Bidirectional level swiching 5V  3.3V
I think the easiest circuit that does what you want would use a N channel MOSFET like the bss138 and two pull-ups, hooked in the following way:



Tell me if it doesnt fit your needs, and why.

Having said this, I have to recommend you to take a look at the MAX3000 series of level converters. I have used them before and they are great bidireccional converters that "simply work" (in apple terms).

Hope it helps.

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