Problem is I don't have any MOSFETs. Can I use NPN transistors instead? or do I better use 2 74HC14 NOT gates as single MOSFET?
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You can try to use BJTs. Here is what I've built and it works (it has some disadvantages though and may not work in all cases):
No, you can't. The "trick" of using MOSFETs in this type of level shifter relies on the fact that MOSFETs are very symmetrical devices — for all practical purposes, the source and drain terminals can be interchanged, and the device works identically (ignoring the effects of the body diode).
BJTs do not have this symmetry property. The base-emiiter and base-collector junctions are deliberately made with different properties, and while you'll get some gain using it in the reverse direction, it's much less than using it in the proper direction.
The 74HC14 has pins that are input only and output only, and can't be used for bidirectional signals at all.