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Design Flaw?

Posted: Sun Jan 06, 2013 2:28 am
by Mike M
Hi Brad,

I wonder if I've stumbled onto a design flaw on another one of your projects?

When I saw the 8x8 Game System I thought it was a very intuitive and logical evolution of the original Great Race project. Driving all of the red and green LEDs on the display as well as adding row driver transistors to increase brightness levels made perfect sense. Then I noticed the current limiting resistors on the outputs of the transistors... Ouch!

I looked at a couple of the Game System programs to see if you were lighting LEDs one-at-a-time, in which case it wouldn't matter if you placed current limiting resistors on the row or the column lines, but that doesn't seem to be the case. The number of lighted LEDs in a particular row will affect the amount of current going through the resistor and the voltage drop across the resistor, which in turn will affect the average current and brightness of the LEDs in that row. You've effectively increased current capability with the transistors and taken it away with the current limiting resistors. Clearly, the current limiting resistors should have been placed on the column lines. May I ask if you noticed brightness level variations across the display, depending on which game program you were running, back when you were actively working on this project? If so, that should have been a clue that there was a problem.

Cheerful regards, Mike

Re: Design Flaw?

Posted: Sun Jan 06, 2013 6:59 pm
by brad
It looks like you've caught me again :)

You're right, It is a design flaw however the screen still looks good and you may notice some variation in brightness depending on how many LED's in a row are lit up.

Re: Design Flaw?

Posted: Wed Jan 09, 2013 1:20 am
by Mike M
Well, I'm sorry to be the one that caught the problem...

Cheerful regards, Mike