Building the NFC reader for the printer

The Snapmaker U1 reads the NFC tags of its spools itself. Other printers cannot, and there you would have to tap “In Drucker einlegen” (load into printer) in the app after every spool change. The NFC reader does that for you: a holder with one slot per printer slot hangs on the printer. You put the sample tile of the type you are loading into it, an ESP32 reads it and tells the server which tile sits in which slot. The server puts a spool of that type into the slot and books the next print's usage onto it.

This guide is the third part after the server and the apps. The sample tiles must already be assigned to a type in the app.

Which variant for which printer

Printer	Build	ESPHome file
one slot, e.g. Prusa MK4S, Mini or Bambu P1S with external spool	ESP32-C3 with display and one PN532, one pocket	fach-1x-pn532.yaml
Prusa Core One with MMU3	ESP32 DevKit and five RC522, one reader per pocket	fach-5x-rc522.yaml
many slots, e.g. Anycubic Kobra X with ACE	ESP32-C3 with display and one PN532 in tap mode: tap the tile, choose the slot with a button	tippen-pn532.yaml

So far the variant with one pocket has been tried on a Prusa MK4S. The MMU3 variant with five readers and tap mode are fully built and compile without errors, but have not yet run on a printer. So it is best to start with a printer with one slot.

What you need

For a printer with one slot or for tap mode:

• an ESP32-C3 board with a built-in 0.42" OLED (often sold as HW-675, 20.7 × 25 mm)
• a PN532 NFC module (the red board with two small switches for I²C and SPI)
• four wires, such as female Dupont wires
• a USB-C cable for flashing and a USB power supply for continuous use

For the MMU3 with five pockets:

• an ESP32 DevKit with 38 pins (ESP32-WROOM-32)
• five RC522 modules
• wires for the shared SPI bus, most conveniently on a piece of perfboard
• a micro USB cable and a USB power supply

For the holders:

• neodymium disc magnets 12 mm in diameter and a good 2.6 mm high; they hold the holder on the printer's frame
• filament without metal or carbon fibre, so PLA or PETG for example; fibres with metal or carbon can dampen the radio field

You also need a computer with Python 3, a soldering iron and your sample tiles with NTAG tags.

1. Wiring

PN532 on the ESP32-C3. First set the two switches on the PN532 to I²C: switch 1 ON, switch 2 OFF. The reader shares the I²C bus with the C3's display, so four wires are enough:

PN532	VCC	GND	SDA	SCL
ESP32-C3	3V3	GND	GPIO5	GPIO6

On the C3 you solder the four wires on directly, without a pin header, so it fits into the holder later. On the PN532 you plug them onto the four-pin header.

Five RC522 on the ESP32 DevKit. All readers are connected in parallel to power and to the three data lines of the SPI bus. Only the SDA line (used here as chip select) goes to each reader separately. RST goes to 3.3 V, IRQ stays free.

RC522	3.3V	GND	SCK	MOSI	MISO	RST
ESP32 DevKit (all readers)	3V3	GND	GPIO18	GPIO23	GPIO19	3V3

Slot	1	2	3	4	5
SDA to	GPIO26	GPIO17	GPIO16	GPIO4	GPIO13

Never connect the readers to 5 V. Both PN532 and RC522 run on 3.3 V here.

2. Install ESPHome

The readers run on ESPHome. Install it once in a Python environment of its own and fetch the repository with the ready-made configurations:

python3 -m venv ~/esphome
~/esphome/bin/pip install esphome
git clone https://github.com/Marcel-B/fil-o-mat.git
cd fil-o-mat/reader

After that the command is ~/esphome/bin/esphome. If you want to call it simply as esphome, add an alias once:

echo 'alias esphome=~/esphome/bin/esphome' >> ~/.zshrc && source ~/.zshrc

The ESP32-C3 needs no driver. If your computer does not recognise the DevKit, it is usually missing Silicon Labs' CP210x driver.

3. Enter credentials and printer

Copy the example file and enter your Wi-Fi and the server's address:

cp secrets.example.yaml secrets.yaml

wifi_ssid: "MyWiFi"
wifi_password: "…"
filomat_url: "http://192.168.1.50:8080"
filomat_api_key: ""
ota_password: ""

For filomat_url use the address in your home network with http, not the Tailscale address: the ESP32 does not join the tailnet. If an API key is set on the server, it goes into filomat_api_key. The file secrets.yaml stays on your computer and is not added to the repository.

Now you need the printer's ID. Open http://192.168.1.50:8080/api/printers in your browser. Each printer has "id": "…" next to its name. With an API key it works in the terminal:

curl -H "X-Api-Key: …" http://192.168.1.50:8080/api/printers

Create a small file of its own for each printer that includes the shared configuration, here mk4s.yaml:

substitutions:
  name: filomat-mk4s
  friendly_name: fil-O-mat MK4S
  printer_id: "ID from /api/printers"

packages:
  fach: !include fach-1x-pn532.yaml

For the MMU3 you include fach-5x-rc522.yaml, for tap mode tippen-pn532.yaml. Because your file only contains the three values, later improvements arrive by themselves with git pull.

4. Flash and check

Connect the ESP32 by USB and run:

esphome run mk4s.yaml

The first run downloads the tools and takes five to ten minutes. If ESPHome asks for the port, choose the USB entry, such as /dev/cu.usbmodem… for the C3 or /dev/cu.usbserial… for the DevKit. If none appears, hold down the BOOT button while plugging in the cable. Later, once the ESP32 is on your Wi-Fi, updates work without a cable: then choose the “Over The Air” entry.

After uploading, the log keeps running. It shows you whether everything is right:

• Connected with an IP address: the ESP32 is on your Wi-Fi.
• In the I²C scan, Found device at address 0x24 (PN532) and 0x3C (display).
• Hold a tile to the reader. A line with Backend: HTTP 200 and "state":"known" follows.

