DIY or Buy Your S0tool

Choose the approach that fits you best: build your own S0tool from affordable, off-the-shelf components, or order a fully assembled PCB from the Huizebruin shop. Both options give you a local, ESPHome-compatible water and energy meter for Home Assistant.

Build It Yourself

Use a Wemos D1 Mini (ESP8266) with a 5V NPN proximity sensor and a 10KΩ resistor — total cost under €10. Perfect for DIY enthusiasts and hobbyists.

  1. Connect one end of the 10KΩ resistor to 5V.
  2. Connect the other end to D2.
  3. Attach D2 to the signal wire of the NPN sensor.
  4. Connect the sensor's GND wire to GND.
Note: Compatible only with Wemos D1 Mini V1 or V3.0 (ESP8266). You will also need a USB cable and 5V/1A power adapter.
Minimum ESPHome: 2026.8.0 — Minimum Home Assistant: 2026.8.x

Alternative: Cyble Sensor V2

Can't get hold of a 5V NPN sensor? A Cyble Sensor V2 reed switch works with S0tool too — it just needs the pull-up resistor wired the other way around.

  1. Connect the 10KΩ resistor between GND and D2 (not 5V and D2 as with the NPN sensor above).
  2. Connect the Cyble sensor's two wires to D2 and GND.
Note: The resistor placement for the Cyble sensor is different from the standard NPN wiring — double-check your connections before powering on.

Shopping List

Part Where to Buy
Wemos D1 Mini (ESP8266) AliExpress · Amazon
NPN Proximity Sensor (5V) AliExpress · Amazon · Opencircuit (NL)
10KΩ Resistor AliExpress · Amazon · Opencircuit (NL)
USB Cable + 5V Adapter Any Micro-B phone charger

Wiring Schematics

Water Meter (NPN)

Water meter schematic

Connection to D2 with a 10K resistor.

kWh Meter (S0)

kWh Meter schematic

Connection to D5 (Ports 20/21).

Complete S0tool PCB

S0tool PCB detail

Detailed overview of s0 port and npn connections.

Click image to enlarge

Wiring Schematic wemos d1 and npn sensor

S0tool DIY Wiring Diagram

Click image to enlarge

Buy a Ready-Made S0tool

Skip soldering and order a fully assembled S0tool PCB. Comes with the 10KΩ resistor pre-installed. Shipping currently available to Netherlands, Belgium, Germany, and Estonia. For other countries, contact us by email.

Order from Huizebruin Shop

kWh Meter Integration

  1. Ensure your kWh meter has an S0 output port.
  2. Connect a 10KΩ resistor between GND and D5.
  3. Connect 5V to port 20 of the S0 terminal.
  4. Connect D5 to port 21 of the S0 terminal.
S0 kWh Connection S0tool kWh Ready

Don't Have a kWh Meter Yet?

Any meter with an S0 pulse output works. Solar panels, heat pumps, and standalone energy meters all qualify — here are a few known-good options:

Rating Pulse Rate Where to Buy
45A kWh Meter 1000 imp/kWh Bol.com
40A kWh Meter 2000 imp/kWh Bol.com
32A kWh Meter 2000 imp/kWh Amazon
40A kWh Meter 1000 imp/kWh Amazon
Understanding pulse rates: a higher imp/kWh rating means finer resolution. 2000 imp/kWh = 0.0005 kWh per pulse (more accurate), while 1000 imp/kWh = 0.001 kWh per pulse. Either works with S0tool — just make sure the pulse multiplier in your ESPHome config matches the meter you buy.