Re: IC / Circuit type help for project

On Fri, 21 Dec 2007 17:20:21 GMT, "patcheswfb via ElectronicsKB.com"
<u39997@uwe> wrote:


Alright, i've been looking into multiplexing more, and I definitly want to
try to do it, but I have a question. The concept is to have rows wired to a
common controlled ground and columns to a controlled source. this way, to
turn on a specific led, the row's ground is connected, and the column's
source is connected. since that specific led is the only one that has both
leads connected, that is the only one that turns on. Say, however, that you
want to have 2 lights on, one in say A1, and one in B2. You connect A to
ground and 1 to source to turn on the first light, and B to ground and 2 to
source to turn on the second light. When you do this, however, wouldn't A2
and B1 also turn on, because those both have both leads connected? there has
to be a remedy for this, every lcd in production uses matrixing. thoughts?

Dan

Multiplexing involves rapid switching, so each LED is only on for a
short period.

Typically, you might apply +V to the column A common lead, and ground
to the common row 1, 3, and 5 leads, to turn on LEDs A1, A3, and A5,
and hold that state for a few milliseconds. Then apply power to
column B, and simultaneously ground rows 2, 4, and 6, for a few
millisecs, lighting LEDs B2, B4, and B6. Continue for as many columns
as you have, then repeat continuously.

Since each LED can only be on for a faction of the time (if you have
10 columns, 1/10) you can drive the LEDs with somewhat more than their
rated continuous current if necessary to get the desired brightness.

The switching frequency should be fast enough that there is no visible
flicker.


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Peter Bennett, VE7CEI
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