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Topic: RGB SMT LED Cube, resistors, drivers, and shift registers. (Read 61084 times) previous topic - next topic


Wow! that was a really good explanation. So, #2, bad idea, on the voltage divider on the shift register power source, I'll look seeing if I can fix that with a VR circuit.

Resistors and caps came in. I can put some filtercaps on my shifters now.

I may pair up some 100 ohm resistors to put on my 3.3v test RGB leds too to test the 3.3v have something setup like the 3rd example.

I may also be able to rig up the cube similar to the 1st example, I have a 7 piggybacked 74hc595 shift tower soldered up for the cube.

but that still has to wait for me to take care of other business first :(


I've been experimenting, before wiring anything up to the cube. I had a rig setup for running 8 RGB LEDs on 3.3v, but I decided to undo that, and set it up for 5 volts, because it wasnt working, and I wanted to start with something easier.
So, re-soldered it up with resistors (8 100 ohm on the red, and 150 ohm on the blue and green). I have the cathodes (the negative side) soldered to a wire, and the anodes soldered to resistors, that are soldered to 3 piggybacked 595's. I soldered up a few .1uf caps to where I hook the 5 volts up to the shift registers. I made a couple jumpers to power the 595s from the 5v and gnd pins on the uno, and a jumper from the cathode to  the gnd on the shift registers.

No LEDs would light up. just to see if If the LEDs would light, i tried hooking up the 5v to the resistors, no luck, but in the process, I touched the cathode wire with the 5v wire, and some leds lit up. I took the ground wire off of the cathodes, and hooked 5v to it, and it seems to work fine (runs the shift pwm demo).

Im confused though. I double checked everything, and my LEDs seem backwards. The schematic shows pins 1,2,3 as cathode, but on all my RGB LEDs, you hook up 5v to that side, and ground on the other side to light them.

I double checked the datasheet online and the only odd thing I can find is the symbol for the LEDs looks funny, on the blue and green leds there is a symbol that looks similar to an LED next to the regular LED symbol.
Im including the image from the ebay auction

Tom Carpenter

Look at the pin numbers on the bottom right diagram (you can ignore the zener diodes). The have drawn the image as if it were from looking at the back.
If you then look at the top left diagram, the cathode is marked with a triangular notch.

The good news is that if you have in fact wired everything as common anode, it is a lot easier to control as you can now use the TLC5940's or any other LED current sink to drive them.


Oh, that was my first thought (that I had wired them common anode), but no matter what side you look at the part, the triangle bit is on the cathode, right? (topside vs upside down). The test unit I made was a 19 gauge wire soldered to the leads that are on the same side as the triangle.

I've checked all the LEDs I've setup, and they all seem to use the + on the side with the triangle, and none work with the - hooked up on the side with the triangle.

I wish I had some tlc chips to test it right now.

Im curious about the funny looking LED symbols in the diagram. Green and Blue look weird, they have a reversed looking diode on top of the regular diode. zener LED? I dont understand.

The LEDs are pretty bright, I think I will need to make tiny diffusers for the LEDs.

Tom Carpenter

Interesting. That means that the diagram is wrong (the mark is the anode not the cathode).

The Zener diodes aren't LEDs. I am not sure why they are there, but I would guess for some sort of overvoltage protection?


You can see the little triangle corners all lined up with the heavy gauge wire. I have a few test setups like this and all of them have all of the cathodes (triangle sides) connected together.

I do have a set of 8 just like these, only with resistors, and they are running on 3 shift registers. I have them wired up rgbrgbrgbrgbrgbrgbrgbrgb, but after reading the code, I would like to have wired them up rrrrrrrrggggggggbbbbbbbb. Its running shiftpwm rgb demo.

I guess one of the benefits of using shift registers and resistors is when you have something goofy like this happen, shift registers will still work.


This is what im working with right now, its a test unit that is setup the same as the ShiftPWM example (even though the LEDs are marked otherwise.)

