Plošná ryba Senzor (PR-SENZOR)
Device page · Flash firmware from the browser
A thermometer for the garden, greenhouse and compost. The stainless probe measures the temperature in compost (or soil), the SHT40 sensor measures air temperature and humidity. The device warns about frost and reports the compost state. Runs on a battery and sleeps between measurements.
- DS18B20 probe in a 30 cm stainless housing on a 1 m cable (1-Wire)
- SHT40 for air temperature and humidity (Qwiic)
- Frost warning with adjustable threshold and hysteresis
- Compost state by temperature
- Measurement interval from Home Assistant
What is in the box
- Jádro board in a waterproof enclosure
- Stainless DS18B20 probe, 30 cm housing on a 1 m cable
- SHT40 air sensor on a cable
- Both cables routed through glands and connected inside the enclosure at the factory
- 18650 cell holder inside the enclosure, wired to the board (the cell is not included, see below)
- Card with a link to this guide
You do not thread or crimp any cables. You only mount the enclosure, the probe and the air sensor. The probe measures compost, the SHT40 air sensor handles the frost warning and the greenhouse.
Enclosure
To stay waterproof, the enclosure has no external USB-C port and no access to the buttons. The top face has no holes, the lid is held by two screws on the underside. The status LED shines through a thin spot in the lid. USB-C and the BOOT and RESET buttons are reachable after loosening both screws and taking the lid off.
Status LED: on while the device is not connected to WiFi (waiting for setup). A short flash every 2 s means Maintenance mode, the device does not sleep. Off in normal operation.
18650 cell
Batteries are not included. Put a common 18650 Li-ion cell with 2500 to 3500 mAh from a known brand into the holder. Both button top and flat top cells fit. Buy cells from a reputable seller. Offers like "9900 mAh" are fakes with a much lower real capacity.
Insert the cell following the + and - marks in the holder. If you put it in the wrong way round, nothing gets damaged, the board has reverse polarity protection and the device just does not start.
Charging: either swap the cell for a charged one (keeping a charged spare is handy, the swap takes a minute: two screws on the underside), or take the lid off and charge the device over USB-C indoors. Never charge a Li-ion cell below 0 °C. For a frost warning that means charging indoors, not outside.
First start
Best done at home on the table, before you mount the enclosure:
- Loosen the two screws on the underside and take the lid off.
- Insert the cell into the holder. The device starts, the status LED is on and it waits 5 minutes for WiFi setup.
- Set up WiFi (see below) and add the device to Home Assistant.
- Put the lid back on and tighten the screws.
If 5 minutes are not enough, take the cell out and put it back, or press RESET.
Connecting to WiFi
- From the browser over USB: open https://plosnaryba.cz/firmware in Chrome or Edge, connect the board with a USB-C cable, click Connect and enter your WiFi name and password (Improv).
- Over Bluetooth in Home Assistant: if HA has Bluetooth (an adapter or an ESPHome Bluetooth proxy), it offers the new device for WiFi setup. Enter the details and confirm the pairing by pressing the BOOT button on the board. Without the BOOT press the device does not accept WiFi credentials.
The device has no fallback WiFi network and no default password. 2.4 GHz only.
Adding to Home Assistant
- Settings, Devices and services. The ESPHome device Plošná ryba Senzor is listed as discovered. Click Add.
- Confirm. If HA offers to set an encryption key, accept it.
- Assign an area (for example Garden). Entities appear on their own.
- Create the Maintenance mode helper (see Maintenance mode), you will need it for updates.
Later the device sleeps most of the time and shows up in HA at its next wake-up.
Installation
Probe (compost)
- Push the whole probe into the middle of the heap. That is where it is hottest, at the edge of the heap you will measure much less.
- The probe cable is 1 m long, so mount the enclosure close to the compost.
- Do not pull the probe cable tight. Take the probe out when turning the compost.
- You can also stick the probe into a garden bed if you want the soil temperature.
Air sensor (frost, greenhouse)
- The SHT40 must be in the shade and protected from rain, but in freely moving air. A small radiation shield (stacked plates, printed or bought) works best. In direct sun it reads several degrees too high.
- For frost on garden beds place the sensor low, 30 to 50 cm above the ground. Ground frost forms near the ground and a weather station at 2 m often misses it.
Enclosure
- Mount the enclosure under a roof or on a wall out of direct rain, glands facing down.
- Mount it so you can reach the screws on the underside when you swap the cell.
Settings
| Setting | Default | What it does |
|---|---|---|
| Frost threshold | 2 °C | Frost warning turns on at this air temperature and below |
| Frost hysteresis | 1 °C | the warning turns off once the temperature rises to threshold + hysteresis (3 °C by default) |
| Measurement interval | 15 min | how often the device wakes up and measures |
The 2 °C threshold is deliberately above zero: when the air at sensor height shows 2 °C, it is often already freezing at the ground. The warning uses the air temperature from the SHT40. If the air sensor is missing, it uses the probe.
The hysteresis keeps the warning from flickering when the temperature hovers around the threshold. The device remembers the state across sleep.
Home Assistant cannot send changes to a sleeping device. So change settings in Maintenance mode (see below) and turn it off again afterwards. During the first start the device is awake for 5 minutes, so you can enter the settings right away.
Compost state
By probe temperature:
| Probe temperature | State | What it means |
|---|---|---|
| below 20 °C | Cold | decomposition almost stopped, the compost needs green material, moisture or turning |
| 20 to 40 °C | Mesophilic phase | starting up or maturing, normal |
| 40 to 55 °C | Active | the compost is working at full speed |
| 55 to 70 °C | Thermophilic, hot | sanitising, kills weed seeds and pathogens |
| from 70 °C | Overheated | too little air, risk of anaerobic spoiling, turn it |
The bands are a guide, every heap behaves a little differently.
