These are the simplest devices. A solar cell powers the object, which can be a motor, a pump, a computer, something that makes sound, or something else that makes sense to operate only during the day or when full sun is available.
An example of a device that only works in direct sunlight is the tiny toy car shown above.
Almost an inch wide, and an inch and a quarter long, this little car has a tiny motor (the type found in cell phones that make them vibrate) that uses very little power. Even so, it needs to be geared down two and a quarter times just to move at all. In bright sun, it zips along very quickly. In the slightest shade, it stops immediately.
If we slide a sharp knife under the solar cell (carefully, so as not to cut the wires) we can remove it from the double sided sticky foam that holds it in place.
We can remove the motor and the solar cell to use in our art projects. If we need more power, we can replace the solar cell with a larger one. We can also leave the motor in the car, and make use of the gearing if we want slower, more powerful rotation.
Of course, you can also simply use the toys as they are in an art piece. Here, I have simply attached wires from the cars to a central pole, to create race cars that travel in opposite directions when the sun is high enough: