Single power source multiple somehow interconnected loads

My little understanding on electreonics led to my first post.
To be exact: I have a 12V output UPS (made of a 12V+ power adaptor and a battery - not much to say here) and a 3.3V powered nodemcu which will com with a relay.
I want to use those 12V though a buck boost converter to drop them down to operational levels for my MCU while I power my 12V device from the other end of the relay. It somehow seems awfully wrong to do so and I think it had to do with common GND and different voltage diffentials.
Does anyone have an idea on how to get on with this?
Thank you in advance.

What kind of relay? Post a link to the seller's webpage.

Panos28:
I want to use those 12V though a buck boost converter to drop them down to operational levels for my MCU while I power my 12V device from the other end of the relay. It somehow seems awfully wrong to do so and I think it had to do with common GND and different voltage diffentials.

Post a diagram (photo of a simple pencil drawing) showing what you have in mind.

It sounds like very normal practice to me - but I would like to see the diagram to be sure.

...R
Simple Image Guide

I haven t bought the relay yet. So I have no specs. I tried to draw it for you.

I think that should work fine.

My only concern is that you seem to have a 12v power supply and a 12v battery. You will need to ensure that the power supply cannot overcharge the battery.

...R

There is a standby voltage spec of the battery which ensures that the unit won t be overcharged. If there is a problem that is the absense of an overdischarge circuit but then I guess there is a solution to that. Thank you Robin! I will post feedback as soon as I finish my project. It is just a v0.1 alarm unit. V.2 will focus on antitampering.

Panos28:
If there is a problem that is the absense of an overdischarge circuit but then I guess there is a solution to that.

With a suitable voltage divider (to ensure that 5v is never exceeded) and if there is a common GND between the 3.3v circuit and the 12v circuit the Arduino could monitor the voltage on the 12v battery.

...R

Keep your grounds all connected together otherwise you will have problems. I would suggest using a Buck converter, they are less money and typically have a common ground between input and output. You must be sure this ground is common or you will probably destroy some parts.

Your sketch looks good, I am assuming the grounds are all connected together.

Have fun,
Gil