I was moderately successful (~ $10,000 a year) writing about tech. Well, many multiples of that if you count the tech work I got because of the writing. But directly, about ten grand.
Almost everything I wrote was stuff I was currently learning about. I wrote about it because tech was what I knew, but also because writing about it made me dig deeper and really understand it.
I'm retired now, not learning a lot about tech, so my writing income is a laughably low number; $10-$20 a month here, $40-$50 elsewhere. For me, that's annoying but unimportant. Annoying because I know how to do more, but don't want to make the effort.
Lazy sod, that's me :)