Shelly1 does have a dry contact output between O(Output) and I(Input)
Only Shelly1 does this though, the others have a voltage output on O
I use Shelly1s this way, running on 12V with esphome (as well as using the SW connection for a binary sensor)
It may work for me then.
All I need to do it power it with a 12v adapter then connect I/O to replace the button on the wireless doorbell transmitter?
When an event occur, it’ll make a close contact acting like pressing a button on doorbell?
Yep I just checked… I was fiddling with mine as a doorbell replacement… so I had an old power supply wired up already… don’t laugh at how old it is … 
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Yep, just set it up like that in epshome or use tasmota or standard firmaware and mqtt
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I have that power supply boxed up somewhere! LOL … you had to buy plug separately for it!
I am a Radio Shack fanboy when they used to sell those red boxed 2 dollars experiment kits. I was always asking for a new radio shack kits for my birthday when I was a young boy 
Also… I had to grind down a pair of tweezers to move the jumper… you may want to buy a pair of tiny ones…
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Yeah that the kind that has interchangeable ends for different size plugs… my dad had about 6 of them I kept
Easier to just pop the lid off (be careful of the attached antenna though)
Thanks for that… now I know for the other one… it seemed like it might have been glued or something
Na, fingernail between the top and bottom and it pops right off. I broke the antenna cable on my first one so be aware of that, its quite short
So my plan was… I have old fashioned doorbell chimes with 10v transformer … I was going to put the Shelly in between the old transformer and the chime, and use the power supply for the Shelly to power the doorbell button (with light inside) … so pressing the button would close the relay and send power to the chime… didn’t look much more into it as i need new doorbell wire before I mess around as the stuff down there now is falling apart… anybody have input if that should work? (I also have front and side doorbells ) so I was thinking maybe Shelly 2.5 I think it is??
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Shelly devices for the most part require mains AC voltage for power, and all of the ones with power monitoring also require mains on the switch input pins (shelly1pm, shelly 2.5, shelly dimmer, others?).
There are many ways to read a doorbell signal. I use a hall sensor, arduino nano, and the HA serial sensor component to do it. This is great because I already use that nano for other analog readings (backup battery voltage, mains status, etc). For a simpler approach if you don’t need the extra IO, I like frenck’s method that uses the bell current to close a relay coil directly, then the relay contacts are measured by an esp:
Now operating a relay based on that doorbell signal is (or can be) totally independent of how you read that signal. All you need here is a relay device (dry contact may not even be needed). Yes most shelly’s may work, but I think the power monitoring and mains switch input are not ideal for this application (former is a waste, latter is a limitation on the install). A better choice IMHO would be a dry contact modified sonoff, or save time soldering and get a shelly 1 (not the PM kind!). Either of those can easily be installed/programmed to operate your flashing lights (via dry contacts) when the doorbell is pressed.
A side note, not sure what doorbell cam device you have or how you configured it in ha, but you might just be able to use the state change in HA to trigger the relay (no additional bell signal reading circuits required). That said, do you now have 2 buttons, the original bell and a camera/bell? If you just have the cam/bell and it’s firing events in HA, then all you really need is the shelly1 or modded sonoff (or similar dry-contact relay device).
Shelly 1 can run on 12v… as shown above… and have a switched input… which means the button press would close the relay…
I already have door sensors by the solenoid in the chime… but they sometimes move and don’t always trigger as they are very sensitive…
I have built this and that is not correct, the ESP reads the doorbell button and the ESP relay essentially replaces the old button. A power adapter is still needed for the ESP relay
That’s right, I do remember something about shelly1’s working in LV applications.
So you are essentially using reed switches to pick up the B-field emitting from the solenoid coils? …or did you stick magnets to the ends of the plungers or something like that? The former seems like it would be really dicey depending on the coils and placement of the reed switch, the latter might have issues with interfering B-fields.
On a related weak signals note…
I mentioned really like frencks insight using the relay coil in series to create a positive binary signal. However, I think that method may not work so well with modern chimes. Lots of newer doorbell chimes use efficient circuits with speakers/amplifiers for output, instead of the classic solenoid and bell. These modern chimes draw an angel’s fart worth of current compared to their solenoid counterparts. Complicating matters, on these modern chimes the current draw may not directly coincide time-wise with button contact (you may read some short blips, rather than a sustained button press). This makes reliably reading bell signals on some chimes very difficult; it’s the reason I had to resort to using a hall sensor for my home.
Ah, lol I skimmed over that a while ago and inferred that’s what frenck was doing there. So I guess came up with the idea of using the coil as a sensor on my own! 
Yeah that being the case, my post directly above is incorrect regarding this working with modern bells (edited accordingly). The only thing that sucks, is lighted doorbells won’t be lit up that way. Perhaps a solution to that, is keeping the button wired to 24VAC, then using a divider on an analog pin together with some basic signal processing to read it. Shouldn’t be too intensive being it’s just 60hz (my hall sensor code only needs one complete wave cycle to read reliably… ~20msec… one could use interrupts instead).
I unfortunately am using Iris 3320-L contact sensors… (because I had gotten a bunch very cheap) and they use a Hall effect sensor instead of a reed switch like most to detect the magnetic field which does make it pretty dicey… which is why I was looking for alternatives… I had bought the Shelly1s in a two pack a long time ago but never used them… so I was hoping to make use of them finally. My doorbell with the iris works about 98% of the time so it’s not a huge deal… but once I remembered the shellys sitting in a box I started thinking of how I could make use of them.
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I am not using it to detect or monitor a doorbell button press
I wanted to something to use as a momentary switch to replace the doorbell button so I can use my old wireless doorbell transmitter to trigger the receiver units in the house and have the hub “press” it when it detect another doorbell being pressed.
I already have a sensor in my doorbell chime which is monitoring it.
Make senses? I know it sound backward but it what I am trying to do for this project …
Does it have to be Zigbee or Z-Wave? An ESP8266-01 and a relay would do what you want using MQTT. For less than $5.
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Aqara sensors can be hacked
The button can do it do