The next step in my project was to test the solar panel to see exactly how much power I could get out of it. To do this, I set up a variable resistor load on the output of the solar panel. Additionally, I used a small resistor in series with the load to measure the current being supplied by the panel. Here's a quick schematic of the setup:
And here's a picture of the circuit (without the meters):
I took this setup outside and measured it under a couple of conditions, cloudy day and sunny day. I used Fluke Series 85V DMMs for the voltage measurements. The one across the 10 ohm current sense resistor is set to mV range to record the voltage across the resistor. The meter across the 100K potentiometer is set to measure in Volts. Here's a graph of the recorded data for these conditions:
So, it looks like I can get about 7mW out of this panel on a cloudy day and about 55mW out of it on a sunny day. To maximize the power from the solar cell, it looks like I need to keep the load voltage in the 1.7V range. However, it looks like anything in the range of 1 to 2 Volts would be reasonable.
Next, I will determine how to use this solar panel to charge a supercap so that I can store that energy for reporting the sensor readings on a regular basis. Stay tuned...