how does MOSFET do 3.3V to 5V logic conversion

[shemo]

G--connect to 3.3V
S---connect to logic level of 3.3V signal.
D---connect to pullup resistor to 5V and the logic level of 5V signal

G is always on.
therefore D signal passed to S.

but who does 5V signal become 3.3V. I don't' see it.

 

[Audioguru]

Mosfets are enhancement mode. If the gate and source have the same voltage then a Mosfet is turned off, not turned on.

If the Mosfet is turned on then the drain pullup resistor will form a voltage divider with the load resistance reducing the 5V to 3.3V.
Pulse-Width-Modulation of the Mosfet can also be used to reduce its average output voltage.

 

[dick_freebird]

You would prefer, in integrated form, to use more than
one MOSFET. There are classical cross-coupled level
shifter circuits which do the job nicely, and can be
implemented in 5V devices really simply; doing the job
with low voltage (3V) devices entails a little more
complexity but has been done as well (on SOI, JI would
have some concerns with cascode device application
ratings).

You can find useful detail in some older level shifter
product data sheets.

Googling

https://www.google.com/#q=cross+coupled+CMOS+level+shifter

yields on the first hit:

https://web.mit.edu/magic/Public/papers/01464775.pdf

Right there on page 1.

 

[erikl]

G--connect to 3.3V
S---connect to logic level of 3.3V signal.
D---connect to pullup resistor to 5V and the logic level of 5V signal

G is always on.
therefore D signal passed to S.

but how does 5V signal become 3.3V?

With one more pullUp at the 3.3V side it seems to work quite well: