The built Solar Engine and an etched PC board. (The tops of the transistors were painted to help me place them more easily, before I'd made a component layout chart).

Here is the foil pattern, magnified (top) and actual size (bottom). It was designed for compactness and modularity. Note the pin headers in the photo at upper left. A solar panel with a connector, and a capacitor can be plugged in on the left, and a motor on the right. Neat and simple.

I will publish the Illustrator or Postscript file for it if there is an interest.

Updated circuit

Since first published, there have been a couple of minor changes to the circuit. Here are Ken's comments:

There has been two optional changes. The first results from building a number of units. It was found that R1 could be replaced with a 680K resistor rather than using a pot. R6 was added to limit the current used to drive the base of Q4 which meant that more current became available for the motor. The 390 Ohms is the smallest value that should be used although it can be increased depending upon the motor and drive transistor (Q4) combination.

There is one further proposed change that could improve the overall efficiency but it has not been tried, as far as I know. Changing Q4 to a ZTX618 improves the voltage across the motor. It is manufactured by Zetex and available from Digi-Key but the trouble and the price (1.08US) may just not be worth it.