You can see my design at this link. The idea is that a falling
weight provides the energy to turn a solar panel or heat engine.
Without the control line, the weight would fall down all at once.
But the turntable is clamped to a control line with a clothespin.
When the clothespin is squeezed, the turntable slips forward. The
force to squeeze the clothespin is provided by a blackbottle/float
combination, and of course, buoyancy. The key is to make it so that
the blackbottle has a blind that just barely shields it from the sun, in
such a way that the blackbottle is exposed just as soon as the turntable
turns too far. A friction brake keeps it from going too far before it can
react to shading. http://home.earthlink.net/~clanl/nonelectricsolartracker.gif