Some day we across 5 mm RGB LEDs that are autofading. Crazy stuff, they look like standard monochrome LEDs but there is an IC melted inside the plastic which is nearly not visible. It does all the pulse-width modulation for you and you don't need an external device. Thus, one LED has only two legs and you only have to connect them with an appropriate resistor to a voltage supply (each LED has 3.2 V forward voltage at about 15 mA). Then the device will automatically start fading. We found two types of them on the market, both only about 0.25 EUR each, fast and slow ones but we only tried slow. The good thing about them is if you start two LEDs at the same time they will start drifting away after about 20 seconds since by some reason they have slightly different fade velocities. But now we needed to build something out of it.
I took the chance to build an strip out of 20 LEDs for my son's room. The strip itself is a standard 15 x 15 mm cable channel shortened to about 1 m. At a 5 cm pitch holes are drilled into it and the LEDs are inserted, banded and shortened.
Now they are a simply connected in parallel with a 120 Ohm resistor before each LED. This nicely fits into the cable channel. The whole strip is connected to a 5 V (5 VA) voltage supply.
This already looks quite nice and was assembled in about only 2 hours. At the end all LEDs are diffused by ping pong ball. Therfore a hole is drilled into the balls with a good old dremel and then glued onto the strip. That diffusion is a matter of taste but pure LEDs look too 'brilliant' for me. And that's the result:
The colors are not that good in the photo and video. In nature it looks really nice and smooth. The whole thing was fun and easy to built, and it was cheap.