The Solar Sensor consists of miniature solar cell and an op-amp. Solar cells have a linear current/irradiation curve. To get a linear voltage the cell has to be operated in short cut mode and the resulting current converted to voltage for the system to read. A little OP-amp on the circuit performs that duty. Even though this sensor is electrically linear, it is sensitive to the direction of the light and an optical or mechanical means of conducting light to it from all angles needs to be used to get a linear system.
If linearity is not that important and you just want to know if the sun is shining a simpler circuit that just heavily loads a solar cell with a resistor will do. You have to size the resistor low enough to load the cell under full sun to at least half of it’s open voltage,or lower. Polymorphous / polycrystalline cells are preferable since their angle at which the output drops to 50% is wider. I have used CIGS cells from Edmund’s Scientific with good results, except that the terminals corroded off too soon. Also the solar cells from solar powered garden light work well, as long as the cell produces about 5 to 6 volts unloaded.
The solar sensor can be integrated into the wind tree if desired.
To “calibrate” the sensor you can adjust the scale factor in the station configuration, so that at midday of the summer solstice, under a clear blue sky, you read about 950W/m² give or take depending on your location.
Eventually I will experiment with flexible cells sometime. Maybe bending them in a curve would eliminate the incidence angle problem?