Mission
Build this
Build a protected temperature probe and a hand-labelled gauge that compares the hidden heap with the weather around it.
Borrowed parts are labelled below and have a fallback. The registered device ID is fixed; sensing and behaviour decisions remain yours.
Bill of materials
Parts
| Source | Part | Fallback |
|---|---|---|
| ● kit | Thermistor and 10 kΩ resistor | — |
| ● kit | OLED | — |
| ● kit | SG90 servo | — |
| ○ fablab | Long insulated leads and probe housing | Measure at the accessible compost surface and label the limitation |
Disconnect USB before rewiring. Motors, the relay, and the servo need appropriate power and a shared ground. Never drive an actuator from the ESP32 3V3 pin.
Your invitation
A compost heap is crowded, hungry, and quietly warm
Nothing inside looks like a flame. Yet millions of small lives are breaking material apart, using energy, and releasing heat. A working heap can carry a warm interior through cold air.
Your probe will visit that hidden interior. The gauge will not declare the compost “good” or “bad.” It will show what the heap is doing compared with the world around it—and how that story changes with depth and time.
“The surface felt cold. Fifteen centimetres below it, the number was still climbing.”
The central idea
Heat is evidence of activity, not a complete diagnosis
Microorganisms use food and oxygen as they decompose the pile. Some of the released energy becomes heat. A warmer core can therefore suggest active decomposition—but temperature alone cannot tell you whether the heap has enough air, the right moisture, or the right material.
The weather surrounding the heap.
Exposed to wind and recent weather.
Insulated from the air and producing heat.
Illustrative readings. Your heap decides its own profile.
The interesting number is often not core temperature by itself, but the difference between core and ambient. A core at 20 °C means something different on a 2 °C day than on an 18 °C day.
How does a thermistor turn buried heat into celsius?
The thermistor resists electricity more when cold and less when warm. Paired with a fixed 10 kΩ resistor, it creates a changing voltage at GPIO 34. Your conversion turns that raw count into temperature.
Calibrate before sealing the sensor into its protective probe. Long leads and the housing can change the reading or slow the response, so repeat one reference check after assembly.
Depth changes the question
Where you measure is part of what you claim
How does weather reach the heap?
Easy to access, but strongly influenced by sun, rain, and wind.
Where does warmth begin?
Useful for comparing the boundary with the interior.
What is the hidden process doing?
Harder to reach and protect, but closer to the active centre.
Mark the probe depth and position. If only the surface is safely accessible, say so and ask a surface question. A limited measurement with an honest label is stronger than a “core” reading taken nowhere near the core.
Wiring
Keep the computer outside and send only the probe in
The thermistor and its insulated housing may enter the compost. The ESP32, breadboard, display, servo, and exposed joints remain dry and accessible.
The power rule: The sensor and display use 3V3. The servo uses a separate 5 V supply. Join the grounds, unplug USB while rewiring, and keep exposed electronics out of damp compost.
Bench referenceOpen the exact wiring map
| ESP32 pin | Part | Part marking | Carries |
|---|---|---|---|
3V3 | Thermistor probe | one lead | 3V3 power — top of the divider, extended on insulated leads |
GPIO 34 | Divider junction | probe + resistor | Analog voltage — ADC1 remains available while Wi-Fi runs |
GND | 10 kΩ resistor | free leg | Ground — bottom of the divider |
3V3 | OLED (SSD1306) | VCC | 3V3 power |
GND | OLED (SSD1306) | GND | Ground |
GPIO 21 | OLED (SSD1306) | SDA | I²C bus — display data |
GPIO 22 | OLED (SSD1306) | SCL | I²C bus — display clock |
GPIO 18 | SG90 servo | signal (orange) | PWM to actuator — moves the compost gauge |
external 5 V | SG90 servo | power (red) | 5V power — separate from the board supply |
GND | SG90 servo | ground (brown) | Ground — join servo and ESP32 grounds |
Use an ADC1 pin such as GPIO 34. Protect and strain-relieve every extended probe joint; a wet or broken connection can imitate an extreme temperature.
- 3V3 power
- Analog voltage
- Ground
- I²C bus
- PWM to actuator
- 5V power
Calibrate the thermistor before enclosing it, then check it again beside a reference after the housing and long leads are fitted.
Invent the dial
Cold, working, cooking—or something more honest?
The servo sweeps a needle across zones your team names. Those labels are interpretations, not properties hidden inside the thermistor.
Your team decides:
- Do the zones use core temperature or core-minus-ambient difference?
- Does the needle react immediately or wait for a trend?
- What display message means the probe has failed or been removed?
Your slow-fire expedition
Build a temperature profile, not one heroic reading
- Check the probe in known air.
Compare it with a reference before and after fitting the protective housing.
- Measure the surroundings.
Record ambient temperature and recent conditions before entering the heap.
- Visit marked depths.
Wait for the reading to settle at each position. Keep the same route and dwell time.
- Return later.
Repeat the profile. A trend earns more trust than one warm afternoon.
| Time | Ambient | Depth | Probe °C | Above ambient |
|---|---|---|---|---|
| ___ | ___ °C | surface | ___ °C | ___ °C |
| ___ | ___ °C | ___ cm | ___ °C | ___ °C |
| ___ | ___ °C | core | ___ °C | ___ °C |
When the heap misleads you
A warm reading can come from the wrong place
The value jumps when the cable moves
An extended joint is loose or damp. Inspect and strain-relieve the leads before interpreting the temperature.
The surface is warmer on a sunny day
Sunlight is heating the probe or surface directly. Compare with ambient and repeat away from direct sun.
The core changes very slowly
The probe housing and still material need time to reach the surrounding temperature. Record settling time instead of rushing it.
The reading fails once Wi-Fi starts
The divider is on ADC2. Move the junction to an ADC1 pin such as GPIO 34.
The gauge resets the board
Give the servo its own 5 V supply and keep its ground joined to the ESP32 ground.
Choose your slow story
What kind of compost observer will you become?
The depth cartographers
Map a full temperature profile and find where the heap separates from the weather.
The careful caretakers
Design dial zones from compost guidance and explain what temperature still cannot diagnose.
The cooling historians
Leave the probe at one marked depth and detect when a sustained warm period begins to fade.
A calm way through the build
Collect four small wins
cc-01says hello.Join the garden before extending the probe.
- The bare thermistor follows warm and cool air.
Build and test the conversion on the bench.
- The protected probe still agrees.
Fit the housing and leads, then repeat the reference check.
- The dial tells a measured story.
Add separate servo power and take the first marked depth profile.
The garden handshake
Share what you found
These names are the rigid part of the project. They let another team find your work without knowing what you called the variables in your code.
Compost temperature
garden/compost/compost-node/cc-01/temperatureUnit: celsius
Process state
garden/compost/compost-node/cc-01/statusUnit: enum
Listen beyond your own device.
garden/outdoor-1/frost-node/fs-01/temperatureOutdoor temperature gives the heap a background. Compare against it before claiming that a warm or cool reading came from decomposition.
Finish line
Ready to introduce to the garden
cc-01 stays online and publishes temperature and process state.
The reject feed stays clear after the final code starts.
The device reads the Frost Sentinel temperature topic.
The gauge moves from real data without resets or interrupted messages.
The build log records both calibration checks, ambient temperature, exact probe depth, settling time, and one limit of the status labels.
Only the protected probe enters the heap; the remaining electronics are dry, stable, and scannable.
The heap is not a machine with one correct temperature. Your device succeeds when it reveals a slow process without pretending to know more than it measured.