching LED. Prove the local wiring and code first, then add displays, logging, or Wi-Fi after the simple version behaves exactly as expected.
Safety Standards
- Unplug USB before changing jumper wires. Recheck 3.3 V, 5 V, and GND before reconnecting power.
- Do not power motors, pumps, LED strips, or servos from the ESP32 3.3 V pin. Use a suitable external supply and common ground.
- Breadboards are for low-current prototypes. Move high-current or unattended builds to proper terminals, enclosure, strain relief, and fusing.
- Li-ion battery builds need a protected cell, proper charger, fuse, and enclosure. Never charge unknown or damaged cells.
What You Will Build
An RFID tracker that scans RC522 tags over SPI, logs events to a microSD card, and uses red and green LEDs for known or unknown tags.
Learning Objectives
- Wire the exact GPIO pins used by the sketch.
- Run the starter code and compare output with the wiring table.
- Explain the main sensor or input signal in plain language.
- Troubleshoot one wiring mistake using Serial Monitor evidence.
Components List
- ESP32 DevKit boardControls SPI, LEDs, and logging.
- RC522 RFID reader module13.56 MHz RFID reader powered from 3.3 V.
- MIFARE-compatible RFID cards or tagsUse test tags for the known UID list.
- MicroSD card moduleSPI storage with CS on GPIO4.
- Green and red LEDs with series resistorsKnown and unknown feedback on GPIO26 and GPIO27.
Bill of Materials
| Part | Qty | Estimated Cost | Notes |
|---|---|---|---|
| ESP32 DevKit board | 1 | Varies | Controls SPI, LEDs, and logging. |
| RC522 RFID reader module | 1 | Varies | 13.56 MHz RFID reader powered from 3.3 V. |
| MIFARE-compatible RFID cards or tags | 1 | Varies | Use test tags for the known UID list. |
| MicroSD card module | 1 | Varies | SPI storage with CS on GPIO4. |
| Green and red LEDs with series resistors | 1 | Varies | Known and unknown feedback on GPIO26 and GPIO27. |
Wiring
An RFID tracker that scans RC522 tags over SPI, logs events to a microSD card, and uses red and green LEDs for known or unknown tags.
-
1
Unplug USB before changing SPI, LED, or SD wiring.
-
2
Connect RC522 SDA/SS to GPIO5, SCK to GPIO18, MOSI to GPIO23, MISO to GPIO19, and RST to GPIO22.
-
3
Connect RC522 3.3 V and GND to the ESP32 3.3 V and GND rails.
-
4
Connect SD CS to GPIO4 while sharing SCK, MOSI, and MISO with SPI.
-
5
Connect green LED through its resistor to GPIO26 and red LED through its resistor to GPIO27.
-
6
Reconnect USB and test one RFID tag before relying on the SD log.
GPIO Mapping
| Signal | ESP32 Pin | Direction | Notes |
|---|---|---|---|
| RC522 SDA/SS | GPIO5 | Output | RC522 chip select. |
| SPI SCK/MOSI/MISO | GPIO18 / GPIO23 / GPIO19 | I/O | Shared SPI bus. |
| RC522 RST | GPIO22 | Output | RFID reset. |
| SD CS | GPIO4 | Output | Separate SD chip select. |
| Green LED / Red LED | GPIO26 / GPIO27 | Output | Scan feedback. |
Circuit Explanation
RC522 and SD share SCK, MOSI, and MISO. Separate chip-select pins prevent both modules from answering at once.
Engineering Explanation
SPI sharing works only when chip-select wiring and code agree. The RC522 is powered from 3.3 V in this tutorial.
Libraries
- Arduino ESP32 coreInstall ESP32 board support in Arduino IDE.
Code
Copy into Arduino IDE. Install any libraries noted in the component guides first.
