Calculating Values of ADC for Phototransistor on Digital Pin
You can simply use a resistor to Vdd of your microcontroller (maybe).



simulate this circuit &ndash; Schematic created using CircuitLab
The resistor should be high enough to give the desired sensitivity but not higher than your ADC can tolerate (usually a few K up to 10K).
To get some idea of whether this will work, measure the photocurrent under the desired range of conditions.