RGB SMT LED Cube, resistors, drivers, and shift registers.

Using Arduino LEDs and Multiplexing
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Something strange happened. When I tried to hook up the 3.3v to the shift registers, i disconneded the 5v, and the cube didnt stop working, it kept on working until I disconnected the ground.

I hooked up the 3.3v and ground from the 3.3v power supply to the shift registers on my old cube, and it didnt work right. its hard to explain how it worked, but it was slowed/delayed, and it seemed to not display stuff. It was just wrong.

Im totally baffled how/why the shift registers and cube were running on 4 wires (clock, data, latch, and ground)

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Clamping diodes on the inputs of the shift register IC. Basically if you remove Vcc, power can flow from an input which is set at logic 1 to Vcc via the protection circuits. This is not good for the IC as the protection circuits aren't designed to power the thing.
If theArduino is running at 5V still you also have a problem as you are running the shift register at 3.3v meanung that the inputs will be far above the absolute maximum allowed voltage of Vcc+0.5V

23

Tom, are you saying that the SRs are getting power from the data/clock/latch line? and that if i put a diode on those lines it will be ok? or that I should resistor the inputs down to 3.3v.

When I made the 3.3v power supply, i was thinking I could run the arduino at 3.3v, but the docs say minimum input voltage is 6v (but isnt the usb power 5v?)

I still need to find some decoupling caps, my shift registers dont have any on them, and I guess they like them.
I have lots of broken stuff i could pull smd caps from, but sadly, i dont know their value, I probably have many many suitable caps on various broken boards.

I also found this website with a 5x5x5 RGB cube thats running on shift registers and ULN 2003 chips. The way they mulitplex uses a lot less pins, but I cant figure out a way to build it, and make it stable with SMD LEDs, and solid uninsulated wire.

They are doing 25 x 15, If I could figure out how to do the SMD LEDs my cube would be 12x16 (28 pins), half as many as ithem currently using 48 x 4 (52 pins)

They use the ULN 2003 on 2 of the shift registers, is this for current limiting? The schematic shows no resistors, but it looks like the PCB has places for 1206 lands, and my guess is those are the resistors.
I also notice that they claim that most RGB cubes are common cathode, but theirs is common anode. That seems odd to me, since I seem to only find drivers for common anode setups.

I also found the rainbowduino (seeedstudio), uses my9221 chip, for their 4x4x4 CC RGB cube. Sadly, i havnt been able to find that part.

I have some ULN 2003s, and from what I see on ebay, they are fairly inexpensive.

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What I am saying is that when you disconnected the 5V line, it stayed working because there are diodes built into the chip. This is a very bad way of operating the chip.

When you connected 3.3v, you need to level shift signals from 5v down to 3.3V otherwise there will be excessive currents flowing through the same built protection diodes which will damage the chip.

Level shifters can range from dedicated IC's to Transistors, to basic resistor potential dividers.

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Hrm.. ok, this is interesting. Could you give me a couple examples that I could start experimenting with ? I have some resistors, I could make a resistor potential divider, What values would be appropriate? Looking over the wikipedia page Voltage divider - Wikipedia

It looks like their example of 6v from 9v, is the same ratio 3.3v from 5 v. In which they say R1 should be twice the value of R2. I've looked, and not found a pair of resistors that are suitable, however I have some tiny resistor packs, and I could bridge 2 resistors to make one resistor thats half the value of the other resistors. They are pretty low resistance (63 ohms).

How about the ics or transistors? I have some routers that I can pull parts from, maybe I have the transistors already?

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1.8k and 3.3k tend to work well.

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I think i have a couple 3.3k, but not 3. and I dont think i have anything close to 1.8k.

I just made up this little schematic to show one layer of the cube.

It shows the cathodes connected (this is physically how they are connected too)

The anodes are connected to the next layers anodes, but im only showing one layer in the schematic, or it would be very hard to read.

640×570 13.9 KB

16 RGB LEDs means 48 Anode pins to control, but only one cathode.

I do like idea of each layer having common R anode, common G anode, and common Blue anode, then it would be 16 cathodes and 12 anodes. I just dont see a way to do that, and make the cube dimensionally stable. The way the cube is built now is pretty dimensionally stable

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The way I have made mine structurally sound is to use a plastic frame, but I have access to a laser cutter at the University where I study so that may not be an option. The other thing I considered doing was to use plastic rod which can be gotten from hobby shops, and then either glue or melt the layer wires into it to give it rigidity.

