Matching brightness across multiple 7-seg LED displays

[Steve]

"Rich Grise" <null@example.net> wrote in message
news:_8pad.348$MY.147@trnddc03...

On Sunday 10 October 2004 01:14 am, Steve did deign to grace us with the
following:

"Rich Grise" <null@example.net> wrote in message
news:Qk%9d.1404$y71.1027@trnddc02...
On Wednesday 06 October 2004 01:15 am, Steve did deign to grace us with
the
following:

I'm driving three 7-seg LED displays from a PIC by mulitplexing
them -
actually "Charlieplexing":
http://korea.maxim-ic.com/appnotes.cfm/appnote_number/1880

I've got a few of these boards and in some cases one or two of the
7-seg displays are clearly brighter/duller than the others. Is this
an
issue with the display ICs (all the same type), the 2N3704
transistors
(all
the
same type) that I use, or something else? FWIW, within each display
IC,
the brightness of each segment is the same. I know it's not the
resistors
since I they reside on a different PCB that connects to this display,
and
so the same resistors are used everytime.

You say, "in some cases." Is it always the same digit that acts up?
Is there any other correlation, like number of segments active? You
could
get that if you've got a common-anode display, for example, with only
one
resistor to +V, rather than the anode to +V and individual segment R's.

HTH!
Rich

Hi Rich and thanks for your advice. With different boards, different
digit
displays appear brighter/duller, e.g. on one board it might be the
leftmost is brighter, and on another board it might be the middle and
the
rightmoost that are brighter. It makes no difference how many segments
are
lit either. I guess that points either to differences in the 7-segment
displays themselves or the drive transistors (2N3704). I'm going to swap
some displays and transistors around to see if I can narrow it down
further (my soldering is lousy, so I've held off this step for a
while...!)

It wouldn't be the transistors, if you're reasonably sure that they're
saturating, which they should.

So, I guess you're stuck either matching displays or designing some kind
of
individual brightness controls. ;-)

Have Fun!
Rich

Yes, as you suspected it wasn't the transistors; swapping them over made no
difference - the same digits were bright and dull. So yes, now need to
decide on matching displays or using trim pots. Many thanks to all who've
helped me out here.

Steve