Here is a cheap and simple project that is also useful. You can make a small electric sky light to illuminate some part of your house that is always dark.
Step 1: The Circuit
It's that time of year at walmart when the left over malibu type solar lawn light go on clearance. Some for as low as $1.50. Also you may have one or two laying around. I do not like having to mow around them. So if you take the internal parts out (circuit board & battery) you have a nice little solar cell that puts out over 3 volts with no load. You can take one and power a led without using resisitor since the load of the one led will drop the voltage down and the lawn light shown in the pictures in this instructable will put out less than 20 milliamps with a led connected directly to it. I decided to use 2 cells in series to get the full 20 ma out at high noon during the summer. I used rtv silicone to glue the solar cells to a sheet of plastic and then glued the sheet to the base plate of my attic fan. I ran cat 5 cable from the cells down to the hall closet and then through the wall into our hallway which is always dark during the day. I used a piranha type led for wide angle illumination. There you have it. An electric skylight for under $10.00.
I think it would also make a nice accent light for a plant or an aquarium.
Step 2: Remove Everything But the Wires From the Solar Cell
Save the circuit board. It can be used to convert 1.5 volts to over 3 volts. I have used these in flashlight circuits.
Step 3: Remove Tube on Covers If They Are There
I drilled off most of the tube and cut the rest off with diagonal cutters.
This is done so the enclosures will sit flat.
Step 4: Wiring
The wiring is very simple.
The black wire from the solar cell on the left is connected to the black wire on the telephone cable.
The yellow wire is connected to the red wire on the solar cell on the left and the other end of the yellow wire is connected to the black wire on the solar cell on the right.
The red wire on the solar cell on the right is connected to the red wire on the telephone cable.
The red and black wire on the other end of the telephone cable are connected to the LED with it’s current limiting resistor. If the LED does not light up with the solar cells in direct sunlight, reverse the wires going to the LED.
The LED in the picture is a common 5mm LED designed to run at 20 milliamps. Since lawn light solar sells differ in size and efficiency the correct resistor is determined by trying different resistors at noon on a sunny day (maximum brightness. Try 150 - 200 ohms to start and then try lower resistances until you hit about 20 milliamps. To calculate the current measure the voltage across the resistor with a volt meter and the divide the voltage by the resistor value (ohms).
After you re-assemble the enclosures you may want to seal the holes in the back with a hot glue gun or silicone sealant.
Update: I did some testing with one solar lawn light and no resistor. It works fairly well also and of course it costs less. I now have the single version lighting up a small area in my garage.