What values of resistor do you have available? If you have enough 3.3k resistors, you can put two in parallel to get 1.65k which would give you exactly 3.3V. Alternatively any three resistors of the same value with the top of the potential divider being two in parallel will result in 3.3V

o-----+--+
      |  |
    R [] [] R
      |  |
      +--+-----o
      |
    R []    Where R = any value - preferably between 1k and 10k
      |
o-----+--------o
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The resistors I have are old leftover ones from who knows... They are partial packets of 5 from radioshack. I dont think I have more than 3 of any one value. But I suppose, If I can find 3 sets of 3 leds, I could make 3 different sets of voltage dividers. I also have some broken routers and things i can pull parts from, but they are tiny (like 0402 or 0603), and hard to solder wires to.

I've been reading about MAX7219/7221 chips, they are common cathode drivers, Is there a good reason why I shouldnt try to find some of those?

Also, the local electronics store only carries NTE parts, so if there are NTE parts that might work, I might be able to get the part locally.

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Am I right in thinking that for each plane you have a sepreate cathode connection and that all the cathodes are not connected together on each plane? Otherwise it would never work would it.

Have you done any tests to see how these LEDs look when they are on, if you need diffusers or what current you need to run them at.
Once you know the current for each LED you can then work out how much current to sink with the cathode sinks, you might need a FET here.

You then can look into how to provide that current at the anodes.

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Grumpy_Mike:
Am I right in thinking that for each plane you have a sepreate cathode connection and that all the cathodes are not connected together on each plane? Otherwise it would never work would it.

Have you done any tests to see how these LEDs look when they are on, if you need diffusers or what current you need to run them at.
Once you know the current for each LED you can then work out how much current to sink with the cathode sinks, you might need a FET here.

You then can look into how to provide that current at the anodes.

You are correct about the each plane having a separate cathode connection (4 cathodes, 48 anodes), but all the cathodes per plane are connected (thats 48 cathodes tied together per plane as the LEDs have 6 leads)

I did many tests on the LEDs at 3.3vs to figure out the forward voltage on the LEDs. I figure at 5vs I would need about 100 ohm on the red, 150 on the blue/green. At 3.3v, I need around 50-75 ohms on the red, and maybe something on the blue, im not too sure. I had a an LED hooked up to the 3.3v with a couple resistors, with the 3 leds lit, it seemed a bit blue, and Im guessing that it should be white.

Nearest I can tell, the forward voltage on the red is 2.15, and the blue and green are about 3.3v (That is to say, if I want 20 ma per LED, then those are the approximate voltages)

I have been experimenting with making some voltage dividers to try to hook up some shift registers, to get them to output 3.3v my lack of parts is making it hard, but I have found bunch of tiny 4.7k resistors, and I'll be messing around with them this morning.

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but all the cathodes per plane are connected (thats 48 cathodes tied together per plane as the LEDs have 6 leads)

You mean anodes here don't you. It is the cathodes that are negative that are connected together.
The LEDs are capable of 60mA so you intend to run them at 20mA, is that right.

Do you know when you start multiplexing them they will look dimmer because they will only be on for one quarter of the time. One way round that is to boost the current.

If you are using resistors then there is no point in running the shift registers at a reduced voltage, you just alter the value of the resistor.
So given a 5V feed what values do you come up with for a 20mA current?

my lack of parts is making it hard

Resistors are less than a penny each unless you pay vastly inflated prices.

I've been reading about MAX7219/7221 chips, they are common cathode drivers, Is there a good reason why I shouldnt try to find some of those?

The reason is that you cannot control the brightness of each individual LEDs. Also they will not work with the way you have wired it up.

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Grumpy_Mike:

but all the cathodes per plane are connected (thats 48 cathodes tied together per plane as the LEDs have 6 leads)

You mean anodes here don't you. It is the cathodes that are negative that are connected together.
The LEDs are capable of 60mA so you intend to run them at 20mA, is that right.

Do you know when you start multiplexing them they will look dimmer because they will only be on for one quarter of the time. One way round that is to boost the current.

If you are using resistors then there is no point in running the shift registers at a reduced voltage, you just alter the value of the resistor.
So given a 5V feed what values do you come up with for a 20mA current?

my lack of parts is making it hard

Resistors are less than a penny each unless you pay vastly inflated prices.

I've been reading about MAX7219/7221 chips, they are common cathode drivers, Is there a good reason why I shouldnt try to find some of those?

The reason is that you cannot control the brightness of each individual LEDs. Also they will not work with the way you have wired it up.

I do mean I have 48 cathodes tied together per plane (aka common cathode) There are also 48 anodes per plane that need to be individually controlled, they are connected to the copper wires in my photo, the silver colored wires are the cathodes. Basically, the cathodes connections hold each plane together, and the 48 copper wires hold the planes up. Using copper for the anodes (+), and zinc coated wire for the cathodes (-), was not an accident, I did that intentionally to make it obvious which is + and which is -.

