I checked an re-checked all over again; I even changed my breaboard.
Finally, I tested all components and surprise: R2 has infinite resistance (should be 12k!).
Everything is fine and the circuit works well now.
Thank you to All for your help.
I add the corrected shema - hope I didn't made a mistake this time.
<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=2707&stc=1&d=121484865 4">
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