I need an LED guru...
 

To assist me with making a circuit board for HP SnapLED's. I can make the clinch-frame, attach the LED's themselves, but I need assistance with creating a control board for them. Pro or amateur help is appreciated!

Do you happen to have the operating voltage and amperage for the LED's? How many LED's are to be used in the array? Are you looking for adjustable brightness?

The simple LED circuit may only require a few resistors. If you're wanting adjustible brightness things get slightly more complex. Since LED's are simply on or off this requires modulating the pulse width rather than typical voltage control.

It will be used for rear combination lights (tail-lights), .5A on the low side, 1.5A on the high side, individual forward voltage 2.8-3.1V, and 25 LED's per circuit in parallel.
It's the power-switching regulator/feedback control where I need help and designing a PCB that will contain the drive circuitry.

I'm not sure where you're getting your amperage rating, but I can't see that being correct. Is that for the stock incandescent lamp assembly? Any particular reason for wanting them run in parallel?

Using 25 in parallel you'll be using much larger resistors. Assuming a 14.1V input and a 3.1 operating voltage you'll end up with 8.2 kO resistor.

If you don't mind running them all in parallel you have quite a number of options. Operating at 3.1v you could do 6 parallel circuits of 4 and one on it's own circuit (1.8kO and 8.2kO resistors). At 2.8v you could do a 5x5 (serial/parallel) array with 68 O resistors. At 2.8v you may lose a little light output. I would test one of the LED's at different voltages to see how that effects output.

I can't seem to find it at the moment, however I remember finding a pretty nifty calculator for calculating an LED circuit based upon all your operating parameters. I have it bookmarked at home. As I get a chance I'll post it.

As far as your brightness control you have two options. Option 1 is split the array and only utilize 1/2 of it for the low side, and all for the high. Option 2 would be a pulse width modulated circuit, using somewhere around 50% pulse width as your "low side" and 100% as your high.

When building my tail lights ('68 Camaro) I opted for PWM using a PIC microcontroller. I've seen others do purely solid state. There are numerous methods of doing PWM, each with their individual benefits. The best bet there would be to look for LED PWM tutorials and work through them. If you're doing it purely in solid state, you likely wouldn't need much power regulation. My LED's themselves are powered by unfiltered vehicle power.
The only thing that needed any special treatment was the microcontroller, for which I added a 5v regulator. I simply built it according to the reference circuit on the regulator data sheet.

As a side note, you may take a look at the laws governing your state (Assuming you're in the US). I know that the NTSB has guidelines for minimum and maximum tail light brightness. It's been a while since I looked into it but they may also differ on the state level as well.

Talk to Michael at Digitails/Spaghetti Engineering. They can help you build what you need. They are also a sponsor here. Great guy!:thumbsup:

The HP Snap150 LED data sheets are where those numbers came from, I'll double-check them. I want to mimic the taillamps of the early 2000's DeVille (they used those LED's) and some brilliant individual just suggested I look for the control boards in the back of those taillamps... Sometimes the easiest answers are the best!
After doing some research last night I'm wondering if the ISIS system I'm going to be using will handle some of those functions and power levels without needing any additional circuits?

They have an awesome product, and one that fits my Firebird, just looking for something a little different and I can imagine one-off's would be a small fortune...

You might be surprised Russ. A phone call is free. If they have a driver already that just needs tweeking or software, no big deal. He is going to do a custom project for me and he is very reasonable. Good Luck! :thumbsup:

Would you mind posting a link to the data sheet? So far all I'm really finding is their marketing materials.

From what I'm seeing each LED is rated for 150 mA. What's not extremely clear at this point is that there are multiple styles of the SnapLED 150, only some of them rated for "dual mode" use. How that dual mode is triggered is beyond the docs I'm looking at.