[Waverley ARS] Drivers for a 8x8 grid addressable array of LEDs

[Adam Carmichael]

Hi Eric and the rest of the list,

Ben VK2XUF pointed out that by using multiplexing on 64 LEDs, the best
brightness I could expect is 1/64th brightness of a regular LED. Given
they are tricolour LEDs I'm going to go ahead even further and suggest
that it will be 1/192 times as bright as it should be, ie almost a
maximum 0.5% duty cycle.

At this point I might not send through the current schematic as this is
quite a critical flaw.

Ben recommended I look into Charlieplexing (named after Chales Allen
from Maxim). It seems with 9 pins, I should be able to drive 72 LEDs -
way more efficient than my 32 pin multiplexing!

This is looking like a really really cool technique right now. "Watch
this space."

73 VK2JSI... Adam

Eric van de Weyer wrote:
> Hi Adam,
>
> Still no attachment as they usually get stripped through the group unless
> they are small. 
>
> There are many chips for such uses. Look at the ULN series such as the
> ULN2003 etc.
>
> 73....Eric VK2VE.
>
> -----Original Message-----
> From: members-bounces at us.cactii.net [mailto:members-bounces at us.cactii.net]
> On Behalf Of Adam Carmichael
> Sent: Tuesday, 13 January 2009 23:14
> To: W.A.R.S. Members List
> Subject: Re: [Waverley ARS] Drivers for a 8x8 grid addressable array of LEDs
>
> Whoops... I forgot to attach the schematic!
>
> Adam Carmichael wrote:
>   
>> Hi all,
>>
>> I've started designing a circuit for a cluster of 64 tri-colour LEDs
>> which I'm going to use to make the next (first) piece of decorative
>> furniture for my bedroom.
>>
>> The project is divided into two boards, the first of which is the
>> "display board" - it will house the 64 LEDs in an 8x8 square. The second
>> board is a control board containing a PIC18F4550 microcontroller,
>> connected via 4 8pin headers.
>>
>> (See the attached image for a schematic)
>>
>> 1 header contains all the cathodes for each row of LEDs, the other 3
>> headers contain the red, green and blues for each column. By example,
>> this means: cathode pin 1 is row 1 and (due to a silly mistake in my
>> schematic which I will fix soon) red pin 8 is "column 1 red". This
>> allows me to turn on any one LED at any instant using just 32 pins total.
>>
>> I can then use a mind hack called "persistence of vision" to turn on
>> multiple LEDs "simultaneously". By quickly flashing them on and off, I
>> can make it appear like multiple LEDs are on.
>>
>> My question to the group is this:
>>
>> What would be the best way to go about driving these, and where would
>> you put the resistors?
>>
>> Right now I am thinking about a resistor for each cathode, and that's
>> where I put, say, 8 BC337 transistors. Does anyone know of an IC package
>> that might be a suitable replacement for the transistors?
>>
>> Thanks in advance,
>>
>> 73 VK2JSI... Adam
>>
>>   
>>     
>
>
>   


-- 
Adam "carneeki" Carmichael - VK2JSI, ex VK2FNRD
p: +61 415 37 1990
w: http://bigneek.com (personal); http://carneeki.net (business)
e: carneeki at carneeki.net
i: 2207644