These are fun and easy to build. The solar engine is made from two transistors and a threshold "switch" that allows the two capacitors to be charged by the solar cell. When the voltage across the capacitors reaches the threshold of the switch, the energy stored in the capacitor is released moving the Roller as current flows to the motor. This allows the vehicle to move in a wide range of light intensities.
Solar Roller and BEAM resources:
- Solar Engine Description, Parts and Instructions at Beam-Online
- Solar Engine free-form wiring instructions at Beam-Online