Go Down

Topic: RGB SMT LED Cube, resistors, drivers, and shift registers. (Read 24 times) previous topic - next topic


it was pretty impressive.

What's really impressive is how the whole thing is wired up. He used a 32-pin mcu for that 8x8x8 cube.


Oct 28, 2012, 09:34 pm Last Edit: Oct 28, 2012, 10:27 pm by Hippynerd Reason: 1

but maybe there is a better way?

Yes you have the LEDs, connect them up with something like a 330R resistor and actually measure the forward voltage then you can do the math correctly to get your current.

I hooked up a 220 ohm on the red, and a 100 ohm on the green. Im using 3.3v from an arduino nano, and with the red LED I get 2 volts, with the green I get 2.7, the green seems pretty bright, but the red doesnt seem very bright.

Calculations Red (3.3-2)/220 = 0.005909091 (or about 6ma)
                  Green (3.3-2.7)/100 = 0.006 (or 6ma)

Green (3.3-2.77)/75 =0.007066667
Green (3.3-2.88)/50 =0.0084

Red (3.3-2.2)/50 = 0.022  (hey, getting close)
Red (3.3-2.1)/75 = 0.016 

Sadly 50 ohms is the smallest resistor I can find.
From that can I calculate what voltage I need for 20ma?


responding to him... (DHenry) is like a battle of wits.... Except you are battling with an unarmed man... Once in  a while he even manages to stay on topic...
Mainly he is just annoying as he knows everything about anything... and usually is wrong. IMNSHO

--> WA7EMS <--
"The solution of every problem is another problem." -Johann Wolfgang von Goethe
I do answer technical questions PM'd to me with whatever is in my clipboard


You are dealing with a few things:

1) different voltage drops causing different If for the same resistors. In your example, your reds have about 2.1v voltage drop and your greens about 2.8v. So to achieve desired If, the reds need to have resistors of (3.3v - 2.1v) / If and (3.3v - 2.8v) / If for the greens.
2) even if the same If goes through all leds, they have different efficiency / light output and our eyes have different sensitivity to different colors.

So play around with different resistors until your eyes perceive them to have same brightness. No point in having very precise values for those resistors.


Oct 28, 2012, 11:47 pm Last Edit: Oct 29, 2012, 12:38 am by Hippynerd Reason: 1
I just did some more testing. i hooked up a 50 to the red, and ran 3.3 to all 3 anodes. It looked a little blue, so I hooked up a 50 to the blue, and a 63 to the red. I then measured the diodes

I found the green, with no resister measured 3.24, red measured 2.15, and blue was 2.7

Red (3.3-2.15)/63=0.018253968
Blue (3,3-2,7)/50=0.012
Green (3.3-3.24)/0= ?
But it still looks like mostly blue

I measured the voltage across the resisters, and the blue measures .49, and the red measures 1.

Im assuming with all 3 LEDs on, it should be white, and any color variance should be adjusted with a resistor.

Go Up