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Topic: 8x8x8 multiplexed LED cube with an Arduino Mega 2560 (Read 42 times) previous topic - next topic

Un4Seen

#255
Mar 28, 2013, 12:09 am Last Edit: Mar 28, 2013, 12:12 am by Un4Seen Reason: 1
Unfortunately I've found an unwanted side effect of my new solution. The problems just don't seem to go away :)

As soon as I've started using the cube in real, multiplexed mode, trying out all LEDs in a test pattern, I've noticed that in one column from the 16 that I have in total, the top 3 LEDs are dimly lit all the time. The lowest LED is fine, the one above it, in the same column, is dimly lit, the third one (even higher in the same column) is dimly lit, but not as much as the second one and the fourth one is also dimly lit, but not even as much as the third one. If I remove the 1K resistor which connects the lowest anode layer to ground, then this unwanted effect goes away, but then I'm back to the ghosting :P
The even more interesting thing is that if I disconnect that column completely from the driving circuit, the top 3 LEDs in that column are still dimly lit, as described above, but when they should really light up at full brightness, they don't (because that column is disconnected). But if the column is disconnected, then it means that they are not grounded at all, so how can they light up, even dimly? And why only this column? And why only when I add a 1K resistor between the lowest anode layer and the ground?  :smiley-eek:
Andras
http://iqjar.com

Un4Seen

#256
Mar 30, 2013, 10:13 pm Last Edit: Mar 30, 2013, 10:18 pm by Un4Seen Reason: 1
OK, all problems are now fixed. After soldering in all the resistors for the anode layers and cathode columns, the ghosting is completely gone with 30 microseconds software delay after turning everything off before turning a new layer on (during multiplexing). I've updated the schematic and the printed circuit design accordingly:







Unfortunately the printed circuit has now grown to 14 x 12 cm because I had to fit 72 more resistors on it. I've already ordered one from a guy in town who makes PCBs, it cost me about 17$, let's hope it's perfect :)

As I wrote earlier, these resistors increase the power consumption of the cube by 72 mA in passive mode (when everything is off), which is something that I'm not happy with at all, but it's still better than ghosting...
Andras
http://iqjar.com

Pedro147

Good to see that you finally sorted out your ghosting problems. It would have been a shame to see all that hard work go to waste  8) Looking forward to more info on the project when you have time, Pedro.
http://www.pedroduino.com

Un4Seen

Yepp :)

Unfortunately the project is advancing slower tan expected. Also, spring is coming, so I'll have less time for it than in the winter. But even if slowly, I will make it happen :) And those articles on my blog will be written as I complete the steps in real life (the driving circuit, the 8x8x8 cube soldering and the software).

I've just found a great youtube video, by the way, in which Kevin Darrah explains multiplexing and bit angle modulation. The part that I found interesting is that the LEDs can be dimmed from software, so I'll make sure to include that in my code:
http://www.youtube.com/watch?v=I0sgqgUwIAQ
Andras
http://iqjar.com

Un4Seen

Hi guys!

Here's the next article in the series: the driving circuit for the 8x8x8 LED cube, based on CrossRoads' design:
http://iqjar.com/jar/building-an-8x8x8-led-cube-the-driving-circuit/
Andras
http://iqjar.com

CrossRoads

Pretty nice writeup! Good looking pictures too.
Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

Un4Seen

Andras
http://iqjar.com

Pedro147

Another well written and informative article Un4Seen. Ten points  8)
http://www.pedroduino.com

Un4Seen

Thank you! :) I do try. the next article will be about the construction of the cube itself. Unfortunately, based on progress so far,  I estimate that it will be done only about 6 to 8 weeks from now  :~
Andras
http://iqjar.com

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