How I Built an RFID Access Thing Using NodeMCU and FastAPI

So a while back I got curious about how those RFID office entry systems work.
Like, you scan a card and it logs you in or opens a door or something.

I had a NodeMCU (ESP8266) lying around, an RC522 RFID module, and some free time, so I just started messing around.

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The Goal Was Simple

• Scan a card

• Get its UID

• Send it to a backend

• Store it somewhere

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That turned into this project:
RFID reader + NodeMCU → sends UID to a FastAPI backend → logs it in a database
Only I (with my admin card) can register new users let’s see that part in Part 2 of this blog 😉

I built it for fun. For learning.
And honestly, for me from 14 months ago, who had no clue how hardware and backend could talk to each other.

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What I Used

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How It Works (In Plain Words)

• I scan a card on the RC522

• NodeMCU reads the UID

• It sends the UID as JSON to my FastAPI backend (over Wi-Fi)

• The backend checks if the UID exists in the database

• If yes → it logs the time and whether it’s a login/logout

• If not → shows “UID not found” in serial

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System Flow Diagram

Here’s a simple overview of how everything connects:

From scanning an RFID card to logging it in a database this is how the system works end-to-end.

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Wiring the RC522 to NodeMCU

Warning: Don’t connect RC522 to 5V it’ll die. I nearly did that lol.

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NodeMCU Code: Scanning RFID and Sending to Backend

Once the NodeMCU connects to Wi-Fi, it waits for an RFID card to be scanned.
When a card is detected, it reads the UID, converts it to a string, and sends it to my FastAPI backend using a simple HTTP POST request.

Here’s the main part of the code:

if (mfrc522.PICC_IsNewCardPresent() && mfrc522.PICC_ReadCardSerial()) {
  String uidString = "";
  for (byte i = 0; i < mfrc522.uid.size; i++) {
    uidString += String(mfrc522.uid.uidByte[i] < 0x10 ? "0" : "");
    uidString += String(mfrc522.uid.uidByte[i], HEX);
  }
  uidString.toUpperCase();

  sendUidToServer(uidString);
}

And this is how the UID gets sent to the backend:

WiFiClient client;
HTTPClient http;
http.begin(client, serverUrl);
http.addHeader("Content-Type", "application/json");

String jsonData = "{\"rfid_uid\":\"" + uid + "\"}";
int httpResponseCode = http.POST(jsonData);

If the backend responds, I print it on the Serial Monitor just to confirm everything’s working.

See the NodeMCU code here

FastAPI Backend: Receiving and Logging UID

My FastAPI app has two endpoints:

• POST /create_user → To manually register a card with a name & number

• POST /access → The NodeMCU hits this to log each scan

Here’s the /access endpoint:

@app.post("/access")
def accesslogs(deets: AccessGive, db: Session = Depends(get_db)):
    return access.verify_user(db, deets)

The verify_user() function checks if the UID exists in the database. If it does, it logs the user as either logging in or out depending on their last access.

Here’s a simplified version of the logic:

if not latest_log or latest_log.is_active == False:
    is_active = True   # Logging IN
else:
    is_active = False  # Logging OUT

It then stores this in the database along with a timestamp, so I know who accessed, when, and whether they were coming in or going out.

See the full Backend code here

I'm using SQLAlchemy + PostgreSQL.
When a UID is scanned, the backend:

• Checks if UID exists in the users table

• If it does → logs the time + whether they’re logging in or out

• If not → responds with “UID not registered”

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Flow Summary

• I register users manually using Swagger UI or Postman

• NodeMCU just scans cards and hits the API

• Backend does the rest no SD cards, no spreadsheets just logs in a DB

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What I Learned

• How to read RFID UID using MFRC522

• How to connect NodeMCU to Wi-Fi and send HTTP requests

• How to create simple APIs with FastAPI

• How to store and query stuff using SQLAlchemy

Also learned how to mess up C++ syntax in Arduino 10 times before it finally compiled 😅

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What’s Next?

• Add a proper “admin mode” where scanning a master card lets me register new users from the scanner itself

• Maybe add a buzzer and LED for feedback

• Will write Part 2 for that once it works

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Final Thoughts

This was just a weekend thing that turned into a fun learning project.
If you’ve ever wanted to connect a microcontroller with a Python backend, this is a great way to try it out.

It’s not fancy, but it works and I built it myself.

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