There are 5 components to the electronics:
- The Arduino
- The LED driver
- Solar Cells
- Battery pack
- Charging/light sensor circuit
In the photo, clockwise from the top left are the solarcells, the battery, the charging circuit and finally the Arduino (actually a boarduino), LEDs, and LED driver chip.
The Solar Cells
I chose some flexible solar cells I had lying around from
www.flexsolarcells.com. It turned out that they are a great choice for generic Arduino use because a single thin film cell generates 7.2 volts which is enough to power an Arduino! In contrast, one of the silicon cells only produces .5 volts (you need from 4.5 to 5.5 volts to power an Arduino).
In fact just one of the cells pictured here can power the arduino even in the shade. You could image one of these on top of a small device... however, they are not ideal for my project because I will have plenty of room under the glass of my table to put a lot of cells. And the silicon cells are cheaper per watt and more efficient per unit area.
I chose 4 1.2 volt Nmh (nickel-metal-hydride) batteries. I chose Nmh batteries because they can be trickle charged with a very simple circuit. They actually generate 1.3 (5.2 total) volts when fully charged. But that is less then the Arduino's limit of 5.5 volts, so I can connect directly to the Arduino's 5v line instead of the Vin. This bypasses the Arduino's voltage regulator and so is much more efficient but a little dangerous. It for some reason the voltage ever does exceed 5.5 volts, I could burn out the chip.
Basically, when the battery voltage is above that generated by the solar cells, current can flow from the top through the base (middle) of the upper right transistor and then backwards through the solar cells. This turns on the transistor, which then turns on the bigger transistor at the bottom and so power can flow through the Arduino. So the solar cells themselves are used both to charge and to sense when it gets dark. In practice, they turn the system on at dusk, so I put the Arduino to sleep for 45 minutes before actually turning on the LEDs.
The diode near the solar cells is so cells can only charge the battery, the battery can't attempt to "charge" the cells!
And finally a close up of the charging circuit itself:
The orange wires to the left go to the Arduino. The upper right ones go to the battery and the lower right to the cells. The wires on the bottom of the photo are basically electrical ground.
I use an M5451 constant-current LED driver chip to light the LEDs. If you are interested in this, there is a lot more information in prior blog posts, or at: http://code.google.com/p/arduino-m5451-current-driver/
Share your solar projects! :-)