With your comments on Hack a Day I found out that there are some mistakes I made in the description and that I forgot some necessary details.
1. The LEDs are mounted on the upper edge of the PCB and the PCB is inserted vertically into the ground. (not horizontally).
2. Making the PCBs waterproof and preventing scratches:
As I mentioned in the comments already: I used a protective varnish called "Plastik 70"
(http://kontaktspray.hu/en/?Products:Protective_coating:Plastik_70_conformal_coating) I'm not sure
what it is... probably an acrylic laquer (?). However,
I'm not sure whether it's available for purchase
in your country. But I'd recommend some kind of coating that can take
temperatures of about
60-80°C at least, since the driveway can get
pretty hot in the summer. And also: Always use a
varnish that is
designed for electronic purposes. I heard about bad test results with
Additionally to the varnish I used a shrink-on tube to protect the PCB from scratches.
So far this protection was able to withstand several (>20x) rainy days and thunderstorms. But I'll still
have to wait to see how well it can withstand freezing temperatures in the winter.
I would have used epoxy in the first place if I would've been sure about whether it would then still fit
in between the stones. Also: I don't know how to make a good case for the epoxy to harden in the
right shape. Any suggestions / experiences?
3. I am soon going to add all the details about the mircocontroller board, the LED-PCB board, and all the electronic parts.