Maintenance mode
After a timer wake-up the device stays awake only until the data is sent (about 3 to 5 s) and goes straight back to sleep. For an OTA firmware update or for fine-tuning settings it has to stay awake. That is what Maintenance mode is for: the device does not sleep and setting changes take effect immediately. Turn it on in one of these ways:
- With a Home Assistant helper (the main way, the enclosure stays closed). Create it once: Settings, Devices and services, Helpers, Create helper, Toggle, name it "Plošná ryba údržba". This creates the entity
input_boolean.plosna_ryba_udrzba. The device reads it right after connecting on every wake-up, so you just turn the helper on and wait for the next wake-up. One helper works for all battery powered Plošná ryba devices. - With the BOOT button while the device is awake. The button is under the lid, so this only works with the enclosure open (for example during the first start).
- With the Maintenance mode switch on the device page in HA, while the device is awake.
In maintenance mode the status LED gives a short flash every 2 s. When you are done, turn the helper off, the device then leaves maintenance mode and goes to sleep. The helper and the BOOT button do not get in each other's way: the helper turned on switches maintenance mode on, the helper turned off switches it off only when you change it while the device is awake. So the helper state right after a wake-up does not cancel maintenance mode started with BOOT, turn that off with the Maintenance mode switch. As long as the helper is on, maintenance mode comes on at every wake-up. The device remembers the mode across sleep and restarts and would drain the battery in a few days.
You can use a different helper entity in your own build by setting the udrzba_entita substitution in the YAML.
Entities
Home Assistant builds the entity ID from the device name and the entity name, typically sensor.plosna_ryba_senzor_probe_temperature. The table lists the suffix after the device name (English firmware build).
| Type | Suffix | Meaning | Unit |
|---|---|---|---|
| sensor | probe_temperature |
temperature from the stainless probe | °C |
| sensor | air_temperature |
temperature from SHT40 | °C |
| sensor | air_humidity |
relative humidity from SHT40 | % |
| binary_sensor | frost_warning |
on at the threshold and below | |
| text_sensor | compost_state |
Cold, Mesophilic phase, Active, Thermophilic hot, Overheated | |
| number | frost_threshold |
warning threshold | °C |
| number | frost_hysteresis |
how far above the threshold the temperature must rise | °C |
| number | measurement_interval |
how often to measure | min |
| switch | maintenance_mode |
device stays awake | |
| sensor | battery |
battery level (estimate from voltage) | % |
| sensor | battery_voltage |
battery voltage | V |
| binary_sensor | charging |
battery is charging from USB | |
| sensor | wifi_signal |
signal strength | dBm |
| sensor | uptime |
time since start | s |
| text_sensor | firmware_version, ip_address |
diagnostics | |
| button | restart |
restart | |
| light | status_led |
status LED | |
| binary_sensor | boot_button |
BOOT button | |
| switch | head_power |
probe power (hidden) |
Automations
Frost notification:
automation:
- alias: "Garden: frost risk"
triggers:
- trigger: state
entity_id: binary_sensor.plosna_ryba_senzor_frost_warning
to: "on"
actions:
- action: notify.mobile_app_phone
data:
title: "Frost risk"
message: >
It is {{ states('sensor.plosna_ryba_senzor_air_temperature') }} °C outside.
Cover the plants or close the greenhouse.
Compost overheating:
automation:
- alias: "Compost: overheating"
triggers:
- trigger: numeric_state
entity_id: sensor.plosna_ryba_senzor_probe_temperature
above: 70
for: "01:00:00"
actions:
- action: notify.mobile_app_phone
data:
message: "The compost has been above 70 °C for an hour. Turn it."
Troubleshooting
| Problem | What to try |
|---|---|
| Device unavailable in HA | It is asleep, that is normal. It shows up at the next wake-up. If you need it to stay awake, turn on the Plošná ryba údržba helper. |
| Probe temperature missing | Take the lid off and check the HEAD connector. Solder jumper JP1 (1-Wire pull-up) must be closed, it is from the factory. |
| Probe shows 85 °C | DS18B20 sends this after power-up, the firmware discards it. If you see it permanently, the probe has a bad contact. |
| Air temperature missing | Take the lid off and check the Qwiic cable. SHT40 has address 0x44, the device log lists the I2C bus. |
| Air temperature too high during the day | The sensor is in the sun. Move it to the shade or into a radiation shield. |
| Humidity 100 % | Normal during morning dew. Permanently 100 % means water on the sensor, protect it from rain. |
| Frost warning does not turn off | The temperature has not reached threshold + hysteresis yet. |
Battery life
Calculated estimate, not a measurement. With the 15 minute interval (default) a 3000 mAh 18650 cell at room temperature lasts about 6 months. A longer interval means a proportionally longer life. Below freezing the cell delivers noticeably less, and that is exactly when you watch for frost. WiFi signal strength and the condition of the cell matter too. The calculation and its assumptions are in docs/baterie.md (Czech).
Put a fully charged cell in for spring and autumn, the easiest way is to keep a spare and just swap them. A forgotten Maintenance mode shortens the battery life a lot.
Safety
- Li-ion cells: do not use damaged, swollen or dented cells. Do not charge below 0 °C or above 45 °C, charge indoors in winter. Do not short the terminals.
- Outdoors: the Jádro enclosure must not stand in water. The probe is stainless and waterproof, the board is not.
- Compost: the probe handles compost temperatures, but keep the pitchfork away from the cable.
- No mains voltage: the device runs on safety extra-low voltage only (USB 5 V, battery 3.7 V). Do not connect anything from the 230 V mains to it.