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Spool Inspector — Build Guide

Everything you need to build a Spool Inspector from the PCB set. If you purchased the fully built unit, skip ahead to the User Guide.

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What You Get with the PCB Set

  • Main PCB — Custom board with HX711 load cell amplifier and SMD components pre-soldered
  • Button PCB — Small breakout board for the two tactile switches

The silkscreen labels on the PCB show where each header goes: U1 (ESP32S3), OLED, LOADCELL, TARE, CAL, BUZZ, and the M2 mounting holes in each corner.

What You Need to Buy

Full BOM with purchase links: 3dspooltracker.com/spool-inspector#bom

Electronics

  • 1x Seeed Studio XIAO ESP32S3 Sense — the brains (WiFi + camera connector built in)
  • 1x OV5640 Camera Module — autofocus camera for QR code scanning
  • 1x SSD1306 0.96" OLED Display (128x64, I2C) — shows weight, status, and menu
  • 1x 5kg Bar Load Cell — the weight sensor (HX711 ADC is already on the main PCB)
  • 1x Piezo Buzzer — beeps when a spool is scanned
  • 2x Tactile Push Button Switches (6x6x8mm) — for Tare and Calibrate

Headers & Connectors

  • 2x 7-pin male headers — for the ESP32S3 (soldered to the PCB, ESP32 sits on top and is soldered to the same pins)
  • 1x 4-pin header — for the OLED display
  • 5x 2-pin headers — buttons and piezo connections
  • 3x 4-pin Dupont housings + 3x 2-pin Dupont housings — for making cables (OLED, buttons, buzzer)
  • ~16x Dupont female crimp pins

Important: The load cell wires are soldered directly to the PCB pads — no header or Dupont connector. See the assembly steps below.

Fasteners

  • 4x M4x8mm heat-set inserts (knurled brass) — press into the scale platform
  • 4x M2x4mm screws — mount the PCB to the housing
  • 4x M4x25mm screws + M4 nuts — mount the load cell
  • 2x M3x14mm screws + M3 nuts — mount the OLED cover
  • 4x M3x12mm self-tapping screws — attach the housing to the base

Wire & Cable

  • 26-28 AWG stranded wire (assorted colors, about 1 foot total) — for Dupont cables and load cell
  • 1x USB-C cable — power and initial programming

3D Printed Parts

You'll need to print these on your 3D printer. STL files are available at 3dspooltracker.com.

Plate 1 — Inspector housing body + lid

Plate 2 — Spool label position template (for aligning QR stickers on spools)

Plate 3 — OLED cover / bezel

Plate 4 — Scale platform (top + bottom), wire tunnel, and inspector base

Tools You'll Need

  • Soldering iron + solder — for headers, buttons, buzzer, and load cell wires
  • Dupont crimping tool (SN-28B or similar) — for making the OLED, button, and buzzer cables (Amazon)
  • Wire strippers
  • Heat-set insert tip (for your soldering iron) — to press the M4 brass inserts into plastic
  • Hex key / screwdriver set — M3 and M4
  • 3D Printer — for the housing and scale parts

Assembly

Important: Direct-solder the load cell. The load cell wires must be soldered directly to the PCB pads — do not use a header or Dupont connector. Dupont connectors cause intermittent contact that leads to weight drift and unstable readings. Dupont cables are fine for the OLED, buttons, and buzzer.

Step 1: Solder the Main PCB

Solder all the through-hole headers onto the main PCB:

  1. Two 7-pin male headers for the ESP32S3 (these get soldered to the PCB — the ESP32 will sit on top later)
  2. One 4-pin header — for the OLED display
  3. Three 2-pin headers — two for buttons, one for the piezo buzzer

Do not install a header for the load cell. The load cell wires will be soldered directly to the PCB pads in a later step.

Tip: Place the headers in the board, flip it upside down on a flat surface, and solder from the back. This keeps everything straight.

Step 2: Solder the Button PCB and ESP32S3

Button PCB:

  1. Solder the two tactile push buttons onto the button PCB
  2. Solder two 2-pin headers on the back side

ESP32S3:

  1. Place the ESP32S3 on top of the 7-pin male headers you soldered to the main PCB in Step 1
  2. Solder the ESP32S3 to the top of those same header pins — this permanently mounts it to the board

Step 3: Make the Cables

Using your Dupont crimping tool, make these cables:

  • 1x 4-pin cable — OLED display
  • 2x 2-pin cables — buttons
  • 1x 2-pin cable — piezo buzzer

Do not make a Dupont cable for the load cell. The load cell wires get soldered directly to the PCB pads for stable weight readings.

Tip: Use different colored wires so you can tell cables apart later.

Pin Mapping

When wiring the OLED and load cell, make sure the pins match up correctly. The PCB labels are silkscreened on the board.

OLED Display

The header labeled OLED on the PCB has this pin order (top to bottom):

  1. VCC — Power
  2. GND — Ground
  3. SCL — Clock
  4. SDA — Data

Important: Many OLED modules have a different pin order on the module itself (commonly GND, VCC, SCL, SDA). Don't assume a straight-through cable will work — check your module's labels and wire accordingly.

Load Cell (Direct Solder)

The pads labeled LOADCELL on the PCB have this pin order (top to bottom):

  1. Power — Excitation positive (E+)
  2. Ground — Excitation negative (E-)
  3. A- — Signal negative
  4. A+ — Signal positive

Solder the load cell wires directly to these pads. Trim the wires to length, strip the ends, and solder each wire to the correct pad. Do not use a header or Dupont connector — loose connections cause weight drift and unstable readings.

Most load cells use these wire colors:

  • Red → E+
  • Black → E-
  • White → A-
  • Green → A+

If your load cell has different colors, check its datasheet. Getting A+ and A- swapped will just give you negative weight readings — easy to spot and fix.

Step 4: Prepare the Scale

  1. Press the four M4 heat-set inserts into the scale platform holes using your soldering iron with the heat-set tip
  2. Mount the load cell between the top and bottom scale platforms using the M4x25mm screws and nuts
  3. Route the load cell wires through the wire tunnel toward the PCB housing
  4. Solder the load cell wires directly to the LOADCELL pads on the main PCB (see Pin Mapping above for wire colors)

Step 5: Install Electronics

  1. Ensure the ESP32S3 is soldered to the main PCB (from Step 2)
  2. Verify the load cell wires are soldered directly to the LOADCELL pads (from Step 4)
  3. Connect the OV5640 camera to the ESP32S3's FPC connector (the flat ribbon cable)
  4. Connect the OLED display via the 4-pin Dupont cable
  5. Connect the piezo buzzer via the 2-pin Dupont cable
  6. Mount the main PCB in the housing using M2 screws

Step 6: Final Assembly

  1. Attach the OLED cover with M3x14mm screws and nuts
  2. Connect the button PCB via the 2-pin Dupont cables
  3. Place the housing lid on top
  4. Mount the housing to the base using M3x12mm self-tapping screws
  5. Connect the base to the scale via the wire tunnel
  6. Plug in USB-C for power

What's Next?

Head to the User Guide to tare the scale, calibrate, and set up WiFi.

Optional Extras

  • Niimbot label printer (B1, B21, etc.) — for printing QR spool labels
  • 50x30mm label tape — compatible with the label printer