Matching brightness across multiple 7-seg LED displays

S

Steve

Guest
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.

Many thanks,
Steve
 
Something crossed my mind that I forgot to mention... Binning on hue isn't so
important for most red devices. Humans are lots less sensitive to wavelength
changes near 700nm. The hue part is more important elsewhere.

Jon
 
"Jonathan Kirwan" <jkirwan@easystreet.com> wrote in message
news:ed8am057k7g4vemhgo1e3vb8pt1q1knust@4ax.com...
Something crossed my mind that I forgot to mention... Binning on hue isn't
so
important for most red devices. Humans are lots less sensitive to
wavelength
changes near 700nm. The hue part is more important elsewhere.

Jon
Click to expand...
Thanks for your info Jon.

Steve
 
Subject: Matching brightness across multiple 7-seg LED displays
From: "Steve" me@privacy.net
Date: 10/6/2004 3:15 AM Central Daylight Time
Message-id: <1RN8d.30$oR1.0@newsfe5-win.ntli.net

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.

Many thanks,
Steve
Click to expand...
I think Don Lancaster suggested tweaking the on-times with software to even
things out for Charlieplexing in one of his columns.

http://www.tinaja.com/glib/muse152.pdf

Good luck
Chris
 
"Steve" <me@privacy.net> wrote in
news:1RN8d.30$oR1.0@newsfe5-win.ntli.net:

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.

Many thanks,
Steve
Click to expand...
I've this problem before and instead of findind what is a fault I simply
insert a 1K trimpot set at minimum resistance in series with the driving
transistors then tune all the displays to match the lowest brightness one .
roma
 
On Thu, 07 Oct 2004 22:40:32 GMT, roma <john-p@shaw.ca> wrote:

I've this problem before and instead of findind what is a fault I simply
insert a 1K trimpot set at minimum resistance in series with the driving
transistors then tune all the displays to match the lowest brightness one .
Click to expand...
Yes. Or, if one is able to consider the idea, PWM all the displays (they are
being used three at a time in Steve's system, so I'm guessing they are already
in a 1/3rd mux situation) and calibrate an adjustment factor for two of the
digits. Something like: assume each digit at 100% of their respective time
slots, select the dimmest (as you suggest), and then tweak down the mux time of
the other two, leaving slight time gaps where no digit is driven at all.

Jon
 
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?
Click to expand...
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
 
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.
Click to expand...
I can't say as I know or care what charlie plexing is. But if you are
multiplexing the displays it might be that your program is doing some
action other than running the displays that causes one or two to miss being
run. Example: it lights the second one, goes and gets data, starts over
with the first one....


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"roma" <john-p@shaw.ca> wrote in message
news:Xns957B9F7502275roma@64.59.144.76...
"Steve" <me@privacy.net> wrote in
news:1RN8d.30$oR1.0@newsfe5-win.ntli.net:

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.

Many thanks,
Steve



I've this problem before and instead of findind what is a fault I simply
insert a 1K trimpot set at minimum resistance in series with the driving
transistors then tune all the displays to match the lowest brightness one
..
roma
Click to expand...
Hi Roma - that sounds a very practical solution. Space on the board is tight
so I'll have a look for some small 1K trimpots.

Steve
 
"Jonathan Kirwan" <jkirwan@easystreet.com> wrote in message
news:3hkbm01jo143b13c755da2fpsfpqv9gonh@4ax.com...
On Thu, 07 Oct 2004 22:40:32 GMT, roma <john-p@shaw.ca> wrote:

I've this problem before and instead of findind what is a fault I simply
insert a 1K trimpot set at minimum resistance in series with the driving
transistors then tune all the displays to match the lowest brightness one
..

Yes. Or, if one is able to consider the idea, PWM all the displays (they
are
being used three at a time in Steve's system, so I'm guessing they are
already
in a 1/3rd mux situation) and calibrate an adjustment factor for two of
the
digits. Something like: assume each digit at 100% of their respective
time
slots, select the dimmest (as you suggest), and then tweak down the mux
time of
the other two, leaving slight time gaps where no digit is driven at all.

Jon
Click to expand...
Hi Jon. Thanks for the idea. The software cal method would be good in that
it won't add any more components. Only problem in my case is that I've
totally run out of codespace, having already crammed in more than I
bargained for. If I found a few bytes of ROM spare I could just about
hard-code the cal factors, but that would mean each PIC would have different
code, which is not great news. To do it a better way, such as holding the
factors in internal EEPROM, would add more code that I really can't squeeze
in as it stands. My fault for not allowing enough slack in the PIC!

Steve
 
On Sunday 10 October 2004 01:23 am, Steve did deign to grace us with the
following:
....
My fault for not allowing enough slack in
the PIC!

I didn't even know they'd ported Slack to the PIC! ;-)
Click to expand...

Cheers!
Rich
 
On Sunday 10 October 2004 01:16 am, Steve did deign to grace us with the
following:

"roma" <john-p@shaw.ca> wrote in message
....
I've this problem before and instead of findind what is a fault I simply
insert a 1K trimpot set at minimum resistance in series with the driving
transistors then tune all the displays to match the lowest brightness one
.
roma

Hi Roma - that sounds a very practical solution. Space on the board is
tight so I'll have a look for some small 1K trimpots.

