My intercom button is 16V AC
I would like to connect a relay to it (it’s just a button that needs to be held in)
What are my options? Are there any relays out there that can be powered by 16V AC?
I would say that official data is 12VAC output…? It’s normal on many devices that voltage is somewhat bigger. Just get 12VAC relay (or 12VDC with graetz - in this case don’t add electrolytic capacitor, only graetz) and you’ll be ok.
I tested it with a Multimeter and I get 15V between the button that needs to be held down to buzz the door.
The building is old, 1920 so I have no idea where to find official data
In any case 12V relay will do just fine, you won’t burn it, don’t worry. I bet voltage will drop a bit when you connect it.
There are 18V relays available, although not as much as 12V ones. 18V relay will also work fine on 15V.
I’ve been looking for AC relays between 12V and 16V but haven’t found any, do you know of any?
There are loads of universal relays that operate from 8V to 240V AC or DC
What kind of form factor do you need?
The smaller the better, it would ideally fit behind a power socket size space.
Zigbee if possible
Well… there’s a big difference between “relay” and “zigbee relay”. Aliexpress is your friend - you can get CLASSIC relay there. AC relays are somwehat bigger than DC, so if you’re lack of space there’s always an option to buy DC relay and add graetz to it.
As zigbee goes - relay modules do exist which have low voltage power supply. Just FIRST hit on aliexpress…
Just add a diode in series with the relay to make it DC (that will also drop the voltage by approx 0.6v, if needed you can even use 2 or 3; with each added diode the voltage will drop another 0.6v)
And you might even need to add a capacitor (just to equalise the ripple); if not the relay might start to bounce (depends a bit on the frequency of the AC). A capacitor of 100 μF/20V should be sufficient.
When using 3 diodes, the voltage would be 15v -1.8v = 13.2v (in which case a 12v relay would work pretty well)
Even better would be to use a diode bridge (which would make the voltage to drop by 1.2v, but reduces the ripple by 50%)
Wouldn’t a 240 (or 120) volt relay work?
I mean it’s only what its rated at, not that it has to be 240 volt.
The relay itself is just a contact so what kind of voltage that goes past it shouldn’t matter as long as its less than the rating.
Right?
Or is the idea to power it from the wires too?
That would be rather risky since you don’t know if the power supply in the house is enough.
I don’t think he meant the relay to push the button, but what he wants is a relay to be powered by the button, so he can use the relay contact for something else 
I’m reading it as he wants the relay to act as the button. So button is connected to com and NO on the relay.
If that is the case, it really doesn’t matter what relay is used, all relays can handle 16v on the contacts 
But that makes me wonder why he needs 16v power 
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Great thanks for the advice, to clarify what I want to do is:
I want to be able to buzz my front door open from Home Assistant.
All I really have to work with is 2 wires, which have 15VAC measured on my multimeter.
When the button is held in, it connects the wires and the door buzzes open.
I have a spare Shelly 1 Plus laying around that I was hoping to use, it needs power.
I also have a spare Shelly 1 Mini Gen3
Not sure what is the best solution. Any more advice is appreciated. I’m doing some more reading into this myself.
I still don’t quite understand you… let me “guess”: so you have those two wires and if you short them door opens. Now you want door to be opened from HA - is that correct ?
If so, using above shelly won’t do since 1. it requires power supply, and 2. it doesn’t have potential-free output contacts. Above shelly requires 110-240V if powered by AC, consequently it outputs 110-240V AC on output pins, while you need only short-circuit without any voltage.
What you need is a relayboard.
Something like this perhaps
ESP8266 5V WiFi relay module Things smart home remote control switch phone APP ESP-01 relay module - AliExpress 502
So this device will need 5 volt on the two pin terminal and the other terminal will have NC - COM - NO and that is what you need for the intercom
A shelly will do that fine, and you can just power it from 220v~ (or 110v~) (if available)
Hm… shelly from above pics? Powered by 230V? Not true unless you want smoke effects… as said, above shelly’s both output 230V, which is not what is needed here…
Correct… it seems that( some models ?) gen3 indeed have dry contact. I checked my shelly’s which are older (gen2) and they have 230V output. So watch out when buying, i guess…