Constant voltage using shunt regulator
What you need is very simple:







simulate this circuit &ndash; Schematic created using CircuitLab

I've left out the ground since you say it is common for the signal and the processor.

When the signal is high, the diode blocks and the MCU input is pulled up to 2.8V through R1.

When the signal is low, it pulls the MCU input down through D1.

You must have 2.8V already at hand since you say the MCU runs on 2.8V.

Much simpler than you idea, and can be managed with parts from the junk box.