Instructables user RDN1 made a tutorial for hacking a solar-powered garden lamp into a light graffiti message projector. He writes:

I recently read this interesting article in Wired magazine about “Light-Graffiti Hackers”. The Problem with light-graffities is that you need a power source to make them permanent, so you usually can’t put them everywhere you like. So I thought about making a small solar powered Light-Graffiti Projector that can be mounted nearly everywhere. The problem is that it mustn’t be very expensive in case it gets stolen, confiscated or whatever. First I thought about using a cheap laser pointer as a projector source, but you can’t make these caps to display different symbol by yourself. They are called “Holographic Optical Elements” and they are very expensive in production if you only need one with your special image (tell me if you know how to make them by your own). So I decided to use a LED. I also noticed that solar panels are still very expensive, but then I found one of these solar powered garden lights for only 5 Euros and transformed it into a “Solar Powered Light-Graffiti Projector“.

Via HackNMod.