LED Color Changing Cross-fader. / Both / One or Other. Need Help

[APAudVid]

Hey There!!! Yes, first post here, been lurking for a while though...
I was hoping to get some input on an automatic cross-fader for an LED project I'm working on... I want to create a circuit that will allow me to (maybe with a couple of switches)... 1) Fade from one color LED to another color LED with a rate of about 2 to 3 seconds. (adjustable?) 2) Have both LED's on. 3) Have one color or the other color on.
Additionally, I will be powering about 50 LED's each with a 12 volt supply (A total of 100 LED's)... I figured out how to do most (not all) of this with a cap timed relay, but thought you guys might have a better way of doing this....
I consider myself electronics knowledgeable, good with schematics, and at one with a soldering iron...
I'm drawing a blank on this one....
I would be honored to get your input.
Thanks in advance!
AP

 

[HarveyH42]

LED Sequencers, Flashers, Clocks, Faders

Look for the circuit 'Fading Red Eyes', you can change it quite a bit, and it's pretty well documented. I used a LM358 opamp, works well.

 

[APAudVid]

Thanks! I noticed for the the cross fading circuit, he's using a quad op-amp... I see in the first drawing how it makes sense (ramp up and down), but in the second drawing (cross fade), using the quad, it almost seems like the second half of the op-amp is not used...
I can see how the ramping driver (in the first drawing) is hitting the transistor, but am unsure of how it is hitting the following transistor in the cross fade drawing... Are they running in parallel with the out of phase being the fact that the LED's are on the opposite sides of the output transistors?
Also, my need to run this at 12v and with so many LEDs, I'm guessing a TO-3, beefy style transistor might be in order... Any thoughts?
Thanks so much for the link! It does provide a good start!
Looking forward to the replies!
AP

 

[HarveyH42]

I believe one side of the cross-fade is off a NPN transistor, the other was PNP. The opamp fade only needs two, think he is either just using what he had handy, or showing that most any will work okay. I have used this circuit in several years, before I got into AVR microcontrollers. I wanted and RGB color fader, and used three of these. Still think it does a much better job, much wider range of colors, compared to the microcontroller and PWM...

To run of 12volts, think you need to adjust the voltage divider, probably better left to somebody who understands that stuff, not up on the math and theory.