I want to know the state of an external Relay connected to my Raspberry-Pi. Basically if there is current flow or not in A-B. A-B should be connected to R-Pi.

I am not sure how to proceed even though I found many similar topics (example: Detect if current is flowing). I don't know if they are compatible with my problem as I don't want to fry my devices ^^"

So the questions are:

  • How to connect the relay properly to R-Pi
  • What extra components are needed
  • How to detect the relay status ON/OFF

enter image description here


I have 2 Honeywell wireless thermostat with their corresponding relay boxes which are meant to be used in stand alone mode as follows, but only 1 can be connected to the boiler:

enter image description here

In the new setup, the goal is to use a Raspberry-PI to coordinate between the two and control the boiler using my own relay and logic (example: only turn-off when both Relays are off, ...) -please excuse the crude diagram-

enter image description here

All components are within 1 meter distance.


I tried to avoid overwhelming you with extra details. But it seems inevitable to have the right answer. Anyhow, the scheme below shows and overview of the overall installation:

  • We have 2 floors each with similar components: relay (controlled remotely), 3-way valve and a pump
  • the relays trigger the heat request
  • the RPI is coordinating how the different components behave based on that trigger (logic yet to implement but that part should be ok)

enter image description here

  • oh, nevermind, your wiring diagram is completely different to ACTUAL wiring of a BDR91 – Jaromanda X Jan 26 at 22:57
  • Is anything else...anything at all...connected to A, B, or C? – Elliot Alderson Jan 26 at 23:00
  • 2
    Your question is easy enough to answer, but before I do, can you please confirm that you know what you are doing by connecting a 230VAC source to your RPi? – Seamus Jan 26 at 23:20
  • @ElliotAlderson I have added more details to clarify – Enissay Jan 27 at 21:47

The relay contacts are just that - a contact closure.

You detect the same way you would any other push button.
There are literally thousands of tutorials in dozens of different languages.

NOTE the relay is shown connected to mains power. There are safety protocols that must be observed to ensure isolation from mains power.

Depending on detail missing from your Question (distance, wiring, need to avoid false triggering) you may need no additional components.

  • I have added more details to clarify – Enissay Jan 27 at 21:45

Note that according to the documentation for the BDR91, the A-B contacts are designed to connect to a 230 VAC appliance:

enter image description here

Your question states that you intend to connect the relay to your RPi, and you ask for help in accomplishing that objective. You must know that there is no 230 VAC input to the RPi - other than the mains input to the Power Supply. You'll need to clarify your question and your intent here as it makes no sense on the face of it.

And since that question is unclear, there seems little point in addressing your other questions.

We're happy to help, but you must be clear in your question. Please edit your question - or simply delete it if you've thought better of this enterprise.


Based on clarification provided by Enissay in his Question, and assuming that the T1-T2 terminals on the Combi-boiler have the required internal connections, no RPi is needed for a solution. This is simply Relay Logic:

enter image description here

Apologies for the hand-sketched diagram, but the site's schematic tool is broken (again).

If the RPi is needed or wanted to perform other functions in the control of the boiler, please advise & edit the question. As shown here, the boiler will be on if either of the BDR91 modules' relays are activated (COM-to-NO), and off if neither are. In other words, the BDR91 relays are wired as an OR gate.

  • I have added more details to clarify – Enissay Jan 27 at 21:47
  • @Enissay: After attempted research on the relay you have selected, I recommend you find another one. There are two problems: 1) The few specs provided indicate the rating is 250VAC - this is too close to your mains voltage of 230 VAC; 2) The cretins selling this junk on Amazon provide no spec sheet. In fact, two URLs are provided in the Q&A, but neither result in a document that can be downloaded. – Seamus Jan 27 at 23:45
  • Many thanks. I have added more details in my edit. As of the relay, I thought it was a good one, specs can be found on their website, here's a direct link to google-drive: drive.google.com/file/d/17WM1SDhoLS0L5OhjDQGIJPk4isnr-Rcn – Enissay Jan 28 at 22:22
  • Else, which other relay can you advise ? – Enissay Jan 28 at 22:23
  • @Enissay: Please read my answer again. I have edited it - you don't need another relay, and you don't even need an RPi. The relays in the BDR91 can do what you want if you'll wire them IAW the diagram. Let me know if that's not clear. – Seamus Jan 28 at 23:57

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

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