Convert a standard clock into one that is recharged by solar. A fairly easy build with optional soldering and low costs make this a good kid friendly project. My son and I built this one afternoon and he was impressed with the idea of easily reusing modular parts of one device to build another.

Needed:
Friendly looking kid to model things
Clock that runs on a single AA battery
Solar powered garden light, the cheap kind with a tiny solar cell and a little LED
Basic tools (screwdriver, cutters)
Soldering iron and heat shrink are optional. (but if not available, some tape, preferably electrical tape, is needed)

Step 1: Breaking Down the Solar Garden Lamp

First things first is getting all the parts out of the doner garden lamp. The necessary pieces are the solar cell, the AA rechargeable battery and the battery contacts.

Using a screwdriver or other necessary tools open the case and take a look around.

The AA rechargeable battery is an easy thing to remove. Do that first and set it aside.

Before cutting anything, check which lead of the solar cell is positive (+) and which is negative (-). Sometimes it will be marked on the cell itself, in which case you can jump to the next paragraph below. Unfortunately sometimes it is not marked. If not, and you have a mutlimeter, jump to the next paragraph. If no meter is handy, you'll need to take a close look at the circuit starting from the positive lead of the battery. Trace it back through the circuit board and to one of the wires of the solar cell. Mark this wire as positive.

The solar cell will be attached to a circuit board with a couple of wires. Simply cut these wires near the circuit board to keep the lengths of wire as long as possible. Then pry, cut or unscrew the solar cell from the plastic cover (different designs use everything from glue to screws to press-fits, just be careful not to put too much force on the cell itself as it is fragile, especially in the very cheap lamps).

The battery contacts will hopefully come easily out of the case as well, and if necessary clip the wires from it that lead to the circuit board. It may require some cutting of plastic to get the contacts intact.

There are some other good parts in here for other projects: switch, photocell and LED are all available for use. The rest is pretty much junk though, don't let it clutter up your workshop.

Step 2: Building the Solar Recharger

Now to build it back up.

If the positive lead of the solar cell is marked (either originally or from you) jump to the next paragraph. Otherwise connect a multimeter to the leads and while measuring DC voltage and shine some light on the cell. The mutlimeter will show a voltage, the magnitude is not important, but the sign is. If the measurement is positive, then the positive lead of the meter is on the positive lead of the cell, if the meter reads negative, then the positive lead of the meter is on the negative lead of the cell. Straightforward right? In any case, mark the leads.

Now connect the battery contacts to the leads of the solar cell. The negative contact is the one with the spring on it. Here is where a little soldering and heat shrink makes a nicer build, but tightly twisted wire and tape will work too.

That's all there is to that part.

Step 3: Finishing the Build

Stick the AA rechargeable battery into the doner clock.

One of the requirements of this build was that my son didn't want me permanently modifying the clock he was so proud of. So the contacts of the doner solar lamp are wedged between the clock and the contacts of the clock itself.

The solar cell can then be mounted on the back of the clock (we did it with tape as a prototype until we are sure everything is right for a permanent mounting). This requires that the clock be hung in a window. Optionally longer wire could have been inserted in step 2 to get the solar cell mounted on the front of the clock. Live and learn.

After that we hung the thing in a place where the cell got some light. And that's all there is to it, outside of some problems that arose later...

Our apartment doesn't get much direct sunlight, a positive thing in the summer but a negative thing for this clock, The battery would slowly run down. We put a data logger on the solar cell for a couple days and found that it had very little charging time during most days and not much power even at those times, so we are now planning on adding solar cells to get a better charge.