Any devotee of electronics will speak fondly of the "blinking LED" experience- that is, the first time a new microcontroller or circuit fires up and you successfully blink an LED. Of course, in order to provide that, we needed to make some large-scale LEDs.
We experimented with a couple of methods- the first being a simple piece of nylon rod with the end sanded into a dome and a hole drilled in the bottom that fitted a smaller LED. While this worked quite well, getting a good dome top proved difficult, and reinforcing the bottom and getting good, solid large scale leads was also challenging.
We experimented also with a couple of other methods- crumpled paper in plastic wrap, a plastic cup, vacuum forming over a machined mold. None of these things provided the cachet we were looking for. Eventually we settled on using a clear plastic resin to cast a larger LED around a smaller one.
The results were spectacular.
 |  |
Cast LED with the lights off. This is at about 20mA drive current- well within the drive range of CMOS logic common in microcontrollers, 555 timers, etc. | Three cast LEDs with the lights on. You can see the rough surface finish which is due to the mold being made from a carved foam positive; some buffing with a Dremel tool cleans thes up nicely. The top center device is a common anode RGB. |
 |  |
Another construction method: machined nylon rod with a hole in the end that has the LED inserted into it. This gives nice results but getting the dome right is hard, as is attaching the thicker leads. | An LED in process. The mold was made by hanging the carved foam positive dome-down in a plastic cup and up-filling with RTV silicone around it. Note that the thick leads are pre-attached to the LED; doing this provides a nice strain relief to the solder joints. |
 |  |
The carved foam positive after it was used to make the mold. This was carved on team member Pete's DIY CNC machine. | Mass cast polyester resin, available from IASCO/TESCO here in Minneapolis (or online). It's noxious stuff- ventilate well! |
We are very please with these results; if cosmetic appeal is less important, other methods can be easily used. Another possibility is the use of jumbo LEDs directly; 10mm devices seem to be reasonable available (the white LED inside the nylon rod is a 10mm unit acquired locally at the Ax-man surplus store).