This instructable primarily shows how to control the charging voltage from a solar panel to an energy storage device (capacitors in this case).
I showed how to construct the light previously http://www.instructables.com/id/Human-Powered-Light-from-my-book-Doable-Renewab/
This solar panel can produce up to 18 volts, but my capacitor bank should never receive more than 15 volts. I want to stop charging at 14 volts–just to be on the safe side.
To accomplish this, I use an Arduino microcontroller which controls a relay. The relay controls power into the capacitor bank. Initially, the Arduino is off and the relay is open–no power to the capacitors. I push a button, which energizes the Arduino and starts its program. (I use the button so that the Arduino is only drawing power while the capacitors are charging–in this situation, I charge for about 10 minutes to get one hour of run time on the light).
The Arduino program keeps the relay energized until the voltage reaches 14 volts. The voltage divided across the 20K and 220K (schematic says 200K, but the resistor is actually 218K) results in about 1.17 volts going into A0 for 14 volts going into the capacitors. The Arduino releases the relay, which stops capacitor charging and powers down the Arduino.
As you can see, the program is rather short. The “valm>239” is what causes the Arduino to shut off the relay when 14 volts is achieved.
1.17/5 = x/1024 This is the voltage in on pin A0 compared to the maximum (5 volt max) equals a portion of “1024”–the number generated when 5 volts is present.
For more detail: Solar Powered LED/Ultracapacitor Arduino Regulated Light