Resistors around here are about 25 cents each, so 48 resistors will cost $12. assuming I only buy the right ones, and dont have any problems. I ordered shift registers online a month ago, and they cost $5/20. A shift register, and a resistor are basically the same cost to me. 48 resistors will make things quite messy too, I would probably want to hide that mess under the cube, but keep the shift registers, and nano, neatly next to the cube, for display, but im still not sure how I will mount/display the cube.

Since there is no way to run the shift registers at the low voltage that the red LEDs need, I will need a minimum of 16 resistors, or a constant current driver. I havnt messed around with PWM yet, but I expect to do PWM, shiftPWM or otherwise...

I havnt done much testing at 5v, my guess is to get 20ma, I will need 100ohm for red, and 150 for blue and green. Im still not too sure about how much current each LED needs for even brightness/color.

The whole point of running the shift registers at lower voltage is to reduce the amount of parts. If I only needed 4 resistors, I could live with that, but 48 is going to expensive, ugly, and more effort.

I have a few of these ULN 2003a chips, they are transistor or diode arrays? Could those be helpful?

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making voltage dividers with 0402 resistors is very difficult, but I managed to make 3 of them with 4.7k resistors, and soldered them up to my old cubes shift registers, and it lights up, but its not working right, its acting crazy. It could be my dividers are not working right, I dunno.

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Resistors around here are about 25 cents each,

WHAT!!!

You need to order them on line, it is sewing your view of electronics and forcing you into making silly design decisions.

It could be my dividers are not working right,

I doubt it, that sounds more like faulty wiring or bad software.

I will need 100ohm for red, and 150 for blue and green.

That sounds wrong.

the forward voltage on the red is 2.15

Again that sounds too high, but assuming it is right:-
So the voltage across the resistor is 5 - 2.15 = 2.85V so to have that voltage with 20mA needs a 2.85 / 0.02 = 142.5 Ohms
Similarly for a voltage drop of 3.3V you need 5 - 3.3 = 1.7 then 1.7 / 0.02 = 85 Ohms

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Doh! i got the values backwards, I meant 150 ohm for the red, 100 for the blue and green.

I couldnt even buy shift registers locally, the only stores that have parts are radioshack, and oregonelectronics, radioshack has almost nothing helpful, and OE only carries NTE parts, and they are expensive, and not very helpful.

I can get resistors at both places, OE is $.25 each, and RS is #1.20/5 pack. We used to have a Norvacs, but they went out of business, probably because there just isnt enough support for them, and they were a bit expensive (about the same prices as OE and RS, but they did have a lot of parts, most of the time I could actually get what i needed, but sometimes you gotta spend $10 for a cap.)

I can order stuff online, but it usually means waiting a week or 3. I also dont like spending $1 on resistors, and have to pay $5 shipping to get it within a week. I'd rather spend $5 on a driver chip that does everything i need, and looks good on display.

Even if I had all the resistors I need for this project, its gunna be ugly trying to figure out where to put them, and I like to use uninsulated wire, which will things difficult to wire, without shorting somewhere.

Im testing the 3.3v on my old cube. The shift registers are on the cathode side on that cube, they get 5v from 4 arduino pins (but are they have 220 ohm resistors) On that cube, when its lighting up all the LEDs on 1 plane, they are dimmer. 16 resistors on the column pins would be better than 4 resistors on the plane pins. If I did the RBG cube the same way, it would be 48 LEDs, and really really dim when all the LEDs on a plane are lit.

I dont have an accurate data sheet, so I had to hook up resistors, and measure and calculate (back on the first page of this thread are all those numbers... Scroll back to the first page, and check my numbers and math there, I think its right, but I could have made a mistake.

The voltage dividers I made are tiny (0402 parts), its very likely I buggered them up with my iron, or solder. No change in the software. Im using the old cube, and its shift registers (that work fine on 5v), to see if I can make them work at 3.3v like I hope to use on the new cube.

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and really really dim when all the LEDs on a plane are lit.

That either means your design is bad or the power supply is not up to it.
There should be no dimming when there are multiple LEDs on.

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Cheap, quick, parts from the USA as far as shipping is concerned. At least that is what I have been told and they shipped quick to me.

http://stores.ebay.com/thaishine

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Thats where i got the shift regitsers. they got here in less than a week.

I ended up ordering 200 (100 ohm, and 150 ohm) resistors. $5 is better than $12 i guess, but I still have to wait till next week probably. Hopefully they will be close enough. I also ordered some .1 uf caps.

I spent the better part of the day wrecking and repairing my old cube, I wont be using it for testing stuff anymore. I also soldered up some RGB LEDs to some shift registers to experiment with 3.3v. I can run the blue and green at 3.3 safely. I still need to make new voltage dividers, I keep breaking the tiny ones I've made.

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I couldnt even buy shift registers locally,

Very rarely you can. But many distribution houses offer next day delivery. Digikey for example does that, as mouser / newark.

I order my parts in hundreds so shipping cost ends up as nothing.