This is a very basic solar engine using a Zenner diode and only one set of NPN and PNP transistors. This circuit is purely analog and so program will crash if you don't pick your zoom and resistance and current properly. The 70 ohm resistor is supposed to be the load. The zenner diode in parallel with the current source is just to limit the voltage of the photocell.
http://solarbotics.net/library/circuits/se_t1_zener.html
