Getting power from and controlling my Doorbell with Arduino?

Other Hardware Home Automation
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Hello Dear Forum,

my wish: getting notification on my Phone, when someone rings my Doorbell, plus a button via the Arduino Cloud to open the door. (atm I have a Arduino Nano IOT, I hope this can handle this project, it runs on 3.3V)
(maybe if im successfull in the future I could upgrade the system to be able to talk over the phone to the person ringing downstairs)

My Doorbell is from siedle and someone on the internet broke down pretty much the whole thing over here

To my knowledge I get 18V DC from the pin 7 and if someone rings the bell there is a voltage drop, which I could read with an optocoupler and a zehnerdiode (pls correct me if im wrong)

now I don't think I need help programming the Arduino, but I lack knowledge in the electronics world, so this is me asking help for the electronic part of the project, but of course if you have coding ideas feel free to share them though hopefully that should be the easy part of my project.

Now the next step is to be able to open the door with my Arduino, but im super confused about the whole button thing. I thought I could simulate the Button press with a relay or a (NPN) transistor but so far all my tries with a SSR or a Transistor have failed.
I think I measured a voltage of 6.25V, when I connected my multimeter to the two sides of the button, not sure if that helps.

Now I think I don't need to know how exactly a relay/transistor works, since I think I understand the base principle, but since I can't get it to work, I think I either need a full understanding of the things, or I need someone to tell me exactly how I need to connect the Arduino to the button to simulate it. I'm happy as long as it works, I don't need to understand it.

now the second big question is, can I power my Arduino from the Doorbell or do I need an external power source?(sadly I don't have a wall socket near the bell so it would be a hassle to somehow get the power from a socket )
Kind Regards Samelmann

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You can power the Arduino most probably by a half-wave rectifier (diode plus a fat capacitor) and voltage regulator. Details depend on the power drop when the door opener is powered.

For adding a door opener "button" the voltage and current through the existing buttons should be known. Perhaps a low power transistor is sufficient, or an optocoupler. Try a diode to close the "button" and find out whether the operating voltage is AC or DC.

Unfortunately I can not determine much from your circuit diagram, a cryptic black box and too many unlabeled pins

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I'm 99% sure it's DC, since I measured -18V when I measured pin 1 and 7 the other way around.
it dropped to 14V when I rang the bell, though only when I rang on my floor (for that I think the ERT pin is for, when I rang downstairs apparently it measured 0.L, which I thought meant overload, but I just googled and it might just be out of range since I had it set on max 20V. so maybe when rung it goes up to 24V, need to check that I guess but im not home atm.
Did you look at the site I linked? it is in German but might clear up some confusion about the circuit diagram, since that's where I got it from.
How exactly would I connect the Arduino and transistor to the button? or do if i need the specific Voltage and current specs first?
thank you for your answer!

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That indicates a weak power source (Klingeltrafo).

Das hätte in der Tat vieles vereinfacht.

Connect an NPN transistor (C-E) in parallel to a button. Connect the Arduino to the base with a current limiting resistor (try 100...1k).

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