Getting closer

Yesterday the last parts for the OpenTherm Monitor arrived, so I could finish that part last evening:

– Adding the last part to the PCB (a 4.7 Ω resistor);
– Removing a diode that was there to power the PCB from the RS232 port that I won’t use;
– Replace the 2nd “RS232 power” diode with a wire;
– Cut away the unused headers.

And this is the end result:

OpenTherm Monitor PCB finished So the OT Monitor part is finished now. As you can see the DTR connection is now unused and the DSR connection doesn’t have a diode anymore.

Next step was to have a close look at the additional parts that are needed to connect it all to a JeeNode:

Additional parts

On this part of the schematic (here’s the complete original), 5 new parts are being used:

  • 2 resistors of 1 kΩ;
  • 2 * 2N7000 MOSFET (available here)

7N2000 MOSFETThese 2 pictures and the the datasheet saying that

pin 1 = Source
pin 2 = Gate
pin 3 = Drain

is enough to make no mistake with the wiring, even if  you have no clue what the 2N7000 does 😉


  • A SN7432N which is a Quadruple 2 input positive OR Gate and can be purchased here


As can be seen in the schematic, a Single 2 input positive OR Gate would suffice just as well; maybe they exist too, but I didn’t bother to search


SN7432N powerBesides the 2 inputs and the output signal, the SN7432N als needs to be powered (pin 7 and 14), so the total number of connected pins results in 2+1+2=5.




With a breadboard, a JeeNode, a 4×20 LCD with a JeeLabs LCD Plug on its back, some wires and the SN7432N I mentioned above, this is the end result for now:

 Almost finished

Yep, I’m waiting for the MOSFETs to arrive. As soon as they arrive and I have some time to finish the breadboard, I’m going to put this OT Monitor between the Honeywell Evotouch Wireless OpenTherm RF module and our Remeha Calenta.

Assuming everything keeps working and that I’m not confronted with errors on the display of the boiler, I can try to have a look at the signal on the data pin – and if that looks OK, the next step will be connecting the data pin to the JeeNode and watch the LCD… exciting!

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11 Responses to Getting closer

  1. Freddy Martens says:

    I am following your blog with a lot of interest during the last month and this item is very interesting since I created a PCB based on the schematic given on the palebluedot site. But I was wondering how much more information I will receive from the heater if would use the opentherm monitor instead of the serial interface of the Remeha Avanta.

    PS: The Remeha diagnostics cable can also be created using a Nokia DKU5 or CA42 data cable. I used an CA42 and it worked like a charm.

    • Hi Freddy,

      I can remember seeing your name in the comments on the palebluedot site. You made a Arduino shield for it, am I right? I saw the pictures, nice! 🙂 I don’t think you’ll get more info with the OT Monitor (this assumption is based on the data IDs I saw passing by on the screen for half an hour or so – I’ll have to start logging them and see what they mean to give a more precise answer). But first I’ll have to get rid of the start/end bit & parity errors I keep getting (a lot!). Somehow, one of the 7N2000 outputs isn’t “clean” – there’s a peak when the other 7N2000 goes from high to low, as if they influence each other. The resulting signal that goes to the JeeNode is messed up by this. I’ll try to explain what I mean tomorrow, with some screendumps that I’ll try to make with my logic analyzer. So keep posted 😉

  2. Freddy Martens says:

    Is there a way to upload the eagle project to this site for others who want to have a go at it?

    • I would, if I could…. but I don’t have the Eagle files, sorry. I bought the finished PCB from someone who had a small number of PCBs produced for himself. He has the Eagle files, which he got from the original creator of the OpenTherm Gateway..

  3. Freddy Martens says:

    Well, I meant to send my eagle project to your website. It might need some validation. It’s the eagle project for the ‘arduino’ shield. It was my first PCB design since a long time so there might be some flaws in it. Maybe others can use the files to save some time.


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  5. CoonHouse says:

    Hello Robert,

    I’m following your OpenTherm project with full intrest. Especially when interfacing with a jeenode!
    Do you have an RSS feed for your blog?

    Best regards,


  6. CoonHouse says:

    Hello Robert,

    Thanks! I was indeed using a mobile browser 😉

    Best regards,


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