I'm following this project which shows how to make an OpenTherm Adapter for an Arduino. It includes this schematic:

schematic diagram

Along with this simplistic PCB diagram:

PCB layout

I am now in the process of making my own version of this circuit on a prototype PCB board. This is my first time making anything more complex than a simple header wire circuit using an Arduino and has therefore posed a bit of a challenge. However, with the help of Fritzing, I have been able to prototype a board design:

Fritzing wiring diagram

I understand this is a messy design and has lots of room for optimisation, but my question is whether or not the design I have made matches the schematic in the first image above. I would really appreciate it if someone could look over my schematic and let me know.

Thank you in advance for you help.

P.S. Here is a link to where you can download the schematic file (.fzz)


The circuit will be connected with a combination of excess solder and wires. Please bear this in mind where it may look like some tracks overlap. Feel free to download the file for clarification.

  • \$\begingroup\$ I see a problem with the rectifier bridge already. All 4 diodes are connected at one side. Exactly how will you lay the tracks? Excess solder, or wires? Since you're using a relatively big board, why not spread things apart (and maybe rotate a few) a bit? At the least it may be easier to connect, debug, replace components etc. \$\endgroup\$
    – Indraneel
    Dec 23, 2018 at 15:14
  • \$\begingroup\$ @Indraneel Ah I see where you mean; I should've made it clearer in my question that the circuit would be connected with a combination of excess solder and wires. As for that rectifier bridge specifically, there would be two separate wires (they look combined in the diagram). The main reason for it being so small is that I was trying to stick as close to the PCB diagram as possible. Any was you think you could improve the layout of the circuit? Thanks once again \$\endgroup\$
    – Rocco
    Dec 23, 2018 at 15:23
  • \$\begingroup\$ Instead of sticking to the PCB, stick to the schematic. Layout will be much much easier. \$\endgroup\$
    – Indraneel
    Dec 23, 2018 at 15:27

2 Answers 2


Lessons to be learnt:

  • to make it neat, like the logic diagram is neat
  • to make neat placement of parts.
  • to connect the dots.
  • to correct your own mistakes.
  • to learn that not all DIY kits are best way to do things.
  • any of these parts will the same or better job.
  • if the schematic doesn't follow global conventions of left to right (even in Asia) , worry about the experience of the designer.

That circuit provides a DC current shunt to an AC rectified source and series load from a 24Vac furnace transformer. All it has to drive is a logic driver to activate the furnace, the same way a reed relay might work.

My suggestion is use an opto-isolated triac for small signal currents. There's many other solutions.

It is better to learn good layout methods to make look like the schematic on a PCB with focus on tight signal loops and heavy power and ground paths with decoupling caps.

It's much quicker to learn how to research how something works than to figure out your mistakes why it doesn't work ( if you designed and built it yourself) yet you won't forget when you experience the pain of the process of learning by trial and error.

Look for an old EE Design magazine 1 page articles that describes in detail how reader design suggestions work. (available at archive.org) Start from the 1960's and move up.

Look on the web at Breadboards and note how many details you can spot that makes it satisfy or not, all the above lessons.

e.g. https://www.instructables.com/id/How-to-use-a-breadboard/

Bottom Line: Research your technology to find how it’s done and try to understand why things are done a certain way,rather than guess.

As posted in comments below my answer I guess the application. Of your circuit wrong, but if you follow the link, you will see there I followed by own advice above and analyzed the application is either for PWM or Manchester BiPhase insensitive to polarity which when used on untwisted pairs will be greatly affected by noise interference. The half duplex nature and method of single wire encoding for each direction was explained . Good luck.

  • \$\begingroup\$ Thank you so much for your in-depth answer. I've taken on board what you mentioned about the lack of experience the designer seems to have. It's been useful to learn from this but now I am looking more into what you have suggested. As for the Optotriacs, would they genuinely do the same job as the complete original circuit? If so, how would they be wired; is there any need for other parts e.g. resistors? I think they could be a great solution to this specific case; if at all possible, please could you elaborate on how exactly to use one for this case with examples? Thanks once again! \$\endgroup\$
    – Rocco
    Dec 24, 2018 at 16:32
  • \$\begingroup\$ Yes the same task can be performed by a Triac with a series Resistor on the gate to limit trigger current. But not exactly the same. en.wikipedia.org/wiki/TRIAC in this case a PhotoTriac for the input, although for the output with DC, a bipolar Phototransistor is still needed with R1 to limit current from 24Vac. It's not necessarily cheaper but simpler. Normally the optoisolation is not needed since the source is transformer isolated. The output only detects that the 24AC is present. \$\endgroup\$ Dec 24, 2018 at 17:07
  • \$\begingroup\$ I offer no training on how to use this tool, which will take time to master, but it can do many things in interactive slow motion. tinyurl.com/ydezzrhm see switch and explore builtin circuits. \$\endgroup\$ Dec 24, 2018 at 17:47
  • \$\begingroup\$ Thank you for the schematic, that idea looks very interesting. Would you be able to make a simple wiring diagram or list of parts as I am fairly new to this sort of electronics. Also, the logic for the circuit on the Arduino will be 3.3v (NodeMCU) if that makes any difference. Best \$\endgroup\$
    – Rocco
    Dec 24, 2018 at 19:44
  • 1
    \$\begingroup\$ @TonyStewartSunnyskyguyEE75 the signaling protocol uses Manchester/Bi-phase-L encoding and really loooong bits for today standards (500μs). There is also a parity bit in every 32-bit frame. Communication is fairly simple and the master usually re-sends the settings continuously anyway. I'd say it's quite robust. My guess is that most incompatibilities are actually software-related and related to the more advanced features. \$\endgroup\$
    – istepaniuk
    Apr 14, 2020 at 19:07

Here's a bit of rerouting of the schematic. Should be able to place the parts on your board just like that. No jumper wires required. Remember that you are using through hole parts, not surface mount. The parts can act as jumpers themselves.

enter image description here

You can fill in the rest. It's not much.

You are using optocouplers for isolation. Ideally, you should have a big hole/slit with no copper under them. Make sure not to route or have any tracks under the optocouplers. If you are unsure, read up on isolation and creepage. You should not have nearby wires/tracks passing from one side of the optocoupler to the other. Keep the area near the optocouplers physically clean (no oil or dirt, or stuff that can attract moisture).

  • \$\begingroup\$ Thank you for you answer, however it has become clear that the original designer of the circuit was not very experienced and, as a result, it has been suggested that I look for an alternative route to solve the 'issue'. \$\endgroup\$
    – Rocco
    Dec 24, 2018 at 16:35
  • \$\begingroup\$ @Rocco Glad I could be of help. \$\endgroup\$
    – Indraneel
    Dec 24, 2018 at 19:07
  • \$\begingroup\$ Sorry, last thing, do you know if the BC558B transistor could work as the transistor in this circuit? Thank you! \$\endgroup\$
    – Rocco
    Dec 24, 2018 at 21:16
  • \$\begingroup\$ @Rocco That would be tough to say without knowing the voltages and current involved. Why don't you simulate it in LTSpice and compare with the datasheet? Remember that the derated thermal dissipation capability will also change with the package type. \$\endgroup\$
    – Indraneel
    Dec 24, 2018 at 22:56

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.