I built a small solar engine circuit from http://www.makezine.com. It basically accumulates energy from a solar panel (or another source) and once there's enough, it triggers a small motor. To trigger an LED, you have to add a small resistor in parallel with the LED.
Changing the size of the capacitor changes the frequency at which the motor is powered on.
More info here: http://www.beam-online.com/Robots/Circuits/1381.html
Comments
Problems with solarengine
Today I tried again the solarengine with various motors. It seems to be pretty capricious: it will stop working depending on the kind of motor and solar cell you are using. One of the reasons I think is that we don't really yet know what are the variables: what really happens? can we drive any motor? can we have a control over the output voltage?
There's a bunch of other BEAM circuits (solar engines and others) available right here:
http://www.beam-online.com/Robots/Circuits/circuits.html
See this one also:
Other kinds of solar engines
Also see these other types of solar engines:
http://www.solarbotics.net/library/circuits/se_t3.html
"Type 3 solar engine designs are charge curve differentiated (i.e., they trigger when the charge rate of the storage capacitor slows down). While theoretically the most efficient, no designs of this type existed until a number were created in just the past few months. This type of solar engine is based on the idea that the solar engine should trigger as soon as the charge-curve slope falls below a certain value (so the "differentiating" is required). If things are done correctly, the resulting trigger point corresponds to the point at which the capacitor "tops off" because the solarcell has done nearly all the charging it can. In bright light that may mean near 3.5V, or 1.2V in dim light. In any case, it should be (at least in theory) the optimum in triggering."