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Help commanding a led matrix

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Member level 3
Oct 31, 2006
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8x5 led matrix

I've got a 8x5 led matrix, with common anodes and cathodes lined up. Now, of course, I can command it from a uc (like a big PIC, 16f877) but it's not too bright, because the PIC can't really source too much current, and it has to be multiplexed.

So, I want to build a power level for it, with transistors. Of course, we can reduce the whole LED panel to a single led, commanding both its anode and cathode. The problem is that, i'm pretty out of ideas. We can wire the led between a draining transistor and a sinking transistor, with its cathode in the first one's sink and the anode in the second one's source. I've tried this, and though it seems the logical way to do it, it doesn't work. Later, a friend explained me, that for this to work, potentials have to be clearly defined.

So, how can I do this the 'right' way, commanding a led (and of course, an array of leds, but that's easier once a led is done) - ie how can I command a led's both anode and cathode?

Another version would be to use a ULN/UDN pair. But for the moment, I want to do this with transistors.


Oh, i've solved it. It appears I had a short. Adding a led with it's cathode into one transistor's emitter and it's anode into another transistor's collector works just fine.

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