Here you can see the prototype for an interactive installation at work. 20 independent cubes that react on light changes were built to see if they could generate some attention. Considering the low budget the results turned out to be pretty cool :-)
By attaching some Attiny85 microcontrollers, LEDs and LDRs to small prototyping boards, the individual cubes are simple enough to build 20 of them for like no money. Even it would have been much more comfortable to have etched PCBs, the wiring was done by using thin coated copper wire. This saved some time, even it was a lot of tinkering work. The LEDs used are diffuse high power RGB ones with a very wide beamwidth. Due to budget reasons the housings were made out of foldable plastic packagings instead of the desired acrylic cubes. In the end these turned out to be surprisingly sturdy and did their job very well.
The whole installation can be triggered by any kind of light input (mobile phones, lighters, etc.) and from any side + cube. Because every cube has 4 light sensors built in, they can be stacked and arranged in every possible way.