user57281: I have no experience about serial communications, with that being said; I have two circuits isolated from each other and each have a ATMEGA8 reading a voltage on their ADC.
I want one of them to send ADC data to another without having a common ground. I know that I can do it with a Bluetooth or Wi-Fi "module" but I'm looking for an easier way, like maybe with an optocoupler but again I'm not aware of the limits.

What's the cheapest and simplest way to do this? and basically how it's done?

There's a lot of information about making two Arduinos talk to each other but I couldn't find much information/documentation on isolated communications.

user71575: [Consolidating an answer from the comments]
A wired connection using Optical isolation (aka Optocouplers) is the recommended way to solve this problem.
There are tutorials available online regarding optical isolation (e.g., https://www.electronics-tutorials.ws/blog/optocoupler.html; https://bestengineeringprojects.com/interfacing-optocoupler-with-arduino/).
One (of many) possible example circuit was noted as:


57281: <p>I have no experience about serial communications, with that being said; I have two circuits isolated from each other and each have a ATMEGA8 reading a voltage on their ADC.</p>
<p>I want one of them to send ADC data to another without having a common ground. I know that I can do it with a Bluetooth or Wi-Fi &quot;module&quot; but I'm looking for an easier way, like maybe with an optocoupler but again I'm not aware of the limits.</p>
<ul>
<li>What's the cheapest and simplest way to do this? and basically how it's done?</li>
</ul>
<p>There's a lot of information about making two Arduinos talk to each other but I couldn't find much information/documentation on isolated communications.</p>
 71575: <p>[Consolidating an answer from the comments]</p>
<p>A wired connection using Optical isolation (aka <a href="https://en.wikipedia.org/wiki/Opto-isolator" rel="nofollow noreferrer">Optocouplers</a>) is the recommended way to solve this problem.</p>
<p>There are tutorials available online regarding optical isolation (e.g., <a href="https://www.electronics-tutorials.ws/blog/optocoupler.html;" rel="nofollow noreferrer">https://www.electronics-tutorials.ws/blog/optocoupler.html;</a> <a href="https://bestengineeringprojects.com/interfacing-optocoupler-with-arduino/" rel="nofollow noreferrer">https://bestengineeringprojects.com/interfacing-optocoupler-with-arduino/</a>).</p>
<p>One (of many) possible <a href="https://github.com/TaaraLabs/OptoIsolatorFTDI" rel="nofollow noreferrer">example</a> circuit was noted as:
<a href="https://i.stack.imgur.com/R66iX.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/R66iX.png" alt="OptoIsolator FTDI Circuit" /></a></p>