Like these?
Click to expand...
http://www.newark.com/product-details/text/CD121/34272.html

Cheers!
Rich
 
"Jonathan Kirwan" <jkirwan@easystreet.com> wrote in message
news:0l2am0ptr2os8scl3ojjrg2uehe0tl1qql@4ax.com...
On Wed, 06 Oct 2004 08:15:57 GMT, "Steve" <me@privacy.net> wrote:

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.

I've actually made testing systems which bin such displays by human
perception
of hue and intensity. These involve optics, spectrophotometer system,
some
sense of your chosen "white point," and some CIE-based software to process
the
data and bin the parts.

If you want to put these into instrumentation and need to have multiple
pieces
"look the same" given the same driving currents, then you need to buy them
already binned or bin them yourself or else calibrate the currents (and
hope the
hue variances are acceptable.)

That is, I think so if I understand your problem.

Jon
Click to expand...
Thanks for your reply Jon. So do you think the problem is variation between
the display ICs themselves, or is it the transistors I'm using? I am
thinking it is more likely to be the transistors, since each segment within
a display has the same intensity, and they share the same transistor.

Steve
 
"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
Click to expand...
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...!)

Steve
 
"Rich Grise" <null@example.net> wrote in message
news:2jpad.350$MY.75@trnddc03...
On Sunday 10 October 2004 01:16 am, Steve did deign to grace us with the
following:

"roma" <john-p@shaw.ca> wrote in message
...
I've this problem before and instead of findind what is a fault I
simply
insert a 1K trimpot set at minimum resistance in series with the
driving
transistors then tune all the displays to match the lowest brightness
one
.
roma

Hi Roma - that sounds a very practical solution. Space on the board is
tight so I'll have a look for some small 1K trimpots.

Like these?
http://www.newark.com/product-details/text/CD121/34272.html

Cheers!
Rich
Click to expand...
Very nice, thanks!

Steve
 
On Wed, 06 Oct 2004 08:15:57 GMT, "Steve" <me@privacy.net> wrote:

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.
Click to expand...
I've actually made testing systems which bin such displays by human perception
of hue and intensity. These involve optics, spectrophotometer system, some
sense of your chosen "white point," and some CIE-based software to process the
data and bin the parts.

If you want to put these into instrumentation and need to have multiple pieces
"look the same" given the same driving currents, then you need to buy them
already binned or bin them yourself or else calibrate the currents (and hope the
hue variances are acceptable.)

That is, I think so if I understand your problem.

Jon
 
On Wed, 06 Oct 2004 09:41:09 GMT, "Steve" <me@privacy.net> wrote:

Thanks for your reply Jon. So do you think the problem is variation between
the display ICs themselves, or is it the transistors I'm using? I am
thinking it is more likely to be the transistors, since each segment within
a display has the same intensity, and they share the same transistor.
Click to expand...
Well, what I was saying is that if the current is identical (and that means
abstracting away from BJT or resistor issues -- I'm talking about identical
current drive) then various LED displays will give different perceived hue and
intensity. Even LEDs cut from the same wafer, I fear. Processes just aren't
able to be controlled well enough -- or at least not affordably so.

You can drive two displays that have the same part number from the same
manufacturer and even from the same manufacturing lot number and use the exact
same currents (say, to within 0.1% for example) and get what looks to a user as
kind of "funky" looking.

This is why manufacturers will sometimes offer a binning service. You might buy
1000 parts, let's say, but ask for them to be binned on hue and intensity
(according to some spec.) Then you will get, let's say, four boxes: 130 parts,
540 parts, 220 parts, and 110 parts -- call them boxes A, B, C, and D. When you
assemble instruments, you will select parts only from the same box. Kind of
like that. Next time you order, you might get four more boxes, but now they are
group A, D, E, and F -- no B or C types, this time. Etc.

Jon
 
"CFoley1064" <cfoley1064@aol.com> wrote in message
news:20041006144518.16253.00000278@mb-m01.aol.com...
Subject: Matching brightness across multiple 7-seg LED displays
From: "Steve" me@privacy.net
Date: 10/6/2004 3:15 AM Central Daylight Time
Message-id: <1RN8d.30$oR1.0@newsfe5-win.ntli.net

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.

Many thanks,
Steve

I think Don Lancaster suggested tweaking the on-times with software to
even
things out for Charlieplexing in one of his columns.

http://www.tinaja.com/glib/muse152.pdf

Good luck
Chris
Click to expand...
Thank you Chris - that's a great read.

It's a very good idea, but unfortunately in my case I've actually got 100
display boards to do in total (and more to come maybe) each with its own
PIC. It could be done but I'd have to mod the code of each one by hand until
it looked ok (there's no free code space, and no spare pins, to do it in a
less painful way unfortunately.).

Steve
 
"me" <me@here.net> wrote in message
news:Xns957E21F501D80meherenet@216.65.98.75...
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.


I can't say as I know or care what charlie plexing is. But if you are
multiplexing the displays it might be that your program is doing some
action other than running the displays that causes one or two to miss
being
run. Example: it lights the second one, goes and gets data, starts over
with the first one....
Click to expand...
Thanks for the idea. In fact, different digits have different intensities on
different boards, so it can't really be the software loop in this case.

Steve
 
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
Click to expand...
saturating, which they should.

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

Have Fun!
Rich
 
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