I thought this might be a good use for that old cell phone you have laying around after you upgraded to 4G LTE, ASAP, PDQ. The amount of work required will depend on what type of phone you have and how much of the insides you want to keep.
What you will need...
resistor 100 - 220 ohm
Electrical tape or shrink tube or Silicone 100%
--- I used liquid tape, very handy.
I had some slow RGB LEDs and a few ideas. I did some tests and narrowed it down to two ideas. I couldn't decide, so I used both.
The first LED pokes through the top lid. With the LED on and pointed at the ceiling in a dark room, you get a Red, Green, Blue light show on the ceiling. The second idea involved shinning the LED on crumpled tin foil.
Remove the battery from the phone. Disassemble the phone, this may be a little tricky as they don't use normal screws. You may be able to use the original battery if you can find a way to charge it. I chose the button batteries and holder I had left over from another project. Collect your parts, switch, battery ect. and plan your placement in the phone.
Next, remove everything you don't need. I wanted to keep the main circut board to hold the key pad in place and add some weight. I added a on/off slide switch to the side. I filed a slot for the switch in the side of the phone and cut away part of the circut board to clear the switch. Then I placed the LEDS. Drilled a small hole in the lid for the top one, and glued it in place with liquid tape. Then placed the second one, and more liquid tape.
Next I made a board to hold the batteries. This was some old board from a TV remote. I filed the traces off the backside and drilled
holes to fit the parts. After the parts are soldered in place I covered the back with liquid tape. Next run wires from the battery/switch location to your LED area. I then partially assembled the phone and soldered the wires to the LEDs, battery and switch.
Before assembling the lid I lightly crumpled some tin foil an placed it to reflect the second LED. I held the foil inplace with... liquid tape!
Test your circut then reassemble the phone.
Where to find parts...
The LEDs were from Ebay. 12 LED + 12 Resistors for about $5
I got the slow change style. They also come in quick change. They are self contained and look like a normal LED.
If you use several together, they will stay in sinc most of the time.
A good place to find flashing items, batteries, LEDs
A good place for small Li-Po batteries
If using Li-Po batteries, make sure you use a Li-Po charger.