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boron Posted - Aug 10 2009 : 11:02:23 AM
Hi all,

I've recently aquired 2 8x8 red-green led matrices and am gonna start using them with a microcontroller but I have a few basic questions that I'm not sure what would be best.

I'm going to use them as a single 16x8 display, having the 8 columns for red being directly driven by a shift register and the 8 columns for green likewise with another shift register.

The common 16 rows will be multiplexed using two 4017's.

My questions are as follows:

1) Since each shift register output only ever has one LED lit its ok to directly drive them. The 4017 cannot however sink/source enough current to light 8 posible LED's all at the same time. I therefore of course need to put transistors. The thing is, since the 4017 has such a small sink/source current at 5 volts, is a base resistor required?

2) My second question is on the other side of the transistor. All the emmitters of the transistors are connected together and the collectors connect to the matrix row, since the rows are common anode. Do I need a current limiting resistor for the LED's for each transistor? Or can I just place one resistor between all the emmiters and V+?

Thanks a lot.
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codingplanet Posted - Aug 10 2009 : 4:48:20 PM
There won't be enough current to "cause any accidents", but it's still good practice to use use resisors there.
boron Posted - Aug 10 2009 : 12:35:27 PM
I've been doing some fooling around with the placement of the resistors and yes indeed if I only use one the brightness varies every time a new LED in the same row is lit. I should have expected that since every time a new LED in parrallel is added the current splits up and the voltage drops of each LED is slightly different causing mayhem etc. When I give each LED in the row its own resistor (this resistor would be driven by the shift registers in the actual circuit) the brighttness is very nice and relatively constant.

As for the base resistor, I thought it would work either way. Since the IC can only supply a small amount of current there's no danger in blowing the transistor. But I suppose I'll put them anyway just in case, so that I don't stress the 4017 or ause any accidents.

Thanks for the reply codingplanet. Much appreciated.
codingplanet Posted - Aug 10 2009 : 11:10:25 AM
1)It should be alright without. However it is good practice to use one, around 1K should be alright.

2)You will need a separate resistor for each transistor, otherwise the LED brightnesses will differ.

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