[Ronja] Luxmeter working

[Karel Kulhavy]

Hello

I have built the new frontend with the logarithmic luxmeter. It produces voltage
0-8.5V according to the current through the photodiode (values predicted from calibration points):

0V 1.2nA
1V 4.6nA
2V 18nA
3V 70nA
4V 270nA
5V 1uA
6V 4uA
7V 16uA
8V 61uA
8.5V 120uA

The dark current of the photodiode (SFH203) is max. 5nA according to the datasheet.
For darkness it shows about 0.1V. When the diode is disconnected it shows even less. That means my circuit is more sensitive than the dark current of the photodiode. For extreme darkness the function is likely to be imprecise due to the -1 term in the Shockley diode equation.

The original idea was to make 10x more light -> 1 volt more. But that was designed for
diode emissivity factor n=1. As it came out, the emissivity factor in this case is n=1.7.Do you know wheter all diodes of the same type have the same emissivity factor?

Karel