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Topic: Multiplexing leds, faint leds (Read 4052 times) previous topic - next topic

Intosia

Sep 25, 2010, 02:23 pm Last Edit: Sep 25, 2010, 02:24 pm by Intosia Reason: 1
Hi, im using 5 digital pins and one 4067 16channel multiplexer to multiplex 16 x 5 LED's. To multiplexer pulls the LED's to ground. (Its for my 16steps sequencer)

The problem im having is that, some led's glow faint when they should be off... I have the feeling im missing some diodes? but dont know where?

here is the schematic, Ow before the LEDS are 220 ohm resistors ofcourse.


Hans K

I had similar problems with a 4051 multiplexer. The answer I got that it was not good with driving LEDs, read this discussion: http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1281382049

Grumpy_Mike

Sorry but that is just not the way to multiplex LEDs or the way to use that multiplexer.
You seem to be feeding 5V into the vase of your transistors with no current limiting resistors at all.
That circuit is just pants.

Intosia

Ok so it need a 1K resistor on the Base?

I dont see the problem with the idea? I got it from this:
http://nzlamb.files.wordpress.com/2010/04/led-multiplex-schematic.png

I replaces the 7seg with led's and hooked the transistors to the 4067.

But like i said i know something is wrong with it, so any suggestions?

Grumpy_Mike

Quote
I dont see the problem with the idea?

Only that what you have is nothing like that schematic you posted, and this is not the way to use an analogue multiplexer.

Intosia

#5
Sep 25, 2010, 03:12 pm Last Edit: Sep 25, 2010, 03:13 pm by Intosia Reason: 1
Ok... Any suggestion how i multiplexer 16x5 leds? And i dont want so daisychain shiftregisters, thats way to slow (updating 80 leds every time).


deSilva

Quote
i dont want so daisychain shiftregisters, thats way to slow (updating 80 leds every time).

In fact it is very fast....
With standard shiftOut() it will take 1 ms (still fast)
With digitalWriteFast() it should be < 50us.

James C4S

Quote
And i dont want so daisychain shiftregisters, thats way to slow (updating 80 leds every time).
Your eyes are are much, much slower.
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Intosia

Hm, well maybe speed isnt the real issue, but its just that i have to wire up all the 80 leds seperatly... If i multiplex them per 5 (per step on the sequencer), i have 5 wires and then i can connect them all to them on the frontpannel.


deSilva

#10
Sep 25, 2010, 04:14 pm Last Edit: Sep 25, 2010, 04:17 pm by mpeuser Reason: 1
To stop Mike's cat-and-mouse  play.

You should use a DIGITAL multiplexer (and of course also base resistors).
The analog multiplexer outputs (well, "throughputs") will become high impedance when de-selected.

You can mend this by base pull-downs (100k or such)

OTOH the internal resistance of the analog channels (100 ohms) results to a current limitation (45mA) that MIGHT be acceptable to your  transistors.


Intosia

#11
Sep 25, 2010, 04:21 pm Last Edit: Sep 25, 2010, 04:22 pm by Intosia Reason: 1
Ok digital multiplexer makes more sence indeed, its just that i have the analog laying around :)

So would need to make a pulldown construction like this? The switch would be the multiplexer then?

deSilva

#12
Sep 25, 2010, 04:28 pm Last Edit: Sep 25, 2010, 04:32 pm by mpeuser Reason: 1
Yes, that should work. One typically would connect R3 directly to the base to stress its intention, but that makes no difference at all.

Edit:
-----
.. and it would suffice to have just ONE base resistor (1k or such) at the 5V INPUT of the multiplexer rather than 8 at the outputs. But you need 8 base pull-downs, of course!

Intosia

Ok thanks all! Ill try some stuff on my breadboard :)

Intosia


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