What to use for detecting AC current and triggering a scene

Other Devices & Integrations Power Management
1

Hello,
I would like to connect some sensor to my doorbell circuit, to trigger a scene when the doorbell push button is pressed.
The circuit being closed by the doorbell push is powered by 8V AC, current is 1A.
I have a fibaro universal binary sensor, but that requires dry, voltage free contacts with low current, so I guess that’s not useful for this purpose.
How can I solve this, what device can I use?
Or, what circuit should I build in order to do this?

An alternative would be to use the door push as a switch for a z-wave relay that powers the 240-8V doorbell transformer, but then I’d have much more current (mains) flowing through the door push, which I don’t think it’s rated for.

Thank you!

2

I’m just designing the same circuit as you, and for the same fibaro door sensor.

My doorbell switches a 8-10Vac supply too.
What I’m doing is getting a small solid state relay which operates at 4.25 to 24Vac.

So the doorbell press operates the relay.

I’ll also link the live to one side of a NO contact to supply the existing bell when pressed.

The Fibaro is then looped through the other contact on the relay.
A simple dry contact then, easy.

If you don’t get what I mean I’ll do a quick drawing when I can get near the computer (and away from the kids).

Chris

3

Wow, thanks!! It’d be fantastic if you could quickly jot down a drawing when you get a chance, as my experience is more on the software side than electronics…
Thank you!

4

Here’s a very quick one.

The coil and two switches represents 2 NO contacts on the same relay

5

To add I just combined the power supply and bell push.

The first lamp represents your existing door bell and the second your fibaro sensor

6

Thank you very much!

And how do you power the Fibaro sensor, since it requires 9V DC? Do you use a separate transformer for that? Or is there a way to rectify+lowpass AC into DC without a change in voltage?

Finally, I’m not sure I understand what the coil is. Is it actually part of the relay, or should I add that as a component to my circuit?

Thank you again!

7

I don’t know where you are getting the 9v figure from, but the binary input in the fibaro door sensor is a volt free contact, so basically you open or short the connection to operate.
You do not need power for it. It monitors resistance.

The coil is an integral part of a relay.

I’ve attached a drawing of a basic relay with a single contact.
In our example we’d want a relay with 2 sets of contacts.
The first to operate the existing bell, the second to operate the dry contact.

The relay coil and first contact get linked on the input so we can power both from our existing supply.

I am looking at a solid state relay (no coil but same principle and connections) as this will have minimal power draw.

8

[quote=“Chrisfraser05, post:7, topic:190575”]I don’t know where you are getting the 9v figure from, but the binary input in the fibaro door sensor is a volt free contact, so basically you open or short the connection to operate.
You do not need power for it. It monitors resistance.[/quote]

True - apologies for the confusion - I meant the supply voltage, not the volt free input. I got the 9V DC figure from the Fibaro manual: Wireless Smart Home and Home Automation | FIBARO
It says “9-30V DC ?10%”.

Thank you!

9

No problem buddy.

I’m new to vera and home automation but I’ve been an electrical engineer and automation/software engineer for years so as much as I’m learning it’s good to be able to give something back.

If you have any questions in future just pm me.

10

Thanks for this thread, really interesting solution - and exactly what I am looking for too.

This seems to follow the same method as the people here.

But I still can’t wrap my head around why a relay is needed - shouldn’t the bridge rectifier be enough? On the (in this image on the bottom) contacts of the rectifier I have 8Vdc - shouldn’t this be sufficient for the Fibaro universal sensor?

11

And as a follow up - if I buy this Solid state relay (Switching voltage
: 0 - 28.8 V AC; Voltage DC: 19.2 - 28.8 V=) I just have to connect my existing door chime to the two AC pins of the relay and the blue/green ends of the Fibaro sensor to the other ones?

12

Pardon my chiming in.

Just a word of caution, depending on the relay used, you may experience chatter in the relay. This would be because the output of the bridge rectifier is pulsating DC (going from 0 to 1.414 * 8, 120 times a second). I discovered this trying to monitor my home heating system (24VAC). If you experience chattering of the relay, you can add a capacitor across the bridge rectifier output. Value could be 10uF or so.

If I may, the answer to your question of why the relay is required is in the Fibaro spec. The words “potential-free inputs” should be interpreted as a mechanical contact with no voltage (aka “potential”) introduced to the inputs.

I believe a mechanical relay (reed relay) will work without any issues. A solid state relay may or may not work depending on the characteristics of the relay output and the Fibaro input. The Fibaro is looking for a contact closure, hence the relay type.

The other benefit you get with a relay is isolation between your doorbell circuit and your Fibaro. This is important for a reliable installation.

Hope this helps.

JohnRob