As I couldn't decide on one color, I picked up some RGB slow rotating color changing LED's... any suggestions on a resistor value to use with it on the RAM cart?
Hmmm ... I wish I'd thought of that idea myself!
The supply voltage from the cartridge board is 5V, so the best value of the current-limiting resistor would depend on the forward voltage rating of your LED. The easiest way to find the resistor value would be to use a calculator like this one.
The LEDs that I used were rated at a maximum forward voltage of 2.4V to 3.4V (depending on the color), and a maximum forward current of 20mA, so I used 82Ω resistors for the blue, white, and green LEDs, and 150Ω resistors for the red and yellow LEDs.