Home Assistant Community

Sonoff sv inside door intercom - no keys anymore! - power issue


Hi. I installed a sonoff sv (after converting it into a dry switch) inside my intercom to be able to control the front door buzzer remotely. This works great with home assistant too, using IFTTT (hopefully I will manage to install tasmota on the sonoff one of these days, so I won’t need IFTTT anymore). Anyway, now I have a cool button on my dashboard and I can open the door of my building remotely… i.e. I don’t need to have the key with me anymore (I already installed a Sherlock on my apartment door, so no key for that one either)!

As you can see in the picture it fits well inside the intercom too, which makes it very neat! Except… that white cable you see going in there. That’s to bring 5V power to power up the sonoff sv. I was hoping that I would be able to find power inside the intercom but either there isn’t any or I’m doing something wrong.

I know this isn’t exactly a HA question, but I was hoping somebody might have an idea. This is the connection diagram (the 1st diagram in this link, type 8875). I used my multimeter and I can detect 5V voltage between some of the cables coming in the intercom. When I connect the sonoff to these cables though it doesn’t power up and when I put the multimeter again there, while the sonoff is connected, the voltage drops to 1V. Is there is a solution to this problem?

There is this solution called Nello and that one somehow gets power from the intercom and works, so i was hoping I could do the same. Nello also has a rechargable battery in it. Could this be how it works? Somehow the battery charges with low voltage and then is able to power up?


Actually did the exact same thing with mine a week ago! Don’t think you can solve the power issue though :disappointed: I got a 12v transformer from banggood small enough to fit in the junction box behind mine https://banggood.app.link/e4mo3Pml3V


Hmm, thanks. So how do you think Nello works then?


Hello, I have the same goal that you have (make my intercom smart with the help of Home Assistant) and I have a Sonoff SV flashed with Tasmota. Can you please explain me how did you connect your Sonoff to the intercom and why did you convert it to a dry switch?
Thank you for your help


Your intercom buzzer works by shortcircuiting (connecting) two of the connectors available. This is what the button does when you press it. To find which of the two connectors, you need to check your manual. If you don’t have your manual, find some kind of model number printed on the circuit bord or plastic cover and google it (in my case I googled elvox 8875). If you find the pair you need to shortcircuit, you connect them to the two connectors of the sonoff. You need to convert it to a dry switch, because you want to use sonoff as a switch (connect the two ends of the intercom).


It worked! I found out that my Intercom (a SIEDLE HT 511-01) opens the door when the port I and C are shortcircuited, so i made exactly as you said:

  1. convert the Sonoff SV to a dry switch
  2. shortcirquit the output port of the Sonoff SV
  3. connect the input of the Sonoff SV to port I and C
  4. power up the Sonoff SV
  5. through the web interface when I toggle the Sonoff to ON it opens the door
    Thank you very much!

Now I would love to interface my doorbell (which gets 11V AC when the doorbell rings) with Home Assistant in order to get a notification (ideally a push notification on my android phone) when someone rings the bell.
Do you have an idea how to do that?


What did you use as power supply of the sonoff?


So far I used an external power supply (I used an old USB phone charger) but I think I will try to power it with batteries and using it in Dynamic Sleep Mode to see how long a battery would last. How did you power it up?


I did the same. I don’t think that you will have much luck with battery, since Sonoff uses WiFi. Had it been bluetooth, maybe. Unless you find a way to charge the battery with the low voltage power supply of the intercom. I still do not know, but I guess this is how Nello does it.