Unidirectional logic shifting (3V3 to 1V8) with minimum components?
Here Q1 acts as a level shifter, R2 and R1 form a voltage divider that produces about
1.8V at the base, so when the input voltage is pulled below about 1.2V base current flows and allows the collector to pull down the output from +3.3V. R3 is optional if you enable the pullup in the RPi.



simulate this circuit &ndash; Schematic created using CircuitLab