[dorkbotdc-blabber] Re: LED Cube Workshop - use an Arduino?

[R. Mark Adams, Ph.D.]

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Peter-

Great analysis!  I was "winging it" based on the 200mA total, and
assuming that we have a bit of a margin (based on those AVR engineers
who do a much better job engineering than I do! :-) )  At any rate, I
want to test it, but it looks like it should at least work, with the
LEDs visible...

I agree with the driver suggestion- either a bank of transistors or a
hex inverter would probably be a much safer way to go!

In terms of "fancy multiplexing" schemes, I was talking about trying to
do all 27 LEDs from 6 pins- the multiplexing that they did with 12 pins
is much easier to implement in software... (For me at least!)

Finally, the reason I am interested in multiple Arduinos is to allow us
to put many of the 27-LED cubes together and have them be able to work
in concert- enabling even bigger displays!  We have so many talented MAX
/ PureData / python / etc. folks, I thought it would be cool to see if
we can get our cubes to talk to each other (and to the outside world) in
a simple and usable way.  WHo knows what kind of neat stuff we can dream
up...

Thanks!

- -Mark

Peter S. May wrote:
> R. Mark Adams, Ph.D. wrote:
> 
>>Peter hit what I was thinking of at first right off the bat.  For
>>driving the LED cube directly from the Arduino, I was thinking the same
>>thing, and assuming (as it looked like Peter did) that the Arduino pins
>>can sink the current from each level, which based on the datasheets, it
>>should.
> 
> 
> Let's examine that a little more closely, because it is too easy to
> assume without scrutinizing the facts.
> 
> The Arduino's processing core is an ATmega168.  The max DC current per
> I/O pin is listed on its data sheet at 40.0mA.  Normally a data sheet
> will also specify both a max operating temperature (125 C for the '168)
> and a thermal coefficient (in W/degC) by which one can determine the
> total current possible through all pins, which is not simply max per pin
> * number of pins.
> 
> Atmel hasn't provided the thermal coefficient, but has specified a max
> current of 200.0mA for Vcc and GND.  Since that current includes
> anything sourced or sunk by the I/O, we can probably use it (after
> subtracting what the chip itself uses.  Also listed under "Notes" in the
> spec is a bit about per-port current limitations.
> 
> In our case, the governing factor appears to be that, if we're using the
> per-layer Z pin to sink, it has to be able to sink from up to 9 LEDs at
> a time.  From an engineering standpoint, you would thus not want to
> drive each LED higher than about 4mA.  Of course, the Maker philosophy
> doesn't encourage paper-pushing, and in reality it will probably work
> okay with higher current (though in the long run it's likely to fail
> sooner if you do).
> 
> The nominal current of an LED is usually something like 20mA, but most
> modern ones will produce acceptable output at around 2mA.  My
> understanding is that the nominal 20mA was set in the earlier days of
> LEDs, when they were less efficient and required a significant current
> just to be visible.
> 
> However, if we really wanted, for example, 10mA on each LED, a much
> wiser idea would be to buffer each Z pin with a transistor (eg PN2222).
> 
> 
>>There should be enough pins on the Arduino to do this without any fancy
>>multiplexing schemes- 12 outputs for the LEDs and the 'usb' port as
>>serial input.  The trick is how to get the serial _output_ to cascade
>>from one board to the next.  I have a couple of ideas, but I need more
>>then the one Arduino board I have right now to try it out...
> 
> 
> I'm not entirely sure what you're talking about here, but it may just be
> a mistake in terminology.  The 12-pin method _is_ multiplexing by
> definition.  A non-multiplexing method would require 27 pins...
> 
> But it kind of also sounds like you're trying to get more than one
> Arduino in on this.  If so...why?  One ought to be enough for 3x3x3.
> Speed probably isn't an issue because you'll get acceptable results with
> a cycle of as low as about 60Hz--you could linger on each of the three
> layers for as long as 5ms, which ought to be plenty of instructions
> (order of 10K-100K) at Arduino's 16MHz clock.
> 
> As always, thanks for indulging
> PSM
> ........................................................................
> .......dorkbot dc: people doing strange things with electricity.........
> ................... http://dorkbot.org/dorkbotdc .......................
> ........................................................................
> 
> 

- --
|                            |   "Information is light.      |
| R. Mark Adams, Ph.D.       |    Information in itself,     |
| Computational Biologist    |    about anything, is light." |
| rmadams at epotential.com     |       - Tom Stoppard          |
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