Bike taillight with a twist

Step 2 Dealing with the LEDs

I won't go into great detail here, as this has been covered elsewhere numerous times. You just have to make sure you don't exceed the micro controller's maximum output current per pin (about 35mA or so for AVRs). The same is true for the LEDs current. As you can guess from the picture, I used one of the SMD LEDs to figure out the resistor ratio to get well balanced white light. There are three 2k something potentiometers on the other side. That's all. In this case I ended up with resistors ranging from 90 to 110Ω, but that depends on the kind of LED you get. Just use a standard multimeter to determine the LED's forward voltages V_led and you're in business.

Using Ohm's Law, you can calculate the values for current limiting resistors for small LEDs like so:

R = ( V_bat - V_led ) / I_led

I_led should not exceed any current limit of the parts you use. Also this approach is only good for low current applications (maybe up to 100mA) and should not be used for Luxeon or CREE LEDs! The current through LEDs is temperature dependent and a constant current driver should be used.

If you need more info on that topic, wikipedia will have some information. Searching for electrical conductivity of semiconductors (low/high doping etc.) or negative temperature coefficient may be helpful.

I've used 6pin SMD RGB LEDs without commond anything. If you google for them, you'll get many results. The magic words are "SMD, RGB, LED, PLCC6 5050". 5050 are metric dimension for x and y in units of 0.1mm. On ebay you'll find them as well for as low as 50¢ per piece for high volume orders. Packs of 10 currently sell for about 10 bucks. I'd get at least 50 ;-)