Tap Reader Build

Building our own tap reader

Our own reader by the door: today a Raspberry Pi 4 with a PN532 NFC HAT, going into a circular 3D-printed case. The RC522 plan below is kept for reference.

Current build (October 20)

  • Reader: a Raspberry Pi 4 with a Waveshare PN532 NFC HAT, plugged straight onto the GPIO header (no loose wires), set to UART mode.
  • Cards: S50 (MIFARE Classic 1K) cards, card-sized, read at 13.56 MHz by a short tap.
  • Software: the same reader.py, now with a PN532 mode (READER=pn532). It runs as a service that starts on boot and posts each tap to the OCS presence API.
  • Tested end to end: tap a card, and the live dashboard shows Present in about 2 seconds.
  • Next: the circular enclosure below, then mounting at the door.

We switched from the RC522 board to a PN532 HAT: same 13.56 MHz cards and the same code path, without wiring. The RC522 plan below is kept for reference.

The finished reader, a Raspberry Pi 3 and RC522 board in a 3D-printed case, mounted on the wall by the classroom door. A student taps the tag on their laptop and the dashboard shows Vibha is present.

The plan: our own reader by the door, a Pi 3 and an RC522 in a printed case.

Exploded view of the reader, top to bottom: 3D-printed case lid, RC522 RFID board, Raspberry Pi 3, 3D-printed case base.

What's inside: lid, RC522, Pi 3, base.

Wiring from the RC522 to the Raspberry Pi 3 GPIO header: SDA to pin 24, SCK to pin 23, MOSI to pin 19, MISO to pin 21, IRQ not connected, GND to pin 6, RST to pin 22, 3.3V to pin 1. The RC522 runs on 3.3 volts, never 5 volts.

Wiring: SPI0 to the Pi header, the mfrc522 library's default pins. 3.3 V, never 5 V.

Software flow: the RC522 reads the card ID, the mfrc522 Python library passes it to reader.py, which posts the tap to the presence API, Flask records it, and the dashboard shows Present. Only the reader hardware changes; the library, reader.py, the API, Flask and the dashboard stay the same.

Only the reader hardware changes. Everything after it stays the same.

Build timeline: RC522 ordered (done), parts arrive, wire it and read a first card, reader.py integration, Shay prints the case, assembly, mount by the door and test, classroom pilot.

Build timeline: the RC522 is ordered, and everything else comes next.

Raspberry Pi 3 Have
RC522 kit · ~$14 Ordered
Cards + fobs In the kit
Case 3D printed
GPIO board + cables Have

Photo of the Pi 3, GPIO board and ribbon cable goes here

Parts: most are already here, and the RC522 kit is on order.

Option D: for

  • Costs a few dollars
  • Works this week
  • No code changes
  • Same fob IDs as today

Watch out

  • Short cable: the Pi sits right behind the reader. Fix: Pi goes in the same case
  • Looks like a prototype. Fix: Shay's printed case

Why Option D, from our options doc: cheap, fast, and no code changes.

Enclosure v2: circular, open only at the tap area

Requirements (mentor feedback, issue #15)

  • The whole enclosure is circular, like the top of a speaker.
  • Closed everywhere except the area directly over the antenna (or thin enough there to read through).
  • Obvious where to tap: a raised tap area, or rings that guide the card to the center.
  • Power, cables, and ports still reachable with the case closed.

Steps

  • Measure the PN532 HAT's antenna area, the Pi 4, its ports, cables, and mounting holes.
  • Test how far cards read through PLA at a few thicknesses: fully open top, or a thin closed top.
  • Sketch 2–3 options (raised tap area or guide rings) and pick one.
  • Model it in CAD with clearance on mating parts for printer shrinkage.
  • Print a quick low-infill test piece to check fit, adjust, then do the final print.
  • Reliability test with the case on: 10 taps per card, several cards, slightly off-center taps.

The next enclosure, due for N@TM: one circle, one obvious place to tap. Printed with Shay.

Talking points and details

Talking points

Why build our own (Option D)

  • Costs a few dollars and can work this week (options doc).
  • No code changes: reader.py already reads cards with the mfrc522 Python library, the standard RC522 library. The fob IDs we've already registered should stay the same.
  • Mr. Mortensen backed it: he's bringing supplies and ordering the RC522, and Shay is printing a case. Building it ourselves also means we control the hardware and can fix it.

What the RC522 is and how it connects

  • An RFID reader board built on NXP's MFRC522 chip. It works at 13.56 MHz and reads MIFARE cards like the S50 (MIFARE Classic 1K) cards and key fobs in the kit.
  • It talks to the Pi over SPI: 8 wires to the GPIO header (see the wiring diagram). IRQ isn't used.
  • Power is 3.3 V only. The chip's supply range is 2.5–3.6 V and the absolute maximum is 4.0 V, so the Pi's 5 V pins would destroy it.
  • SPI has to be switched on in the Pi's settings (raspi-config → Interface Options → SPI) before the library can see the board.

Who's doing what

  • Mr. Mortensen: supplies, and the RC522 order.
  • Shay: prints the case and helps with assembly.
  • Us: wiring, software, and testing.

How we'll test it (planned, not done yet)

  1. First card read: the Pi prints the card's ID. Check it matches the ID already registered for that fob.
  2. End to end: tap, then the dashboard shows Present. Same checks as the Postman demo.
  3. Repeated taps: tap many times and count any missed or doubled reads.
  4. Read distance: measure how far away a tag still reads, both bare and stuck on a laptop.

Open questions

  • Network: the Pi 3 will hit the same school-wifi problem as before (devices can't reach my laptop). Options: Ethernet, a phone hotspot, or hosting Flask somewhere both can reach.
  • PN532 HAT: do we need it at all, or is the RC522 enough?
  • Read range through a laptop or case: laptop bodies (often metal) can weaken 13.56 MHz reads. We need to test where the tag sits.

Sources

Not verified on hardware yet: the wiring, the first read, and read range. All of it is planned until the parts arrive.

← How It Works
Project TeamRuta Sirdeshmukh, Vibha Mandayam, Kush Shah
MentorDr. Douglas Fenner
Working prototype · UHF pilot in progress
View Repo