I'm working on a midi-controlled light system with lots of different color LEDs. Basically I'm trying to use a TIP120 as a master fader for a bunch of ULN2803-controlled lights, so that I have digital control over each light, and analog control over all of them.
For digital control of each group of LEDs I'm using shift registers (74HC595) and darlington arrays (ULN2803A).
For global analog fading control I'm using PWM and a TIP120.
I've got these two things working independently, but I can't figure out a good way to combine them, so I can turn lights on or off with the digital control, then control the intensity of large groups of LEDs with the analog control. Everything I've tried so far either turns on all the lights a little bit, or limits the current too much so the lights can't get as bright.
Any tips for combining ULN2803A and TIP120 transistors with separate inputs?
Thanks for any help.