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FAQ

The questions that come up again and again, on Discord and on MakerWorld.

Building and hardware

My scale or NFC reader does not work. Where do I start?

With the hardware, in this order. Almost every report so far came down to one of these three, and not once was it the firmware:

  1. The DIP switches on the PN532. They are the most common cause by far. See the next question.
  2. The cables and plugs. A plug that is not fully seated, a cold solder joint, two wires swapped. See how to check the wiring.
  3. A faulty part. When everything above is right and it still does not work, it has almost always been a defective module. See I have checked everything.

The scale checks itself on every start and says in the status bar what it found. Each message, with the likely cause, is under What the scale is telling you.

How do I set the DIP switches on the PN532?

The PN532 has to run in I2C mode: SW1 = ON, SW2 = OFF. In any other position it does not answer on the bus at all, and the scale reports the NFC reader as missing.

  • Look closely: the switches are tiny, and one resting halfway between the two positions is the classic case. Push each one firmly all the way.
  • Set them before soldering and assembly if you can. Details and photos on The parts in detail.
  • The module only reads the switches when it powers up. After changing them, unplug the scale completely and plug it in again.

How do I know my wiring is right?

  • Plugs fully seated. The small JST connectors need a firm push until they click. Half-seated plugs are behind many "it worked yesterday" reports.
  • Solder joints checked while the plug is in. A cold joint can measure fine on the bench and open as soon as the cable is under strain.
  • Nothing swapped: SDA on pin 3, SCL on pin 4, GND on pin 2, 5V on pin 1. Every pin is in the Wiring tables.
  • PN532 straight to the WT32, not through the NAU7802's second STEMMA QT socket: that one only carries 3.3V, and the PN532 needs 5V.
  • Look at the boot line. On every start the scale logs which chips answer on the bus: I2C_EXT scan: 0x24 PN532, 0x2A NAU7802 is what you want to see. You find it in the serial monitor (115200 baud) or on the Logs page of the web interface. A missing address points straight at that module's cable.

I have checked everything and it still does not work

Then the most likely cause is a faulty part. The modules are cheap, and a bad one in a batch happens: a PN532 that answers on the bus but never reads a tag, a NAU7802 whose readings drift, a load cell that does not settle.

Swapping the suspicious module is usually quicker than hunting further. If you have a second one, try it. If you are still stuck, ask on Discord and bring the boot line from the log; it tells us a lot.

Which parts should I buy?

The full list with links is in the Bill of materials.

Prefer everything in one cart? A community member put together an AliExpress list with parts that work well too: 👉 SpoolmanScale parts list on AliExpress

Do I need a Raspberry Pi?

No. SpoolmanScale is a standalone device. It needs a filament manager somewhere on your network - Spoolman, FilaMan or BamBuddy - and that can run anywhere: a NAS, a server, an existing Pi, a Docker host.

SpoolmanScale Pro turns a Raspberry Pi into such a server, set up and kept up to date from a web page.

Does it work without a load cell?

Yes. Switch off Settings → Scale → Scale fitted and restart. The device then works as a pure tag terminal: it reads, links and writes tags and sets locations, and the weights leave the screen.

Tags

Do I have to write anything to my tags?

No. The scale identifies a spool by the tag's factory UID, and the link between UID and spool lives in your inventory.

Writing tags is optional - useful if you want other readers, or an Anycubic ACE, to understand the tag too. See NFC tags.

Which tags should I buy?

NTAG215 if you want the scale to write them. NTAG213 reads perfectly but its 144 bytes are too small to hold a record. MIFARE Classic cards work too, by their UID.

Full detail and the compatibility table on NFC tags.

Can I use my Bambu Lab spools?

Yes. Bambu's own tags are read and decrypted. They are never written - that is deliberate.

Can a spool have two tags?

Yes, one on each side, so the spool is found whichever way round it lies. Right after linking, the scale asks for the second one. See NFC tags.

What about Snapmaker, Creality or Prusa tags?

Snapmaker tags can be read (an option in the web settings), Creality tags by their UID. Prusa's OpenPrintTag uses a radio standard the PN532 cannot read.

Why is my tag read unreliably?

Most often because it is too close to the reader, which is the opposite of what people expect. Add 5 to 20 mm of distance before replacing the tag. The physics behind it is on NFC tags.

Everyday use

Can I switch backends later?

Yes, any time, and your entries for the others are kept. The display clears and the spool on the pad is looked up again at the new backend.

Why does it say six digits instead of grams?

The scale has never been calibrated, so it is showing the converter's raw value. That is normal on a new build. See Scale not calibrated.

Is it accurate enough?

A 24 bit ADC on a 2 kg cell resolves far below a gram. What limits you in practice is the reference weight you calibrated with, so use a good one. Readings are shown and stored as whole grams.

Can I use 5 GHz WiFi?

No. The ESP32-S3 has a 2.4 GHz radio only, and a 5 GHz network will not even appear in the scan.

Do I have to type my WiFi password on the scale?

No. The web flasher asks for it right after flashing, and on the scale Set up by phone lets you enter it on your phone instead. Typing it on the touchscreen still works, with every character. See Step 2 - WiFi.

Does it need the internet?

No. Everything runs on your own network, with no cloud, no account and no subscription. Only two things reach outside when there is internet: the clock is set from a public time server, and once a day the scale asks GitHub whether there is a new firmware. That check can be switched off with Auto check on the firmware update screen.

Can two people use the web interface at once?

It is a small embedded web server on a microcontroller. It works, but it is built for one person at a time. The settings and maintenance areas are off by default, and you can protect them with a password (4 to 8 digits).

Which label printer can I use?

For now the Phomemo M220 over Bluetooth, as a first version marked beta. More printers are planned. See Bluetooth & labels.

Updates and bug reports

Why does 0.8.0 need the USB cable once?

0.8.0 divides the scale's memory anew, which only works over the cable. It is a one-time step for scales coming from 0.7.x, all settings are kept. See Updating the firmware.

How do I send a useful bug report?

Open the Logs page in the web interface and set the scope to Verbose, then repeat what went wrong. Download the log and post it on Discord or in a GitHub issue, together with the firmware version and which backend you use. No SD card needed: the log can live in the scale's internal memory.