Backend: HTTP 200 {"slots":[{"slot":0,"tag":"0423BC53CB2A81","state":"known","label":"eSun PLA #1",…}]}

The display shows “Eingelegt” (loaded) with the type and the spool number, and in the app the spool is now in the printer. With the MMU3 you hold a tile to each reader in turn. It must appear in the right slot in the log; if it shows up elsewhere, two SDA lines are swapped.

You can look at the log again at any time:

esphome logs mk4s.yaml

5. Print the holders

The holders have a pocket for the sample tile at the front and a compartment for the reader behind it. Magnets in the base hold them on the printer's frame. They sit below the reader and so outside its antenna, because right behind it they would disturb reception. They are printed standing, base on the bed; you need no supports.

An important measure is the distance between tile and reader. The PN532 reads a tile only from about one centimetre away and reliably from a good two centimetres. The holder for the PN532 therefore keeps a distance of 25 mm and is 41 mm deep as a result. The ESP32-C3 sits right in the block in between: it slides in from the left, the display looks out at the top, the USB-C socket on the left. Below it there is room for the wires, and a slot leads them back to the reader.

Part	For	Size (W × D × H)	Files
PN532 reader holder with snap catch: two catches hold the C3	one slot, tap mode	45 × 41 × 57 mm	3MF, STL
PN532 reader holder with plug: a second part with a USB hole closes the opening	one slot, tap mode	45 × 41 × 57 mm, plug beside it	3MF, STL
RC522 reader holder, five pockets	MMU3	209 × 17 × 73 mm	3MF, STL
Case for the ESP32 DevKit, box with lid	MMU3	72 × 63 × 28 mm	3MF, STL
Tag holder without reader, one pocket	just keeping tiles, e.g. at the U1	43 × 11 × 46 mm	3MF, STL
Tag holder without reader, five pockets	MMU3 without reader	209 × 11 × 46 mm	3MF, STL
Fit test for the magnets: five slots, tighter from left to right	before the first holder	85 × 4 × 18 mm	3MF, STL

You open the 3MF files directly in your slicer; for the holder with plug and the DevKit case both parts already lie side by side. The STL files contain the same parts.

The holders with a built-in C3 and the RC522 holder are new and not yet proven in continuous use. Print the fit test first and check with a reader whether the range reaches through the printed holder.

This is how you assemble the PN532 holder:

1. Press the magnets into their slots from below.
2. Slide the PN532 into the rear compartment from above, components facing back, the smooth side towards the tile. The pin header sticks out at the top.
3. Slide the C3 with its soldered wires into the block from the left until the catches engage or the plug sits, and lead the wires through the slot to the pin header.
4. Put the holder on the frame and insert a tile into the pocket.

6. Adapt the holders to your parts

All parts are generated by small Python scripts. Every measure is a parameter in them that you can change when calling the script. You need the packages manifold3d and numpy:

python3 -m venv ~/halter
~/halter/bin/pip install manifold3d numpy
~/halter/bin/python leserhalter.py                          # PN532 with snap catch
~/halter/bin/python leserhalter.py lock=stopfen             # PN532 with plug
~/halter/bin/python leserhalter.py reader=rc522 count=5     # five RC522 for the MMU3
~/halter/bin/python leserhalter.py sep=20                   # less distance between tile and reader

Script	Generates
leserhalter.py	reader holder with PN532 or RC522, optionally with built-in C3
esp_gehaeuse.py	case for the DevKit (board=devkit) or a single rail for the C3 (board=c3)
taghalter_coreone.py	tag holder without reader, count=5 for five pockets
magnet_passungstest.py	fit test for the magnets

The parameters are explained at the top of each script, such as mag_d and mag_h for other magnets or slot_w and slot_h for larger sample tiles. The result is a 3MF file in the current folder each time.

7. How you work with it

• One pocket per slot: load the spool, put the tile into the pocket, done. The display shows the type with the spool number. Pulling the tile out changes nothing in the stock. The spool stays loaded until another tile comes or you move it in the app.
• Tap mode: hold the tile to the reader, the display shows the type. With the BOOT button you choose the slot, each press one further (on the Kobra X 1–4 in the printer, 5–8 in the ACE). Four seconds after the last press the spool goes into that slot. Without a button press the reader gives up after 15 seconds.
• Unknown tile: the display shows “Unbekannt” (unknown), and with ntfy set up a notification arrives on your phone. Assign the tile to a type in the app and insert it again.

When something does not work

What you see	Cause	What helps
command not found: esphome	ESPHome is not on the path	call ~/esphome/bin/esphome or add the alias
ESP32 does not appear when flashing	not in flash mode, or a charge-only cable	hold BOOT while plugging in, try another cable
no 0x24 in the I²C scan	PN532 switches not set to I²C, or SDA and SCL swapped	switch 1 ON, 2 OFF; swap the wires on GPIO5 and GPIO6
tile is not recognised	too close to the reader, or a magnet in front of the antenna	increase the distance, keep magnets outside the antenna
display “nicht erreichbar” (unreachable)	wrong address, or the Tailscale address entered	home network address in secrets.yaml, then flash again
display “Fehler 404” (error 404)	server too old, or wrong printer_id	update the server, check the ID under /api/printers
display “Fehler 401” (error 401)	API key missing or wrong	filomat_api_key in secrets.yaml
one MMU3 slot flickers in the log	neighbouring RC522 interfere with each other	more distance between the readers

The address and all other values are written into the firmware when it is built. So after every change to secrets.yaml or your printer file, flash again with esphome run. More details on the wiring and on how the server behaves are in the reader's README (in German).