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

Hippynerd

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)

Tom Carpenter

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
~Tom~

Hippynerd


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.
http://gemlit.com/howto/
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.

Tom Carpenter

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.
~Tom~

Hippynerd


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.


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 https://en.wikipedia.org/wiki/Voltage_divider

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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