A reliable sunshine detector can be made using a bolometer sensor. Nearly
all the other methods will react to fake light from street lighting,
thunder, advertising etc.

Automatic lights use a CdS sensor usually. They are pretty dumb and react
to birds and other obstructions.

I think that one can use a CdS photoresistor and a microprocessor to do
some filtering to remove the unwanted stimuli. I also think that using a
clock in the microprocessor to tell it when daylight should be there will
help a lot to avoid blinds closing when lightning hits in the middle of
the night or when passing cars illuminate the sensor for a few seconds at
night. Same for opening them because a bird is plucking its feathers while
sitting on the sensor. I am speaking from experience here. Three CdS
photoresistors in series, spaced out about 20 cm from each other in a
horizontal translucent (PE) tube oriented NS did it for me. Better incline
the tube to your latitude's sun angle away from the pole in your
hemisphere. The inclination helps dirt to slide off. I still had to do
some filtering and it used to be fooled by things like fireworks at night
and heavy clouds during the day. A CdS sensor can be used with the AN512
scheme.

Remember that exposure to strong light will cause a CdS to temporarily
change its Rdark.

Peter

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