FAQ

S0tool works with most water meters that have a pulse output or a reflective disc that an NPN sensor can detect. Check the compatibility list for known working meters. If yours is not listed, it may still work — ask on Discord or test it yourself.

Use a 5V NPN sensor — specifically the LJ18A3-8-Z/BX-5V (M18, 8mm detection) or the smaller LJ12A3-4-Z/BX-5V (M12). Make sure it is rated for 5V, not 6V or higher. See the DIY or Buy page for purchase links.

The sensor should be placed against the glass or plastic window of the water meter so it can detect the rotating reflective disc inside. Most meters have a small metal insert or star-wheel visible through the window — point the sensor directly at that.

Most water meters generate 1 pulse per litre of water used. Some models use different ratios (e.g. 1 pulse per 0.1L or 10L). Check your meter's datasheet or the label on the meter itself. The S0tool firmware defaults to 1 pulse = 1 litre but this can be configured in ESPHome.

An S0 port is a standardized pulse output found on many energy meters (kWh meters). Each pulse represents a fixed amount of energy — typically 1000 pulses per kWh. The S0tool reads these pulses on pin D5 and translates them into energy readings in Home Assistant.

Yes! Any device with an S0 output can be connected, including solar inverters, heat pumps, and EV chargers. Connect the S0 output to pin D5 as described on the DIY or Buy page.

Yes — that is exactly what the Standard configuration does. The water meter NPN sensor connects to D2 and the S0 kWh input connects to D5. Both are monitored simultaneously.

Currently only the Wemos D1 mini V1 or V3.0 (ESP8266) is supported. The ESP32 is not supported at this time.

The DIY version costs less than €10 in parts and works great if you are comfortable with basic wiring. The ready-made S0tool PCB (€18–€30) comes fully assembled with the resistor already soldered on, and is easier to install. Currently ships to Netherlands, Belgium, Germany and Estonia — more countries coming soon, contact by email to ask about yours. Both versions run the same firmware.

Yes, for the DIY build. The resistor acts as a pull-up between 5V and D2 (water) or between GND and D5 (S0/kWh). The ready-made PCB already has this resistor built in.

In the Energy Dashboard settings:

• Water consumption: search for watermeter Totaal
• Water flow (new in v3.6.9): search for watermeter flow and add it under Water flow (Waterdoorstroom). If HA shows a unit changed warning, choose to relabel l/min to L/min without conversion and click Repair.
• S0 port / kWh: search for Totaal opgebracht

This works from Home Assistant 2022.11 and S0tool v22.10.20+. The flow sensor requires v3.6.9+. See the Troubleshooting page for screenshots.

Use the Home Assistant service call to set the correct value. Go to Developer Tools → Services and call the S0tool calibration service. Instructions are on the Troubleshooting page.

Open the ESPHome add-on in Home Assistant, find your S0tool device, click the three dots, select Validate, then Install. You can also use the web installer to flash the latest firmware over USB.

Yes! Import the update notification Blueprint into Home Assistant. It creates an automation that alerts you when a new version is released on GitHub. See the Troubleshooting page for the import link.

No. S0tool runs entirely on your local network. All data stays on your Home Assistant instance. No cloud connection, no account, no subscription required.