The logic is simple: you need to have a two-wheels vehicle-like frame with two small solar-panels and two electric motors (one for each wheel), such that the left solar-panel connects to the right motor and vice-versa. This crossed configuration ensures that when the sun illuminates the left side of the toy, the right wheel spins forward rotating the toy leftward (toward the sun), and likewise when the sun illuminates the right side. The result is an automaton which tends to move/orientate towards the sun.
You can see some videos below:
(notice how it "avoids" the shadows while keeping a more or less stable course)
I guess it might be possible to obtain different phototropic motion patterns by changing the position of the solar-panels (thus altering its relation with the direction of the sun).
One interesting aspect of this toy is that it shows how "purposeful"-like behavior might emerge from (very) simple circuits that, nonetheless, establish a link between efferent (sensor) and afferent (motor) components in such a way as to be tuned to a particular environment. Readers of J. J. Gibson might find this toy particularly appealing as it illustrates how an orientation behavior can emerge from the close coupling between the sensing/moving "being" and ecological variables.