Multiplexing

Discussion in 'Electronic Design' started by roxlu, Jun 1, 2006.

1. roxluGuest

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

2. Rene TschaggelarGuest

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

3. LuhanGuest

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

5. roxluGuest

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,

Gr. Roxlu

6. John FieldsGuest

---
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?

7. roxluGuest

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.

Greetings Roxlu