Senzor: a garden temperature sensor for Home Assistant
Senzor measures temperature with a stainless DS18B20 probe plus air temperature and humidity with an SHT40. Use it in a compost heap, as a frost alert for seedlings or in a greenhouse, with everything reported to Home Assistant.
- Works with Home Assistant
- Open source
- No cloud
A pre-order costs nothing. You pay after we e-mail you that your device from the first batch is ready to ship, and you can cancel until then.
Illustration of use. Photos will follow from the first batch. See the enclosure
How it works and how it connects
- A stainless probe measures temperature in the middle of the compost or in the soil, and an SHT40 sensor measures air temperature and humidity at plant height.
- Both cables run through cable glands into the box: the probe into the keyed 6-pin JST-PH connector, the air sensor into a Qwiic connector.
- Inside the box is the Jádro board with an 18650 cell; it sleeps between readings.
- Readings go over WiFi to Home Assistant at home, no cloud.
- A frost warning on your phone, a temperature chart, and an alert when the compost overheats.
What it will look like
3D design of the enclosure, which you can also print yourself. The product is not manufactured yet, the design may change after the first test prints.
Connect to Home Assistant in three steps
Insert the cell
Take the lid off (two screws on the underside) and insert an 18650 cell. The status LED is on and the device waits 5 minutes for WiFi setup.
Set up WiFi over Improv
Over a USB-C cable from Chrome or Edge, or over Bluetooth from Home Assistant. You confirm Bluetooth pairing by pressing the BOOT button on the board.
Adopt it and mount it
Home Assistant finds the device through the ESPHome integration. Set thresholds and interval, close the lid, push the probe into the compost and put the air sensor in the shade.
One thermometer, three garden jobs
In the garden, temperature is often the one number worth knowing, but it is always somewhere different: the middle of the compost heap, leaf height next to freshly planted tomatoes, the greenhouse air at 4 a.m. That is why Senzor has two sensors: a stainless DS18B20 probe you put where it matters, and an SHT40 for the air around it.
Nothing is switched. Senzor sends all values at once and in Home Assistant you use the ones that fit the job.
Compost, frost and greenhouse
Compost. The 30 cm probe on a 1 m cable reaches the core of the heap. The firmware sorts readings into bands at 20, 40, 55 and 70 °C, so Home Assistant shows the phase as well as the number. More on the compost thermometer page.
Frost alert. The SHT40 air sensor at plant height and a binary sensor with hysteresis. It turns on below the threshold and stays on until the temperature climbs back above it by a set margin, so you are not spammed all night. More on the frost alert page.
Greenhouse. Air temperature and humidity from the SHT40, to know when to ventilate and how cold the nights get. More on the greenhouse thermometer page.
Installation and power
The probe and the air sensor come on cables already routed through the glands and connected, you do not thread or crimp anything. Keep the box in the shade, ideally under a small roof or on a stake below leaf level. It is printed in white PETG, has no external port or buttons and the lid is sealed with a TPU ring. The shade also helps because PETG becomes brittle in direct sun over time. The seal has not been tested yet, so we do not state an IP rating.
It runs on a common 18650 Li-ion cell, which is not included. It deep sleeps between readings and stays awake only a few seconds to send the data. By calculation (not measurement) a 3000 mAh cell lasts about 6 months at a 15 minute interval and room temperature, noticeably less below freezing. The calculation is in the manual. Charge it by swapping in a charged cell, or over USB-C under the lid indoors. You change the interval and other settings from Home Assistant in maintenance mode, when the device does not sleep.
Open hardware
Senzor runs on the same Jádro board as our other devices. Firmware is ESPHome under MIT, hardware under CERN-OHL-P-2.0. Want to add a soil moisture sensor over Qwiic? Edit the config and push it over the air. No cloud, no app, the data stays on your network.
Senzor costs 1 090 CZK, is available for pre-order and ships to Czechia and Slovakia via Packeta once the first batch is produced.
What is in the box
- Jádro in a PETG enclosure with a TPU seal ring and a printed lid
- Stainless DS18B20 probe, 30 cm on a 1 m cable
- SHT40 air temperature and humidity sensor
- Both cables routed through glands and connected inside the enclosure at the factory
- 18650 cell holder wired inside the enclosure (cell not included)
- Link to the manual and source files
Specifications
| Product code | PR-SENZOR |
|---|---|
| Probe | DS18B20 in a 30 cm stainless sheath on a 1 m cable |
| Air sensor | SHT40 (temperature, relative humidity) |
| Compost | probe temperature and Compost state (bands at 20, 40, 55 and 70 °C) |
| Frost | air temperature from the SHT40 and Frost warning (binary sensor with hysteresis, adjustable threshold) |
| Greenhouse | air temperature and humidity from the SHT40 |
| Power | 18650 Li-ion cell in a holder inside the enclosure (cell not included), charged over USB-C under the lid or by swapping the cell |
| Battery operation | deep sleep, interval set from Home Assistant |
| Battery life | calculated estimate about 6 months at a 15 min interval (3000 mAh cell, room temperature) |
| Chip | ESP32-C6-WROOM-1 (WiFi 6, BLE, Thread) |
| Firmware | ESPHome, MIT licence |
| Enclosure | printed PETG, sealed |
| Warranty | 24 months |
| Price | 1 090 CZK |
FAQ
Which probe is included?
One stainless 30 cm probe on a 1 m cable that reaches the middle of a compost heap. Frost alerts and greenhouses use the SHT40 air sensor, so we do not offer a short probe.
Do I have to switch between compost, frost and greenhouse?
No, nothing is switched. Senzor sends all values at once and in Home Assistant you use the ones you need: probe temperature and Compost state for compost, air temperature and Frost warning for frost, air temperature and humidity for a greenhouse.
Why two sensors?
The DS18B20 probe measures wherever you put it (compost, soil). The SHT40 measures the surrounding air. The difference between the two is often more useful than either number alone.
Will the enclosure survive outdoors?
It is printed in matte white PETG and the lid is sealed with a TPU ring. It has no external port or buttons, the top face has no holes and the lid is held by two screws on the underside. PETG becomes brittle in direct sun over time and softens around 70 to 80 °C, so keep it in the shade or under a small roof and out of direct rain. The seal has not been tested yet, so we do not state an IP rating.
How long does the battery last?
By calculation, not measurement, a 3000 mAh 18650 cell lasts about 6 months at a 15 minute interval and room temperature. Noticeably less below freezing, a longer interval gives a proportionally longer life. The calculation is in the manual.
Is a battery included?
No. The enclosure has a wired holder for a common 18650 Li-ion cell with 2500 to 3500 mAh from a known brand (button top or flat top). Buy from a reputable seller, cells labelled 9900 mAh are fakes. A cell inserted the wrong way round does no harm, the device just does not start.
Do you ship outside the Czech Republic?
We ship to Czechia and Slovakia via Packeta. For other EU countries, e-mail us at info@plosnaryba.cz and we will find a way. Prices are in Czech koruna (CZK).
When will it ship?
Senzor is on pre-order and ships once the first production batch is made.
Docs and source files
Device documentation · Firmware from the browser · How it works
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