Sourdough Monitor

A sourdough starter tells you when it is ready by how far it has risen and how fast, and both of those depend almost entirely on how warm the kitchen is. This is a printed lid that sits over a 40 oz starter jar with a 3.5in colour touch screen on top: temperature and humidity in big type, a chart of the last few hours under them, and a Home Assistant device behind it so the same numbers end up on a dashboard and in automations.

The interesting part is what happens at boot. The ESP32 keeps 48 samples in RAM and nothing on flash, so two seconds after the API connects it fires an event at Home Assistant asking for its own history back. The recorder is the storage; the jar is just the display.

Read The lid and Firmware before you order anything. The two STLs in this folder are byte-identical, the merged mesh is a sealed box that cannot be printed in one piece, and the ESPHome config in esphome/ stops partway through — it has no i2c:, no sensor: and no display: block. This page is honest about all three. It is a parts list and a set of measurements, not a config you can flash tonight.

Parts

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3.5in 320×480 touch screen with an ESP32 on the back — screen, touch panel and radio on one board, which is the only reason this fits under a 121 mm lid. This is a substitute, and you need to know how. The shipped config declares board: esp32-s3-devkitc-1 with octal PSRAM; this Hosyond board is the classic dual-core ESP32-WROOM version of the same ST7796U panel, with no PSRAM. Build on it and you change the esp32: block to a plain ESP32 board key, delete the psram: block, and check the backlight pin against your board before you trust the GPIO14 in the config. If you want the config’s platform block to be true as written, buy an ESP32-S3 3.5in 320×480 display board instead.
SHT31-D temperature and humidity module, 3-pack — I²C, ±0.2 °C and ±2% RH, and it does not mind living in the damp air over a jar. This is the sensor to use for anything where the number drives a decision; a DHT22 reads to a whole degree at these temperatures and a starter’s whole behaviour sits inside a five-degree band.
VL53L0X time-of-flight laser module, 3-pack — this is what measures rise. It looks straight down through the lid’s port at the surface of the starter and reports millimetres, with nothing touching the dough and nothing to clean. Also I²C, at 0x29, so it shares two wires with the SHT31-D at 0x44 and needs no multiplexer.
40 oz sourdough starter jar — straight-sided, wide mouth, and it comes with the marked feeding band that makes the screen’s chart checkable by eye. The lid’s skirt is bored 114.8 mm; measure the mouth of whatever jar you own before you print, because there is no thread here and the fit is friction.
JST 1.25 mm connectors with pre-crimped leads — two sensors, four wires each, inside a 31 mm cavity. Plugs let you lift the screen out without desoldering anything. Dupont jumpers work if you would rather not crimp, and 24 AWG silicone wire is the flexible stuff for the tail that has to bend inside the lid.
5 V 2.5 A USB supply and a right-angle 1 ft cable — the board runs off its own USB socket. The lid’s cable window is 12 mm wide and 7.5 mm tall, so a right-angle plug is the one that goes in and does not lever the lid off the jar.
PETG for the cup and PLA+ for the cover — see Print settings for why the two halves get different plastic.

There are no screws in this build. The CAD has no bosses, no threaded-insert holes and no fasteners of any kind — the only round hole anywhere in it is the 12 mm port through the floor.

If you want the barometric pressure the config’s header comment mentions, a BME280 module gives you temperature, humidity and pressure on the same two wires and replaces the SHT31-D outright. The globals in the shipped config only ever track temperature and humidity, so pressure is a line in a comment and nothing more — but it is a two-dollar upgrade if you want to log it.

The lid

Everything below is measured off cad/smart_jar_lid_v3.step, which is the authority. Two things about the files first, because they will both cost you a print if nobody tells you.

There is one lid design in this folder, not two. smart_jar_lid_v3.stl and smart_jar_touch_lid_v1.stl are byte-identical, and the two STEP files carry the same geometry under different product names. Open either one.

The STL is two solids merged, and it is not printable as it stands. The STEP carries two bodies — a cup and a cover — and the exported mesh is their union: a sealed 31 mm chamber with a closed floor under it and a closed disc over it. Slice that and you get supports trapped in a box you cannot open. Load the STEP or the .f3d in Fusion, separate the two bodies, and export them as two parts.

The shape

The body is a fluted cylinder, 121.2 mm across the ribs and 119.6 mm across the flutes, 45.8 mm tall. The grip runs the whole height, which is what lets you twist it off a jar with one flour-covered hand.