Im running the 8 RGB LEDs off the arduino, The power supply is via USB (using a usb phone charger, I think its 500ma)

it seems to me that I should be having power issues when its running all the LEDs at 100% (24 LEDs * 20ma =480m. Thats about 160ma per shift register, and about 1/2 amp on the arduino.

according to this site:
thats not really a problem? I also noticed that there is a picture of 16 RGB LEDs, which is what I will need to do for the RGB cube,

I do have the cube mounted on a case, I have to solder up 48 resistors, and 6 shift registers, which is going to be kinda messy.


This is the next test Im doing, its running the LEDs on a 3.3v power supply, and possibly running the arduino off the 12v that powers the 3.3v circuit, but probably running on the 5v from the usb, I dunno.

The one Im wiring up actually has 100 ohm resistors, I didnt have any 47 ohm resistors.
The wiring between the shift registers and LEDs looks crazy, but i think it will be easier to solder up.


Heres a video of the 3.3v with voltage dividers (like the 3.3v setup in the previous post:


As you can see, it didnt work right, it did make the LEDs light up, but It was pretty wrong.

Heres a video of the 5v test setup (it uses 8 RGB LEDs, 8 150 ohm resistors, 16 100 ohm resistors, and 3 shift registers.) In this video I also include the 3.3v setup, without the voltage dividers, it runs pretty good w/out the dividers, but It may be causing some kind of damage, i dunno.



Instead of soldering up a bunch of resistors, and shift registers, I did a little googling instead.

Check out what I found.

This guy built a 4x4x4 RGB cube using 64 LEDs, some wire, perf board, and a nano. No resistors or transistors or chips of any kind, just some wire, leds, board, and nano.

The way he solved the problem was like using geometry to solve a accounting problem.


I did a lot of soldering yesterday. I found some unused xmas lights that had wires in groups of 3, about the right lenght for my project. I cut, stripped, tinned, and soldered them onto the resistors that I had previously soldered onto the column wires on the cube. I ended up having to untwist the wires to move them through the case, and it ended up pretty ugly. I will probably end up replacing those wires, but they will work for now.
Before green wires:

After green wires:

The horror!

Soldering away...

Which brings us to here:

Now that I have the shift registers wired up to the ribbon cable (clock, latch, data, ground), and wired to the LEDs, I need to figure out how to source current to the common planes. 

In my first cube, I used 20 pins, 4 pins sourced 5v (through 220 ohm resistor). I upgraded that cube to using 2 shift registers to control the sink, and sourced 5v via 4 pins through 220 ohms.
I recently upgraded that cube to 16 100 ohm resistors on the column(sink) pins, and removed the 4 220 ohm resistors on plane(source) pins.
In my RGB shiftpwm tests, I sourced 5v to the common pins via the 5v pin on the uno (Originally I had it from a 5v power supply, but later I moved it to the uno for portability). 

It seems to me, each of those are drawing too much current through the arduino.

Im considering a few different approaches, I could add a shift register (this is what i had originally figured that i would do). using 4 bits to source the current, and having the other 4 for added features, like buttons? or sound?, I dunno really.
Then I realized that mixing those bits in with the column pins may make shiftpwm, or other however I may control those bits more difficult, and a separate shift register may be better.
I figured, I could use 2 bits for sourcing the current per plane (1 8bit chip to source 1 48 LED plane), but it seems to me running the current for 48 LEDs through 2 shift register pins will be too much for it.

In my previous post, with the 4x4x4 RGB cube, they are able to do this with 64 RGB LEDs, and no other parts. I guess they are able to do all the current control via PWM, and since the red LEDs need a lot less voltage than the other LEDs, it somehow must adjust.

Also, Ive noticed other people posting about making 4x4x4 RGB cubes, if you are making a similar cube, post in this thread with how you are doing it, and what features you are thinking about.


