@GoForSome, Using a LED as a sensor you mean?. I think the photosensitive area is pretty small here (and then the current it generates will be in the range of 1pA or even less), but it would be interesting to test them as their are super cheap and you can get them with any spectral range you want (different led colors).
The TSL237 is one of the most sensitive (well, tied with the TSL238, which is just the same but in other package format) photodiodes with integrated amp op and voltage to freq in the market now, but you will probably want to use some kind of IR-cut filter to cover the sensor because it's sensibility peaks on the red - near IR. The Sky Quality Meter (SQM) uses it as it's sensor, I think Corpze just want to build one of those himself.
One of the problems of these sensors is the huge range of frequencies you want to measure, from ~ 0.1 Hz (or even less in 'pristine skies' !) to nearly 1MHz.
I'm also doing some test in this field with the TSL238 and arduino, using FreqCounter (High Frequencies) and pulseIn for low frequencies, but this mix is far from perfect and it's giving me some headaches hehe.