piclist 1998\07\21\145039a >
Thread: loads of flashing LEDs
face BY : paulb@midcoast.com.au

William Chops Westfield wrote:

> Ok, I'll bite.  I thought in a traditional matrix arrangement, you
> needed the diode characteristics to avoid hidden current paths.
> Having bidirectional pairs would defeat that,

 Ahh!  No, you mistake something here.  It is not the uni-directional
characteristic of the LEDs which is brought into play, but their
threshold characterisitic, i.e., 1.4 volts or so.

 Take a four-point matrix (i.e., two "rows", two "columns") of back-to-
back LEDs (i.e., eight of them) and energise one row-column pair.  The
other three pairs are indeed in series, paralleled across this, but as
their threshold voltage is three times the first, they do not light.
It generalises for M by N.

 To do this with incandescent lights properly, you need to add the
diodes.  You can use either diodes or threshold phenomena, LEDs offer
both so you can drive those tri-colour (R-Y-G) matrices quite easily
this way.  That said, it is actually *easier* to drive them if they use
the three-wire versions of tri-colour LEDs.

 Using PICs, you can drive a column of data at a time, directly from
the PIC with a column driver (NPN emitter follower) to take the heavier
current.  You can do this without TRIS, but using TRIS you can use the
same drive pins (bar 1) to select columns as well.  This allows n(n-1)
LEDs to be driven brightly.

       Paul B.

<35B4E252.3B68@midcoast.com.au> 7bit

See also: www.piclist.com/techref/displays.htm?key=leds
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