Go Down

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

Hippynerd

Oct 28, 2012, 12:51 am Last Edit: Oct 28, 2012, 12:58 am by Hippynerd Reason: 1
I made a new cube using some SMT RGB LEDs, Im using these 5050 SMT LEDs I found on ebay.
This is the specs i have on the LEDs, I've written the ebayer, but they wont be back for a couple weeks.

RGB PLCC -6 5050 3-CHIPS SMT SMD LED Light
VF Forward Voltage
Min2.9   Max 3.6
Luminous Intensity
Min 4000   Max 5000 mcd
Power Dissipation 200 mW
DC Forward Current 60 mA
Reverse Voltage 5V

I've built the cube using common cathode, assuming thats how most of other rgb cubes are setup.
The LEDs have 6 leads, so I could build it either way, or even use them in series.


I had planned on making a matrix, but it turned out to be too difficult to solder up, and i found that I can just buy a nice SMT RGB LED matrix for under $10 so I abandoned that idea, but I made up a 32 bit shift register out of 4 74hc595s.


I figured i'd use it with this cube, but then I realized that I will need to control one plane (48 LEDs) with it, and 32 bits wont work unless I cut the planes in half, which would be difficult. Had I made 2 1/2 cubes, and set them next to each other, it would work... but I have 1 cube.

So, I built a big 56 bit shift register tower (7 chips tall) I figure I will use 48 bits for the LED anodes, and 4 bits for the planes (cathodes), and have 4 dummy bits (but they could be used for additional features).

If I use shift registers, i will need resisters, and im not exactly sure what the forward voltage is on these leds since the only info I have lists one voltage. I tested the leds with 3 volts, and the red ones were really bright, too bright, but the didnt blow from quick testing 3v. Based on looking over specs on  several 5050 leds, my guess is the forward voltage is 2.2v Red, 3.2v Blue, 3.2v Green

From 5 volts, 100ohm should get me close on the blue/green ones, and 150 on the red.
Since I dont really know what these LEDs should use, I figure i can experiment with resisters on a spare LED to try to dial in the right resisters for each color, but maybe there is a better way?


I have also been revisiting the idea of running the shift registers at 3.3v instead of 5, reduce the 5vs to 3.3, and resister the data pin so that it gets 3.3v from the arduino. I was thinking maybe having a voltage regulator (like an lm317) drop the voltage from 5 volts to 3.3. Then I would have to resister the red LEDs, and run the other leds on 3.3v

Is that a reasonable solution?

Im also considering just using LED drivers instead, just to be easy... There seem to be many good LED drivers, which would be a good one, and why?

I've read some posts here that mention TLC594?, max232,ws280?

I have an uno for developing it, but I plan on running it off of a nano, and hopefully a small inexpensive power supply (like used for charging cell phones)

I also have a real nice power supply from a DVD player, it puts out 5, 12, and 24v, but i was hoping to save it for something that needs a lot more current (like a burning laser!)


So... im interested in ideas about how to control the cube best, I have shift registers, and can maybe scrounge up resistors (or just order some online...), but I will probably have to order resisters online, and maybe i should just order some inexpensive driver chips instead?

Oh, i just realized that I have some ULN2003 chips (they are in circuits, but I can unsolder them.)


Grumpy_Mike

Quote
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.

Quote
I have also been revisiting the idea of running the shift registers at 3.3v instead of 5, reduce the 5vs to 3.3, and resister the data pin so that it gets 3.3v from the arduino. I was thinking maybe having a voltage regulator (like an lm317) drop the voltage from 5 volts to 3.3. Then I would have to resister the red LEDs, and run the other leds on 3.3v

Is that a reasonable solution?

Absolute crap solution.
1) The shift registers cannot supply the current.
2) You always need a current limiting device with an LED. http://www.thebox.myzen.co.uk/Tutorial/LEDs.html

Quote
maybe i should just order some inexpensive driver chips instead?

No as well.

dhenry

Quote
but the didnt blow from quick testing 3v.


Told you so, :)

Quote
Is that a reasonable solution?


Yes. Essentially the output resistance of those shift registers act as the current limiting mechanism here.

The risk you run, however, if the leds have wildly differing forward voltage drops, you may not be able to turn some of them on.

dhenry

Here is an interesting design that you may want to think of. It is for singles but notice that a) it is done without the use of shift registers - think very hard about that; b) it is done without resistors either.

http://www.doyoung.net/works/CUBE8/index.html

Youtube has a video. Check it out.

I like the implementation of touch-based power button. I think its light-sensing can be improved.

Hippynerd

#4
Oct 28, 2012, 02:19 am Last Edit: Oct 28, 2012, 02:20 am by Hippynerd Reason: 1
Thats quite the interesting cube. 128 control lines to control 512 LEDS.

I see the touch button thing, it looks like they use a photo resistor, and maybe capacitive touch on the other button.
Heres a video of a guy that made circuit boards on glass slides, They use capacitive touch, and you can operate it from back of the glass (not the side with the traces).
https://www.youtube.com/watch?v=5ZH1-eScY0M
I thought it was pretty awesome just because the circuit is on a see through board, but its even cooler as a input device!

I have to salvage some resistors to do some testing, and see if I can figure out the appropriate voltage for each LED.

I did read your webpage about LEDs Grumpy Mike, I assumed as long as you had the right voltage to feed your LED, you dont need to worry about current.  As a test, I ran a blue LED on 2 D cells, Im not sure what the forward voltage on the LED is, but my guess is close to 3v. The circuit was simple, 2 batteries and an LED,  no current limiting resistor, or any other fancy bits. Based on your webpage, it sounds like I have stressed the LED, but it did not seem weaken the LED any. Im confused, or conflicted, im not sure...

The Shift register way seem a bit more clunky, but thats also kind of appealing. The components will be on display with the cube, so a stack of shift registers, and resisters might be extra cool looking, but not as nice of a way to control the LEDs. I think its kind of interesting how I got 2 totally different responses. Mike, why is the shift register way crap? Im not criticizing, im curious about what is good and what is bad about things. Im also unclear on what you mean by "no as well"


Go Up