Thank you for your reply.
Answers 3 and 4 not possible!!! I have a BLACK box, and have no access to the led itself. Cannot place any light sensor or something similar.
The answer 2 is also discarded because I must keep the component count and size at a minimum, and I can not aford adding another IC to the board.
So I have one solution left.
I've tried the first one by simulating it in pspice, but I don't know why, the transistor gets fired too early, before Vs reaches the led's drop voltage.
Could you explain me exactly how would you implement this solution?
Thanks a lot!!!