// ESP32 RFID Inventory Tracker - Beginner
// RC522 tag scan -> SD card CSV log with known/unknown LED feedback
// Libraries: MFRC522 by GithubCommunity, SD by Arduino
#include <SPI.h>
#include <MFRC522.h>
#include <SD.h>
#define RC522_SS 5
#define RC522_RST 22
#define SD_CS 4
#define LED_GREEN 26
#define LED_RED 27
MFRC522 rfid(RC522_SS, RC522_RST);
// Known tag UIDs (hex strings) mapped to asset names
const char* KNOWN_UIDS[] = {"A1B2C3D4", "11223344", "DEADBEEF"};
const char* ASSET_NAMES[] = {"Laptop-01", "Camera-01", "Toolbox-01"};
const int NUM_ASSETS = 3;
String uidToHex(MFRC522::Uid *uid){
String s="";
for(byte i=0;i<uid->size;i++){
if(uid->uidByte[i]<0x10) s+="0";
s+=String(uid->uidByte[i],HEX);
}
s.toUpperCase();
return s;
}
void flashLED(int pin){ digitalWrite(pin,HIGH); delay(300); digitalWrite(pin,LOW); }
void logScan(const String &uid, const char* name){
File f=SD.open("/inventory.csv",FILE_APPEND);
if(!f) return;
f.printf("%lu,%s,%sn",millis(),uid.c_str(),name);
f.close();
}
void setup(){
Serial.begin(115200);
SPI.begin();
rfid.PCD_Init();
SD.begin(SD_CS);
pinMode(LED_GREEN,OUTPUT); pinMode(LED_RED,OUTPUT);
Serial.println("RFID Inventory Tracker ready. Scan a tag...");
}
void loop(){
if(!rfid.PICC_IsNewCardPresent()||!rfid.PICC_ReadCardSerial()) return;
String uid=uidToHex(&rfid.uid);
const char* name="UNKNOWN";
bool known=false;
for(int i=0;i<NUM_ASSETS;i++){
if(uid==String(KNOWN_UIDS[i])){ name=ASSET_NAMES[i]; known=true; break; }
}
Serial.printf("UID: %s Asset: %sn",uid.c_str(),name);
logScan(uid,name);
flashLED(known?LED_GREEN:LED_RED);
rfid.PICC_HaltA();
rfid.PCD_StopCrypto1();
delay(1000);
}Code Explanation
The sketch defines RC522_SS on GPIO5, RC522_RST on GPIO22, SD_CS on GPIO4, and LEDs on GPIO26/GPIO27.
Expected Output
Serial Monitor reports scans and SD logging status. Known UIDs flash green, unknown UIDs flash red, and the SD card receives CSV rows.
Troubleshooting
- RFID tags are not detected Check RC522 3.3 V power, SPI wiring, GPIO5 CS, and GPIO22 reset.
- SD logging fails Check SD format, GPIO4 CS, and shared SPI wiring.
- Known tag shows unknown Copy the UID from Serial Monitor into KNOWN_UIDS.
Common Mistakes
- Powering RC522 from 5 V.
- Using the same chip-select pin for RC522 and SD.
- Mixing up MOSI and MISO.
Testing Checklist
- Upload the sketch with only the documented circuit attached.
- Open Serial Monitor and confirm the expected messages.
- Move or trigger the input and watch the output change.
- Power-cycle the project and confirm it starts safely.
Engineering Tips
- Change one variable at a time.
- Keep wires short and labeled while testing.
- Record any calibrated threshold before installing the project.
Upgrade Ideas
- Add Wi-Fi and NTP timestamps.
- Show asset names on an OLED display.
- Add a small web page for latest scans.
Real-World Applications
- Classroom lab build
- Bench prototype
- Local ESP32 learning project
FAQs
Why do RC522 and SD share SPI?
SPI supports multiple devices when each has its own chip-select pin.
Can I use my own tag IDs?
Yes. Read the UID from Serial Monitor and replace the starter values.
Should RC522 use 5 V?
No for this tutorial. Wire RC522 power to 3.3 V.
Review, Testing, and References
Author: Abdul Mubeen and the ESP32 Engine editorial team. Last updated: 2026-06-27. Reviewed: wiring logic, Arduino code structure, beginner safety, and learning sequence.
Educational level: Intermediate. Estimated completion time: 90-120 min. This project is for learning and prototyping; production or unattended hardware needs additional engineering review.
Project Complete!
You completed ESP32 RFID Inventory Tracker as a real ESP32 engineering build with matching wiring, code, testing, and troubleshooting.
- Read the GPIO map
- Verify code against hardware
- Troubleshoot with Serial Monitor