20 dollar circuit problem continued

Remember the 20 dollar circuit problem? Last Friday I tried to bug Prof Liew for the answer, but he told us to wait for 2 more weeks. Ok if you can't bug him for solution, at least bug him for clues.

Here's the clues: you are not supposed to build/add anything across the boxes. ie, everything has to be done inside the boxes. And the AC source is important, had it been a DC source it would be a totally different solution.
Hoho I can solve the problem already. Think of 8 diodes.

EDIT: someone did it with 6 diodes, kudos to you!