@reversEMF - right!
I only want to show that this way of obtaining energy is feasible using basic resources, it is easy to make, easy to maintain.
[maybe OT: so many times I saw projects (including my projects :-)) never getting to an end because of to ambitious goals. It is crystal clear in my mind that Arduino is about learning and teaching in the first place. Of course, real life projects can be done in the Arduino ecosystem (or alike), but myself I am not in this.]
Thank you for the reccomandation! It is possible to use a trimer (aka variable capacitor) for the capacitor you suggested? If so, what value do you think?
I can find here easy a 4-20 pF and a 7-100 pF.
The 7-100 pF has a larger range so might be handy. Then I can go (hopefully :-) ) by trial and error.
The coil- yeah, I have a bunch of torroids and drossels on my desk. I cant measure them by my multimeters and honestly the devices to measure capacitance and inductance are insanely expensive.