An open-source hardware interface designed to safely connect microcontrollers (like ESP32 or Raspberry Pi) to the EMS (Energy Management System) bus used by Bosch, Buderus, Junkers, IVT and Nefit heating systems.
- Galvanic Isolation: Full optical/digital isolation protecting your microcontroller and smart home infrastructure from high-voltage spikes or ground loops on the boiler side.
- 4-Pin Output Interface: Simple connection via
GND,Rx,Tx, and3.3V. - High Compatibility: Designed to be fully compatible with popular open-source firmware EMS-ESP32.
- Compact DIY Design: Two-layer PCB created in Autodesk Eagle, optimized for easy manual soldering or cheap fabrication (JLCPCB, PCBWay).
- Input: 2-wire EMS Bus (Non-polarized)
- Output: 3.3V Logic Levels (
Rxfrom EMS to MCU,Txfrom MCU to EMS) - Power Supply:
- EMS Side: Powered directly from the EMS bus line.
- MCU Side: Powered from the connected microcontroller via the
3.3Vpin (requires approx. 10 mA).
- Isolation Voltage: Min 3 kV (achieved via optocouplers 6N137 and 6N136).
/hardware- Original Autodesk Eagle design files (.sch,.brdand.lbr)./gerber- Production-ready Gerber and Drill files (Excellon) for easy PCB manufacturing./bom- Bill of Materials with component specifications./assets- Images and pdf files.
You can notice the strict galvanic isolation of both parts. They are bridged only by optocouplers.
- Manufacture the PCB: Download the ZIP file from the
/gerberfolder and upload it to any PCB manufacturer (e.g., JLCPCB, PCBWay, OSH Park). Choose a standard 1.6mm thickness, 2-layer option. - Buy Components: Refer to the BOM.csv or BOM.pdf file for the exact part list.
- Assembly: Solder the components starting from the lowest profile parts (resistors, diodes) to the highest (connectors, optocouplers).
Connect the 2-wire terminal to your boiler's EMS service jack or thermostat line.
| Interface Pin | ESP32 Pin | Description |
|---|---|---|
| GND | GND | Ground reference (isolated side) |
| Rx | GPIO 5 (Rx) | Connect to ESP32 Rx pin |
| Tx | GPIO 17 (Tx) | Connect to ESP32 Tx pin |
| 3.3V | 3.3V | Power input for the isolated side |
This hardware is verified to work with:
- EMS-ESP32 Firmware: Set the board profile to Custom and configure your chosen Rx/Tx pins in the web interface.
C12 - 2200µF/25V
Originally I wanted to use a capacitor with a capacity of 1000µF. During testing, everything seemed to be fine. However, for dimensional reasons, I also tried a capacitor with a capacity of 470µF, because it fit into the intended box even in the normal position ("vertical"). However, in practice it turned out that with this capacitor the communication error rate increased sharply. Of course, this was not desirable, so I used a larger capacitor (with sufficient margin).
OC1 and OC2 - 6N137 and 6N136
For the Rx direction (EMS -> ESP32), I chose an optocoupler of the 6N137SM-ISO type. It is fast enough and guarantees problem-free communication. In the opposite direction, the cheaper 6N136S-TA1-L is more than enough. Both optocouplers are in an SMD8 package with gull-wing pins.
R2, R3 and R4 - 68Ω
The resistors are in a R2010 type case, so they should last with a large margin. Regular resistors in a R0805 case would be overloaded if used in this way and the connection would not be reliable in the long term.
U1 - LM393
I used the LMV393IDR type.
It is fast enough, suitable for 5V power supply, has low consumption and has open collector outputs.
U2 - JW5017S
You can also use the (perhaps more affordable) RY8310.
Finally, you can see some photos of the fully assembled PCB including the installation in the box and the connection to the ESP32-S3 development kit. I have also added one screenshot showing the statistics of Data Traffic after about 90 hours of operation.
This project is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License (CC BY-NC-SA 4.0).
- Non-Commercial: The hardware design files were created using the free/personal-use tier of Autodesk Eagle and are strictly intended for non-commercial, educational, and DIY purposes.
- ShareAlike: If you remix, transform, or build upon the material, you must distribute your contributions under the same license.
Disclaimer: Working with heating appliances carries inherent risks. Improper connection can damage your boiler's mainboard. Use this hardware at your own risk. The author is not responsible for any damage caused by using this design.