Multiplexing

Jun 01, 2006 6 Replies
R

Hi!



I'm working on a LED display and I want to see how much LEDs I can ultimately drive using a 5v, pic16f628. Now, I'm using 2 shift registers and multiplexing to drive 64 LEDs. Now I want to extend the amount of LEDs I'm using.



I'm "scanning" through the columns and only one column is on at a certain moment. Per column I put on the rows for which I want the LED to be on.



I heard that when I want to use more than 10 columns, I must push more current through the LEDs. I can imagine that this must be done because the time when the LED is on decreases when the number of columns increases. And pushing more current trhough them will let the shine brighter, so our eyes won't see a difference.... Is this why they need more current? And how many columns or rows can I use?


Greetings



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R

The more leds you drive, the more area you want to illuminate. From a certain area up, you need to dim the lights to see it.

Rene

Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
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L

You can just increase the current as you decrease the time each LED is on - until you reach the peak current spec of the LED. Then you fry them.

Luhan

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D

Huh?

You can control 132 LED's using twelve port lines with NO external multiplexing at all.

See

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R

Hi Luhan,

That technique sounds promising but I've also heard that just using multiplexing can be used to drive hundreds of leds. As I'm working to a display with at least 512 LEDs, I want to start with that.

Gr. Roxlu

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J

If you use more than _one_ column you must push more current through the LEDs, the reason being that since all of the LEDs in column 1 will be off when column 2 is active, then to keep column 1 as bright as if the LEDs were being driven CW you\'ll need to increase the current into the column 1 LEDS by a factor of 2 during the time they\'re lit. The same goes for column 2. If you increase the number of colums to 10, then you\'ll need to increase the current through the LEDs by an order of magnitude. For 20mA LEDs that means you\'ll need to push 200mA through them during the time they\'re lit, so you can see that it won\'t take too many columns before you\'ll exceed the peak current allowed through the LEDs. I noticed from another post that you want to expand your array to 512 LEDs. How do you plan to dimension the array?
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R

Hi Jonh,

Actually I'm not sure how many LEDs I want to drive exactly because I don't know how much I can drive with one pic16f628. But I'm going to try to drive 512 as my first step.. Than I want to try even more.

Greet> >

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