Step 1: What it can (and can't) do
There are 15 levels of digital control of LED brightness, and a SINGLE resistor to set the maximum brightness of all of the LEDs on one chip.
The processor can be any standard Arduino (Atmega 168 or 328) and with the ICSP connector, you can burn your own bootloaders.
The input interface uses a simple current limiting resistor and two diode clamps to the 5V rail. DON'T make a habit of driving into the clamps !
There are no input scaling resistors on the circuit at the moment.