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Topic: 8x8x8 cube common anode (Read 1 time) previous topic - next topic

deepds14

I was curious about LED cube, so started a project of building an 8x8x8 LED single colour cube. Used references to make that. Made an 8x8x8 single colour LED cube.

The problem is I mistakenly connected all the anodes instead of cathode. Now there are common anodes instead of common cathodes base.

Is there any way to glow the LEDs using the common anode and finish the cube, or I have to redo the cube again?

I've used the following components:

1:LEDs x 512 
2: 21 AWG  copper wire
3: Wires
4: PN2222 transistors x 16
5: 220 ohm resistors x 64
6: 74HC595 shift registers x 9
7: Arduino Uno
8: Push button



https://www.youtube.com/watch?v=pxLvk3Zn8jo

PaulRB

#1
May 30, 2018, 03:31 pm Last Edit: May 30, 2018, 03:51 pm by PaulRB
Can you post a schematic?

I am not sure why there are 16 transistors. If there were connected as one transistor per layer, only 8 would be needed. If there were one transistor per column, 64 would be needed. It could be that the transistors are used in pairs because of the max 600mA collector current.

I think that the design that you have tried to follow is not a good design. It overloads the shift registers with too much current. Assuming the leds are red with a forward voltage of 2V, with 220R series resistors they will draw 14mA each, which means that each 74hc595 will need to source up to 110mA when its maximum is 70mA. To avoid this problem, you would need to increase the 64 resistors to 390R. This will reduce the brightness of the cube. To understand how bright this will look connect a single led to 5V with a 3K3 series resistor.

I mistakenly connected all the anodes instead of cathode. Now there are common anodes instead of common cathodes base.
Can you clarify what you mean by "cathodes base"? All cubes have both common anodes and common cathodes. Otherwise there would be 1024 wires connecting to the cube. Are the 8 layers of the cube connected to anodes or cathodes?

To answer your question directly, no, you do not have to re-make the cube, but you will need to change the driver circuits and also make changes to the code. But by re-designing the driver circuits, you may be able to improve the brightness and avoid overloading the components.

wilykat

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I am not sure why there are 16 transistors.
Could be following this: http://www.instructables.com/id/Led-Cube-8x8x8/ which used 2 transistors per layer to handle more current.

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The transistor array is responsible for switching on and off GND for each layer in the LED cube.

Our first attempt at this was an epic fail. We bought some transistors rated for over 500mA, thinking that would be plenty of juice. We don't remember the model number.

The LED cube worked, but it wasn't very bright, and the brightness was inversely proportional to the number of LEDs switched on in any given layer. In addition to that, there was some ghosting. Layers didn't switch completely off when they were supposed to be off.

Needless to say, we were kind of disappointed, and started debugging. The first thing we did was to add pull-up resistors to try to combat the ghosting. This removed almost all the ghosting, yay! But the cube was still very dim, bah!

We didn't have any powerful transistors or MOSFETs lying around, so we had to come up with another solution.

We posted a thread in the electronics section of the AVRFreaks.net forum, asking if it was possible to use two smaller transistors in parallel. This is the only option available to us using the parts we had on hand. The general response was, this will never work so don't even bother trying. They even had valid theories and stuff, but that didn't deter us from trying. It was our only solution that didn't involve waiting for new parts to arrive in the mail.

We ended up trying PN2222A, NPN general purpose amplifier. Ideally, you'd want a switching transistor for this kind of application, but we needed 16 transistors of the same type. This transistor was rated at 1000mA current, so we decided to give it a try.

For each layer, we used two PN2222As in parallel. The collectors connected together to GND. The emitters connected together, then connected to a ground layer. The base of each transistors was connected to it's own resistor, and the two resistors connected to an output pin on the ATmega.

We soldered in all the transistors and turned the thing on again, and it worked, perfectly!
If this is the one OP used, I'd ditch those transistors and look into MOSFET. They can handle a lot more power and is more efficient.

As for fixing LED cube, yeouch.  Changing plane transistor would be easy (swap NPN transistor to ground with PNP from 5v) but for the individual driver ICs, reversing logic might work. But it may be easier to swap the driver IC.  What schematic did OP follow so we could see what can be done?

PaulRB

So I think what the OP means is that in his cube, the 64 columns are cathodes and the 8 layers are anodes. This means the layer driver transistors should ideally be p-channel MOSFETs. If the series resistors are increased to 390R, then the 74hc595 chips can still be used. But to improve brightness, various other chips could be used including tpic6c595 or even dedicated led driver chips, which would remove the need for the series resistors, and some have 16 outputs per chip, which would need only 4 chips instead of 8.

Grumpy_Mike

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we used two PN2222As in parallel.
More instructables crap. Why do these people bother when they know so little? Transistors simply do not share current, FETs yes but bipolar transistors like that one no.

It shows they have no idea.

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