I would have probably used a phototransistor instead, they usually have somewhat better sensitivity. An analogue comparator is the only device you need to implement the rest of the logic.
There are a number of IC comparators available, in the worst case most 16 bit PIC processors have at least one comparator on the chip.
But in your pre-dawn low light situation even that wouldn't be enough I guess. The problem would be that dark current would be very similar in the magnitude to your light signal. You will be detecting noise rather than the required amount of light. I know what I am talking about, because I tried building a low-light light meter last year. I would recommend using TSL230R Light to Frequency Converter from The Texas Advanced Optical Systems - it is the only thing that worked for me in the end for that amount of light.
PS: PM me if you need more details - my light meter worked in the end
Last edited by vyshemirsky; 10-31-2011 at 07:11 PM. Click to view previous post history.
Reason: added PS