Filament weight scale spool holder - Prusa CORE One

Spool holder with Integrated filament weight scale for Prusa CORE One

This is a spool holder for the Prusa CORE one with an integrated load cell to measure filament spool weight. It takes up no extra space* compared to the stock filament spool. It's connected to a micro-controller flashed with ESPHome.

I also added a PN532 NFC reader to tag my spools but this is completely optional. I have not yet added instructions or files for that but included my config.

* The electronics take up space

NOTE: Don't expect great precision. While calibration can help, there is going to be drift in the measurements especially as the chamber temperature heats and cools. The intent of this project is to be a general measuring system – not a precision scale. 

Hardware required

• 5kg load cell, straight bar (pictured). Dimensions are 80mm long, 12.7mmx12.7mm. 2xM4 holes on the load side and 2x5mm holes on the static mounting side.

• HX711 Load cell ADC
• ESP32-C6 recommended but anything that is supported by ESPHome
• QTY 2, M4x16mm button head fasteners
• QTY 2, M5x16mm button head fasteners
• M3 x 5 x 4mm heat set inserts (Voron style)
• M3 x 8mm Torx T10 fasteners (these are reused from the original spool holder)

Instructions

1. Print the parts. Minimum 3 walls, 20% infill. PETG or ASA recommended
2. Program your micro-controller using the config below, replacing the values as required
3. Insert the heat set inserts to the bottom half
4. Insert the load cell into the bottom half with the wires leading out of the back. (Towards the printer chamber). The arrow should be on the opposite side. 
5. (Optionally) Tape down the wires with some Kapton tape
6. Screw in the M5 fasteners to secure the bottom half of the load cell
7. Attach the top half of the spool holder with the M4 fasteners
8. Affix to the CORE One, while keeping the wires out of the way and avoid tangles.

Programming the microcontroller for ESPHome

In my photos I used a Wemos D1 Mini but this is the ESP8266 and not recommended by ESPHome. If you are buying a new microcontroller for this project, I would recommend an ESP32. The config below is one for the ESP32-C6-DevKitC-1.

Open ESPHome Builder and use the following config, modifying the pins for those matching your microcontroller:

substitutions:
  spoolman_url: "http://192.168.1.110:7912" # replace this with your Spoolman server and port

esphome:
  name: prusa-core-one-filament-scale
  friendly_name: Prusa CORE One Filament Scale

esp32:
  board: esp32-c6-devkitc-1
  flash_size: 8MB
  variant: esp32c6
  framework:
    type: esp-idf

logger:

api:
  encryption:
    key: "<there's_a_random_key_here>"

ota:
  - platform: esphome
    password: "<there's_a_random_password_here>"

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password

  ap:
    ssid: "prusa-core-one-filament-scale"
    password: "random_characters_here"

captive_portal:

http_request:

sensor:
  - platform: hx711
    id: gross_filament_weight
    name: "HX711 Value"
    dout_pin: GPIO0
    clk_pin: GPIO1
    gain: 128
    update_interval: 10s # Change this to a longer value once you've calibrated the HX711. 30s is good
    filters:
      - calibrate_linear:
          - 23000 -> 0
          - 61550 -> 100
          - 155000 -> 200
          - 235000 -> 500
          - 439000 -> 973 # These are example values. You will need to calibrate these
    unit_of_measurement: g

number:
  - platform: template
    name: "Spoolman Spool ID"
    id: spoolman_spool_id
    icon: "mdi:printer-3d"
    min_value: 0
    max_value: 999999999
    step: 1
    restore_value: True
    optimistic: True

button:
  - platform: template
    id: send_to_spoolman
    name: "Send weight to Spoolman"
    on_press: 
      then:
        - if:
            condition:
              - switch.is_on: toggle_spoolman_update
            then: 
              - http_request.send:
                  url: !lambda |-
                        char buf[256];
                        sprintf(buf, "%0.f", id(spoolman_spool_id).state);
                        return ((std::string) "${spoolman_url}/api/v1/spool/" + buf + "/measure").c_str();
                  method: PUT
                  headers:
                    Content-Type: application/json
                  json:
                    weight: !lambda "return std::to_string(id(gross_filament_weight).state);"

switch:
  - platform: template
    name: "Send to Spoolman?"
    id: toggle_spoolman_update
    icon: "mdi:server"
    restore_mode: RESTORE_DEFAULT_ON
    optimistic: True

#### Optional LED on the devkit board             ####

light:
  - platform: esp32_rmt_led_strip
    rgb_order: GRB
    pin: GPIO8
    num_leds: 1
    chipset: ws2812
    name: "Board light indicator"
    id: board_led

#### Everything below here is completely optional ####
#### Remove everything below if you are not using ####
####    an NFC reader                             ####
i2c:
  sda: GPIO4
  scl: GPIO5
  scan: true
  id: bus_a

#### Tags have a single NDEF text record. It is   ####
####    formatted as:                             ####
#### spool_id:<id>                                ####
#### Example:                                     ####
#### spool_id:37                                  ####
pn532_i2c:
  - update_interval: 1s
  - on_tag:
      then:
        - light.turn_on:
            id: board_led
            brightness: 100%
            red: 0%
            green: 100%
            blue: 0%
            flash_length: 5s
        - lambda: |
            if (!tag.has_ndef_message()) {
              return;
            }
            auto message = tag.get_ndef_message();
            auto records = message->get_records();
            for (auto &record : records) {
              std::string payload = record->get_payload();
              size_t pos = payload.find("spool_id:");
              int spool_id = atoi(payload.substr(pos + 9).c_str());
              id(spoolman_spool_id).publish_state(spool_id);
              ESP_LOGI("custom", "Scanned spool ID: %d", spool_id);
            }
            return;
        - button.press: send_to_spoolman

Connecting hardware

Connect the load cell to the HX711 ADC board following this wiring scheme:

Red → E+

Black → E-

White → A-

Green → A+

Wire the HX711 to 3.3V power (VCC), ground(GND), clock (SCL/SCK) and data (DOUT/DT).

If you're adding a PN532 NFC reader, then you'll need to share the 3.3V power with the HX711. This will require some soldering or crimping multiple connectors. I tried the HX711 with 5V with no issues but it's likely going to cause premature failure.

Calibrating the scale

The printer should not be printing while doing this or else it could throw off the values. It should also be around room temperature.

1. On a kitchen scale or similar, measure 5 known spool weights, including an empty spool (lightest you can find). Write these measurements down and keep them in mind.
2. After flashing the micro-controller, assembling everything and mounting the filament holder to the printer, load up the logs for ESPHome.
3. Note the initial value you're receiving from the HX711. In my case it was around 23000. This is your tare weight (IE: your zero). Write this down as your 0.
4. Load up the empty spool. Wait around 60 seconds.  Write down that value.
5. Repeat this step with the other spools waiting ~60 seconds for the weight to “settle.”
6. Fill in the values in your ESPHome config. Example below:

      - calibrate_linear:
          - 23000 -> 0
          - 61550 -> 101
          - 155000 -> 220
          - 235000 -> 550
          - 439000 -> 973

Here's a little chart you can use to write things down:

This should be enough to get you started.

This is a CC0 model. Remixes welcome!

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