Mike, K8LH

Hello Gentlemen,

Can anyone suggest how I might choose current limiting resistor values to equalize the brightness of the red, green, and blue elements of an RGB LED without sophisticated test equipment? Do I simply "eyeball" it and try different resistor values?

TIA for the help/suggestions... Regards, Mike

Attached Images

rgb_led_pwm_1.jpg (70.9 KB, 477 views)

Oznog

Afraid not. But looking at the spec sheet and using the specified power for each channel I think usually makes a white since that's what most people want.

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I thought what I'd do was I'd pretend I was one of those deaf-mutes.

williB

mike , very interesting project you got there..
question : you are using the same pwm ratio for the three colors right?
you just want to make the blue a little dimmer i suppose??
you could use a varible resistor on all three till you find the right combination..

Mike, K8LH

Hi Willi (and Oznog too),

Yeah, I thought about adjusting the PWM to compensate for brightness levels between elements... Though, I'd still like to get as close as possible with the resistor values...
I don't have any pots' but I've got a good range of resistor values... Perhaps I'll just put a couple small machined pin sockets on the first prototype board to plug resistors into...

I'll probably put another three 2N7002's on the board to drive the LED too... I don't want to burn out the 12F683...

Thanks for the suggestions guys... Regards, Mike

<added>

Anyone out there with PC programming expertise willing to write a little PC control app' with slide controls for the three LED values, etc.? I plan to implement a simple RS-232 command language, perhaps something as simple as sending strings like; "7F,FF,00,<cr>" for the Red, Green, and Blue values... Maybe even add some effect commands like, 'fade', 'from value', 'to value', 'delay', 'repeat'...

williB

Mike, Hows that project coming along..??

Mike, K8LH

Hi Willi,

I think I've got a handle on the resistor values... Thanks...

I'll be playing with code this weekend and perhaps get a start on the next variation (below)... I'm also looking for recommendations on bright wide angle RGB LEDs... The little SMD LEDs should work fine for super low profile strip light fixtures for shelf lighting but I'd like to use something heftier for ceiling fixtures and above cabinet fixtures...

Regards, Mike

Attached Images

rgb_led_pwm_2.jpg (103.7 KB, 434 views)

williB

http://www.laminaceramics.com/products/bl2000.aspx
was just looking at this, in another thread , thank oznog..

Mike, K8LH

Thank you (and Oznog),

Gosh, those are beautiful (and expensive)... Guess I better start saving my pennies (grin)...

Regards, Mike

Oznog

BL2000 RGB is under $20 from Mouser. You need to pay attention to whether your device is a common anode or common cathode and if your driver design supports that type. The solution will vary, especially switching converters usually needed for this kind of power level. A switching converter needs to be a completely different design for common anode vs common cathode.

__________________
I thought what I'd do was I'd pretend I was one of those deaf-mutes.