Many years ago I built a small device that follows the sun without any electronics. With my new found solar panels and some nice precision gear motors I built a cute completely solar powered devise that looks to the best source of green energy we will ever have, the sun. The baby's eyes look at it's generations best hope for a kind world.
The dual cubes are set up so that the opposing solar panels current cancels when they both are evenly illuminated by the sun.The resultant voltage is zero. When the sun moves to the west, the panel on the west side gets more direct sun, and the east side gets less sun.The increased current on the west side brings the voltage across the cells up with the polarity of the west panel. This drives the motor to the west until a new null is located. The vertical axis works the same.

The shades amplify this unbalancing effect near the null point for more accurate tracking.

When you bend over to read the sign, your shadow makes the device look away. When you move again so it gets full sun, it leaps back quickly to point directly to the sun. Almost seems alive. A cool toy and science project.
The two solar panels on the back assure 360 degree recovery. A simpler dual panel tracker works just fine except for when the sun is behind both panels.
I discovered another cool use for the device. A solar puppet. I stand at a distance, and using my hand/fingers to cause a shadow on either the horizontal or vertical cells, I can make the device shake up and down for a yes, and left to right for a no.
Is solar energy our best source of green energy? I shade the vertical cells to make the eyes go up and down for a "YES". Do we have a lot of time to switch to alt energy sources? A shading of the horizontal cells makes the eyes move left and right to say "NO"

The things to be aware of if you want to try to build your own version.

The low voltage gear motor with the ideal gearing is not something you can buy easily or cheaply.
The quickness of the movement and the accuracy of tracking, heavily depend on the balance of power available from the cells, and the gear motor characteristics I was lucky to find the ideal motors at a swap meet, and have not found gear motors that work as well again.

I have some cheap non geared motors that respond sluggishly, and do not track very accurately, but demonstrate the principal.
I have some very high quality gear motors that are very positive in their response, and do track accurately, but do not move very fast, so you don't get the alive like response that I get with the motors on the puppet.

Some day I will put up a video that explains all of this and shows the difference that the choice of motors brings.
The motors I used that work so well,if purchased new would cost over $100 each, taking this out of the cheap solar demonstrator category.

The ideal motor is low voltage (will run with 1V)has a 4-6:1 gear ratio, and requires very low current(<10mA) to start and operate