I need to check the state of a voltage source, and I'm trying to see if I can simplify a circuit that I previously built. The purpose of the circuit is to check if there is enough voltage at a source (Vline). The original circuit is as follows.

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I've already built the circuit shown above and it is working just fine. As can be seen, a low-side nFet is in charge of activating a high-side pFet, which in allows current to flow through a resistor voltage divider and a zener prevents the output voltage from going too high for an MCU to read as a logic signal.

What I'm trying to do is simplify things and do without the pFet if possible, using a single "mid-side" nFet instead. So I've simmed the following circuit, and it seems to work, in the sim at least:

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My questions are:

• Will it work in real life? Or is the only way to find out is to actually build it? I'm gonna build it anyway, but I'd like to know at least what to expect.
• I'm trying to bring its maximum output up to 3.3V. But it seems that even if I remove the zener, its maximum output remains the same, why?
• And if I were to lower the value of R1, its maximum output also remains the same, why?

 

But it seems that even if I remove the zener, its maximum output remains the same, why?

Because of the Vgs(thr) value of the FET. Swap the FET for a different type and note the difference.
If your MCU also has a 3.3V logic high, then the whole circuit could be replaced by a simple emitter follower (which would limit the output voltage to ~ 2.5V) unless you actually need some measure of Vline.

 

Why not use a comparator powered with 3.3V?

 

Why not use a comparator powered with 3.3V?

forgot to add... the circuit needs to draw zero power when not active.