Feature	Measurement
Outside diameter	121.2 mm over the ribs, 119.6 mm over the flutes
Overall height	45.8 mm
Wall thickness	3.2 mm
Skirt bore	114.8 mm for the first 5 mm, then a chamfer out to a 118 mm counterbore for the next 6 mm
Internal floor	1.6 mm thick, 11 mm up from the bottom rim
Electronics cavity	114.8 mm across, 31 mm deep
Cable window	12 × 7.5 mm through the side wall, 4 mm above the floor
Cover	114.8 mm disc, 1.2 mm thick, dropping into a 1 mm rebate at the top

That 118 mm counterbore above the 114.8 mm bore is a groove, not a mistake. It is where a gasket or an O-ring goes if you want the lid to seal rather than sit.

Two openings go through or into the floor, and they are not the same kind of feature. A 12 mm round port, 24 mm off the axis, goes all the way through — it is the only hole between the cavity and the jar, so it is the window the distance sensor looks down and the vent the humidity sensor reads through. On the axis there is a 24 mm square well, 5 mm deep with a closed bottom: a locating pocket for a small breakout, not a through-hole. Don’t drill it out expecting a second port.

A starter jar has to vent. A fed starter makes real volumes of CO₂, and a lid that seals over one will either pop off at 3 a.m. or bulge the jar. The 12 mm port is the only path out of the cavity apart from the cable window, and both are small. Leave the gasket groove empty unless you have added a vent of your own.

Print settings

Setting	Value
Layer height	0.2 mm
Infill	20%
Supports	Cup only, inside the bore
Material	PETG for the cup, PLA+ for the cover
Orientation	Cup skirt-down; cover flat

Print the cup with its open skirt end on the bed. The internal floor then lands 11 mm up and needs support across the bore — but that support is 11 mm tall and pulls straight out of the open bottom when the print comes off. Any other orientation buries it. The cable window starts 16.6 mm up and bridges 12 mm, which prints clean with no help.

The cover is a flat disc. Print it face down, no supports, and give it four or five perimeters — at 1.2 mm thick and 115 mm across it is mostly perimeter anyway, and a couple of extra walls is what stops it bowing.

PETG for the cup. It spends its life over a jar of warm, wet, actively fermenting flour, and it sits under a screen that makes its own heat. PLA creeps under exactly those conditions and a lid that has crept no longer grips the jar. The cover is a flat disc in open air and PLA+ is fine for it.

Nothing printed touches the starter. The lid sits over the mouth of a glass jar and the plastic never meets the food, which is the only reason a printed part belongs anywhere near this.

Wiring

The shipped config names exactly one pin: GPIO14, an LEDC output at 1000 Hz driving backlight_pwm, wrapped in a monochromatic light called Display Backlight with restore_mode: RESTORE_DEFAULT_ON. That is the whole pin map in the file.

The screen’s own bus, the touch controller and the two sensors are not in it. On an all-in-one display board the panel pins are fixed by the PCB and you take them from the board’s own documentation, not from here — and the two I²C sensors go on whichever pair of pins the board brings out to a header. Write those four lines yourself and write them down, because a confident wrong pin number is worse than no pin number.

Firmware

esphome/bruh-sourdough-monitor.yaml is in the download section below. Here is what is actually in it.

The node is smart-jar-2, friendly name Sourdough Monitor, on esp-idf with psram: mode: octal at 80 MHz. WiFi, OTA and the API key all come from secrets.yaml; there is a captive-portal fallback AP called Smart-Jar-2 Fallback Hotspot.

Three services are exposed to Home Assistant:

Service	Argument	What it does
`force_display_update`	—	Redraws the screen now
`set_chart_hours`	`hours` (int)	Sets the chart window and asks HA for that much history
`set_brightness`	`level` (float)	Writes the backlight PWM directly

The globals are the interesting half. history_data is a float[48], so the chart is 48 points wide however long the window is — at the default eight hours that is one sample every ten minutes. chart_hours is the only global with restore_value: yes; everything else, including the history itself, is thrown away on reboot. last_temp, last_humidity and last_display_value are three separate values, so the charted quantity is not temperature and not humidity. That is the rise.

The colour table names five temperature bands — cold, cool, ideal, warm, hot, running #2196F3 blue through #4CAF50 green to #F44336 red — and three humidity bands. The thresholds that pick between them live in the display lambda, which is not in this file.

The config is truncated. It ends after the color: block. There is no i2c:, no spi:, no sensor: and no display: section, even though the API service and the debounce script both call component.update: smart_jar_display. ESPHome will refuse to compile it as it stands. What is here is the skeleton — the platform, the services, the globals, the fonts and the palette — and the sensor and drawing code is yours to write against your board.

Home Assistant

The device has no long-term storage, so it asks for its history back. Two seconds after the API connects, and again every time set_chart_hours is called, it fires this:

event: esphome.smart_jar_request_history
data:
  device: "smart_jar_2"
  hours: 8

Something in Home Assistant has to hear that and answer it. That automation is not in this folder — it is the other half of the design and it does not ship here. It needs to read the recorder for the requested window, resample to 48 points, and hand them back to the node.

If you rename the device, change the device: field in both homeassistant.event blocks as well. The node’s own name and the string in that payload are set separately, and a responder matching on smart_jar_2 will silently ignore a jar that calls itself anything else.

Assembly

Print the cup and the cover from the two separated bodies and dry-fit the screen board in the cavity before you wire anything. There is room across 114.8 mm of bore, but the cable window has to line up with the board’s USB socket, so check that first and rotate the board until it does.
Wire the two sensors on flying leads long enough to reach the floor with the screen lifted out. Both are I²C and both live on the same pair of wires; run one four-way plug up to the board and split it at the sensor end.
Mount the VL53L0X over the 12 mm port so it looks straight down the jar, and keep it square — a tilted time-of-flight sensor ranges to the jar wall instead of the starter and reads a constant.
Put the SHT31-D near the port too, but not in it. It wants the jar’s air, not a bead of condensate. Anywhere on the underside of the floor within reach of the vent is right.
Flash and bring it up on the bench with the jar empty. Confirm the entities arrive in Home Assistant and that the distance reading changes when you put your hand under the port, before you close it up over live starter.

Troubleshooting

The chart is empty after every reboot. That is the design working and the Home Assistant side not answering. history_data has restore_value: no, so the node comes up with nothing and waits for the event responder. Check the automation first, then check that the device: string still matches.

The distance reading is constant. Either the sensor is tilted and ranging on the glass, or its window has fogged. Condensation on a ToF lens inside a jar of fermenting starter is the failure mode of this whole build — it is warm, wet air against a cold plastic lid, and it will happen every time the kitchen cools overnight.

Humidity pegs at 100%. Same cause, one component over. A starter jar’s headspace really does saturate; if you want a number that means something, read the room rather than the jar, or accept that this reading is a condensation alarm and not a measurement.

It compiles with an error about an unknown ID. smart_jar_display is referenced by the API service and by the debounce script but never defined. See the caution under Firmware.

The screen is dark but the node is online. The backlight is a light entity, not a fixed output — something turned Display Backlight off, or set_brightness was called with 0. It restores to on at boot, so a power cycle proves it.

Files & downloads

Printable parts

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smart_jar_lid_v3.stl 502 KB
smart_jar_touch_lid_v1.stl 502 KB

Parts marked from CAD are tessellated from the STEP files below, for previewing only — approximate where the STEP is exact. Download the STEP for the real model.

ESPHome configuration

Copy this into your ESPHome directory and adjust the substitutions at the top. Secrets are referenced by name — see secrets.yaml.example .

bruh-sourdough-monitor.yaml 213 lines

# BRUH Sourdough Starter Monitor
# ────────────────────────────────────────────────────────────
# Smart jar monitor for sourdough starter fermentation tracking.
# Features TFT display with historical graphs, temperature and
# humidity monitoring, with Home Assistant integration.
#
# Hardware:
#   - Board: ESP32-S3
#   - Display: TFT screen
#   - Sensors: Temperature, humidity, pressure sensors
#   - PSRAM: Octal mode for data buffering
# ────────────────────────────────────────────────────────────

esphome:
  name: smart-jar-2
  friendly_name: Sourdough Monitor
  on_boot:
    priority: -100
    then:
      - wait_until:
          api.connected:
      - delay: 2s
      - homeassistant.event:
          event: esphome.smart_jar_request_history
          data:
            device: "smart_jar_2"
            hours: !lambda 'return id(chart_hours);'

esp32:
  board: esp32-s3-devkitc-1
  framework:
    type: esp-idf

psram:
  mode: octal
  speed: 80MHz

logger:

api:
  encryption:
    key: !secret api_key
  services:
    - service: force_display_update
      then:
        - logger.log: "Force display update requested"
        - component.update: smart_jar_display