I havnt lit this cube yet, Im still not sure how i want to source 5 volts to the 4 planes. It seems to me that I should be similar to my other cube, but I need to deal with 3 times as many LEDs.

I have built an SMD version of the asher glick cubes spire. 

I also came up with a way to do this with 8 RGB LED Spires, but I havnt figured out how to do the 8 led spire in SMD.

I've figured out 2 ways to do the 8 LED spire. One way is to simply build 2 4 LED spires, and rotate the top spire 45 degrees from the bottom spire. Instead of 4 leads, you will have 8.

The more interesting way is instead of using the pi/2 rad formula for rotating the LEDs, use pi/4 rad, and iterate it 8 times (instead of 4).  This should end up with 8 leads and 8 charlieplexed RGB LEDs in one column. I have to wait for LEDs to arrive before I can build a spire.



Nov 18, 2012, 07:55 pm Last Edit: Nov 18, 2012, 07:59 pm by Hippynerd Reason: 1
I just read up on the ULN 2003 chip, and it looks like a great way to source 5 volts to each layer. It says it can source upto 500ma, that sounds like plenty compared to a single arduino pin. It looks like I hook ground up to pin 8, 5v to pin 9, and use the first 4 pins for input, and last 4 pins for output to each plane. WahBam plenty of current, with a few pins unused. If I had a ULN2803, I could pair up the 8 arrays into 4 arrays for twice the current.

This way, I can modify my old software for the cube to shift out 48 bits instead of 16, and it should work on this cube. I think i can probably use shiftpwm with this cube too.

Still waiting for new RGB LEDs to arrive... They are common cathode, I will use them to make a charliecube like asher glicks. I cant wait to compare the 2 cubes :D


An update on the 4x4x4 RGB Cubes.

The charliecubes seem to be working great. I have built a couple, and have been running them 24/7 on 5 volts via USB. I have also run them via the Vin with 18v, and they work fine, but the chip gets warm, so I went back to 5v.

The more proper cubes are still under development, one uses TLC5490, and the other uses shift registers and resistors. I built a 48 bit shifter for the anodes, and am using 2 shift registers to sink the 4 planes. I think I need almost 1 amp, and 2 shift registers wont be enough for that probably, so I may need to modify it.

I have a few ULN 2003a chips, and I had thought they would work for sourcing, but that may not be the best solution.

I have been investigating using p-channel and n-channel mosfets. There are a lot of them, and I dont know which ones would be appropriate. It seems the ones in the smaller packages are too small (1/4 amp), and the bigger packages are way bigger than my needs (4 amp).

What are good ways of supplying sink/source to the planes? Is one way vastly superior to another?

Another consideration is circuit board space, im using very small boards with little space, so space is as much consideration as cost (ideally both would be small.)

Shift registers, mosfets, darlington arrays are what I have been looking at, but maybe I should look at other options too?


I think i have found a good current source for my TLC5940 cube,
Manufacturer Part No: IRF3205

Mosfet Type N-channel
Current Rating 110A
Rds (On) 8m?
Voltage Rated 55V

They are under $1 each, and seem to be a good n-channel source. I need about 1amp max.

Im still uncertain about sinking on the shift register cube. I had originally built it with 2 shift registers to sink, but I think I will need lot more than 2 shift registers for that.

I've ordered some of these p-channel mosfets, we will see if they will work.
Manufacturer Part No: IRF9540

Transistor Type:MOSFET
Transistor Polarity:P Channel
Drain Source Voltage, Vds:-100V
Continuous Drain Current, Id:-23A
On Resistance, Rds(on):117mohm
Rds(on) Test Voltage, Vgs:-10V

Im still interested in other options, ULN ?

For what im doing, I only need 1amp max, are there other parts that might be more suitable? These mosfets seem way bigger than I need. I think I could also do the same thing with a pair of ULN2803s,  but I think that would be less energy efficient, and probably a worser way to do it if its running on batteries.

What are some good ways to source/sink

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