low power sensor output interface to MCU

[littleboot]

Hello,

I'm currently working on a project and I need to connect a sensor output to the MCU but I would like to make this as power efficient as possible.
I attached a image of the sensor out from the datasheet.
the second image is a configuration were I connected a resistor across the base and emitter of the transistor. This circuit seems to work inside the simulation but I don't really get why it works.
R5 10000K = I/O imput impedance
R1 25K = interal pulldown (can't be turned off in MCU stanby mode)

the last image is how I connected it the first time, this circuit also works but uses more current.

Could some explain which circuit config is superior, and why.

thanks in advance

[Marco]

What is happening in the first circuit is that the transistor base-emitter junction acts like a diode with the sensor delivering all the current ...

The second is better, but you don't need R4.