    - service: set_chart_hours
      variables:
        hours: int
      then:
        - logger.log:
            format: "Setting chart hours to: %d"
            args: ['hours']
        - globals.set:
            id: chart_hours
            value: !lambda 'return hours;'
        - homeassistant.event:
            event: esphome.smart_jar_request_history
            data:
              device: "smart_jar_2"
              hours: !lambda 'return hours;'

    - service: set_brightness
      variables:
        level: float
      then:
        - output.set_level:
            id: backlight_pwm
            level: !lambda 'return level;'

ota:
  - platform: esphome
    password: !secret ota_password

wifi:
  ssid: !secret wifi_ssid
  password: !secret wifi_password
  ap:
    ssid: "Smart-Jar-2 Fallback Hotspot"
    password: !secret ap_password

captive_portal:

globals:
  - id: history_data
    type: float[48]
    restore_value: no

  - id: history_count
    type: int
    restore_value: no
    initial_value: '0'

  - id: history_min
    type: float
    restore_value: no
    initial_value: '0.0'

  - id: history_max
    type: float
    restore_value: no
    initial_value: '100.0'

  - id: history_loaded
    type: bool
    restore_value: no
    initial_value: 'false'

  - id: last_display_value
    type: float
    restore_value: no
    initial_value: '-999.0'

  - id: last_display_minute
    type: int
    restore_value: no
    initial_value: '-1'

  - id: last_temp
    type: float
    restore_value: no
    initial_value: '-999.0'

  - id: last_humidity
    type: float
    restore_value: no
    initial_value: '-999.0'

  - id: chart_hours
    type: int
    restore_value: yes
    initial_value: '8'

  - id: display_update_pending
    type: bool
    restore_value: no
    initial_value: 'false'

script:
  - id: update_display_debounced
    mode: restart
    then:
      - delay: 100ms
      - component.update: smart_jar_display

output:
  - platform: ledc
    pin: GPIO14
    id: backlight_pwm
    frequency: 1000Hz

light:
  - platform: monochromatic
    output: backlight_pwm
    name: "Display Backlight"
    id: backlight
    restore_mode: RESTORE_DEFAULT_ON

font:
  - file: 'gfonts://Roboto'
    id: font_chart
    size: 14

  - file: 'gfonts://Roboto'
    id: font_label
    size: 16

  - file: 'gfonts://Roboto@medium'
    id: font_status
    size: 18

  - file: 'gfonts://Roboto@bold'
    id: font_top_row
    size: 28

  - file: 'gfonts://Roboto@bold'
    id: font_env
    size: 28

color:
  - id: color_black
    hex: '000000'
  - id: color_white
    hex: 'FFFFFF'
  - id: color_gray
    hex: '888888'
  - id: color_dark_gray
    hex: '444444'

  - id: color_chart_line
    hex: '64B5F6'
  - id: color_chart_dot
    hex: '2196F3'

  - id: color_temp_cold
    hex: '2196F3'
  - id: color_temp_cool
    hex: '4FC3F7'
  - id: color_temp_ideal
    hex: '4CAF50'
  - id: color_temp_warm
    hex: 'FF9800'
  - id: color_temp_hot
    hex: 'F44336'

  - id: color_humid_dry
    hex: 'F44336'
  - id: color_humid_low
    hex: 'FF9800'
  - id: color_humid_ok
    hex: 'FFEB3B'

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CAD source

The design itself, if you want to change it rather than print it. A STEP file is exact geometry and opens in almost anything; a Fusion 360 archive keeps the modelling history, so you can go back and edit the sketch that made the part.

smart_jar_lid_v3.f3d 325 KB Fusion 360 archive — the editable design, with its history
smart_jar_lid_v3.step 271 KB STEP — opens in Fusion 360, FreeCAD, Onshape, SolidWorks
smart_jar_touch_lid_v1.f3d 323 KB Fusion 360 archive — the editable design, with its history
smart_jar_touch_lid_v1.step 271 KB STEP — opens in Fusion 360, FreeCAD, Onshape, SolidWorks

Every file for this project on GitHub →