Basically how do I wire one LED to be powered externally and have the Arduino control its function.
That chip is basically a bunch of transistor switches to ground, with build-in base current limiting resistors.Arduino pin (digital, or PWM for dimming) can be connected directly to a darlington input (1B-8B).Led, with current limiting resistor, between LED supply and darlington output (1C-8C).The darlington diodes (COM) must be tied to that same supply, but that can be ignored for resistive loads and LEDs.Note that a darlington has a high volt-drop (loss) when 'on'.That makes them unsuitable for power LEDs.Leo..
So I wasted my money then?
Depends on which LEDs you're talking about.Leo..
Do I understand that your LEDs will be supplied with current from a supply higher than the Arduino 5V?If so you have not wasted your money.The ULN2003 is ideal for doing what you need with an array of LEDS.
More than 1 LED. Some circuits are looking at 2-4. Not counting a shift register to free up pins
Ok I need to describe what the shift is doing because there is a lot going on with this build. The Shift register is going to handle 12 LEDs. 6 LEDs on either side of this model I'm building. At one time no more than 4 LEDs will be lit on both sides and only two to a pin(1 red and 1yellow so low voltage). If I numbered them 1-12: LED1&4 and 7&10 would be on, then off as it moved to 2&5 and 8&11 and then 3&6 and 9&12. The Darlington is so I can handle a programed blinking sequence with a Red and Green Led as they are wired in series, as well as the 7 interiors white LEDs which will be static.