The following is an ascii schematic of a solar-engine, the heart of many solar-powered robots. The idea is that available solar cells do not supply enough power to overcome friction in a typical robot design. So the power is stored until enough is available and then used to operate a small motor. The result is a continuously operating (well, as long as there is light) drive system. When coupled with a clever design, it can produce robotic "plankton" which display emergent behavior. A more detailed set of plans is available in the complete guide to the 1995 Games.
|-------------------O-------------O-----------------|
| | | |
| |-------| | |
| < mot. > | + |
| |-------| | --- design
p 2.2k-10k | + | ------- circuit
n|--O---/\/\/------O ----- >2200uF --- >2v solar
P | | ----- capacitor ------- cell.
| | N - | ---
|----|------------|p | -------
| n | - |
/-/ | | |
^ Zener | | |
| mc4742 | | |
|--------------O-------------O-----------------|
Legend:
| |
collector- P N |
base- -|n PNP trans. -|p NPN trans. ----O---- wire join.
emitter- p n
| |
Essentially it's a modified SCR design with supercritical feedback. The zener starts clamping at vcc-.7v and will eventually trigger the PNP, triggering the NPN, and will lock so long as the motor is moving. The beauty of this design is it's incredibly high impedance until tripped. For transistors, 3904s and 3906s are typical, but for higher power designs, the NPN must be replaced with something like cascaded 2n2222s, or high gain power transistors, like TIP 31s. The zener should be rated to trip at 2 volts below your solar-cell voltage rating to get maximum power.
This circuit is called the BEAM solarengine. I developed it for the Robot Olympics, used to build a solaroller device which covers a 1 meter distance in pure sunlight using only a cell 2-1/2" by 1/2". I promote it in my BEAM robotics package because of it's simplicity and cheapness of parts.