Uh, well, thank you everybody for taking over my topic. Just got back from office, I need to focus a little on your request(s).
First at all, trivial experiments and (mostly) read your posts made me clear that "LOW" on "LOW LEVEL TRIGGER" doesn't refer to voltage, but to the state needed to *activate* the relay. That's a fairly bad news to me, quite unused to such a mental contortion. Being on this world by long time, I should have known...
Now I have cleared up my previous mistake, and I managed to have the damn thing working (for the record, it's sold as 12V Arduino 2channel LOW LEVEL TRIGGER, 3393):
1) this is a 12V driven board, nothing to deal with Arduino unless by-other-means connected, see further;
2) input pin is already "pulled up" quite as Crossroads said. Should be grounded to see the relay to react;
3) to answer romanz, and after having de-soldered the relay from board, the coil hasn't burned after all and the relay is still intact. The
circuit has failed somehow, but probably it's been due of powering from the "wrong" end (the coil itself). No further investigation I'll take, as there are plenty of more interesting stuff to learn on the world but still was a quite unusual happening. I'd file it under "another bad day";
4) rather than throwing everything off the window, while being the electronic waste quite regulated in northern Italy, I chose to (still) drive the damn thing using Arduino by the mean of a single PNP lying around (BC238). It works somehow BUT...
...at any give voltage provided to the base, even touching the contact with my finger, providing 5V or grounding, the relay gets crazy and turns on and off at a high speed. I suspect there's need for a pull-up/pull-down resistor somewhere, but I haven't figured out how, why and, especially, where.
(oh, schematic... shame on me, I've been spending the last HALF AN HOUR just to figure out where the ground is on the "easy" circuit editor. Imagine a PNP, have the emitter to the input pin, collector to ground and base to 5V, nothing else. Half an hour and still no go! Puah.. such a waste of time, I'd better go hunting ghosts!)
Of course, connecting Arduino thru a safety diode doesn't make the relay clicking when I expect to do.
Any clue? [sincere thanks in advance!]