Another answer:
Bear with me... a little model would make things easier!
On the panel that must point at the sun: Mount, in the plane of the panel, 4 phototransistors, arranged in a square, about 5cm apart.
Attach two vertical "fins" at right angles to the plane of the panel and at right angles to each other, such that each phototransistor is in a "corner" of the "cross" the fins make if viewed from above.
If I've managed to descrive that, then you will see that whenever the panel isn't right, one or more of the phototransistors will be in a shadow.
But all of it isn't really necessary.
You can program the panel to point to where the sun SHOULD be. You don't need to actively "follow" it... any astronomical almanac (or re-invented junior version) will tell you where the sun will be. All you need is date and time of day.
And you don't need half of that.
The problem is in two parts.... tracking across the sky daily and "up" and "down" the sky by season.
The former: by Arduino driven timer and electro-mechanical means.
The latter by a crank you turn by hand once a month. You'll want to check the panel from time to time anyway, won't you?
Of course, active tracking would be more fun!
