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'[EE] driver for 7 segments display'
2009\12\07@132229 by Cristiano Cesaretto

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Hi all.
I was wondering if someone has an advice for a driver for a 7 segments
display. I need to be able to adjust the brightness of the display "in
software", with a command. The display drivers I have found so far expect an
external resistor to set the brightness level.

Other approaches are ok, if I can get the same result. I also have enough
pins to connect directly avery single segment of the display, if this helps.

Thank you.

2009\12\07@133035 by Marcel Birthelmer

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On Mon, Dec 7, 2009 at 7:22 PM, Cristiano Cesaretto
<spam_OUTcristianoTakeThisOuTspamcesaretto.it> wrote:
> Hi all.
> I was wondering if someone has an advice for a driver for a 7 segments
> display. I need to be able to adjust the brightness of the display "in
> software", with a command. The display drivers I have found so far expect an
> external resistor to set the brightness level.
>
> Other approaches are ok, if I can get the same result. I also have enough
> pins to connect directly avery single segment of the display, if this helps.

The way to go would be to use a PWM signal to strobe the LEDs at a
frequency of 100Hz or so, with a duty cycle proportional to the
desired brightness. You can accomplish this either by driving a
transistor in the common anode/common cathode of the LED display, or
by using AND gates to AND the PWM signal to the drive inputs of each
of the segments. If you have multiple digits that you're looking to
multiplex, it gets more interesting. I've used a NAND gate that fed
four logic-level FETs connected to each of four common cathode LED
digits, so that each digit could be turned on/off at will and also
brightness-regulated via the PWM.

2009\12\07@134924 by Richard Prosser

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Cristiano

If you only need a couple of levels, you can use a transistor to
switch in/out another external resistor.  Or, if the display is driven
from a separate supply, you can change the voltage of this.
If you need to vary the brightness over a range, then PWM is the way to go.

RP

2009/12/8 Marcel Birthelmer <.....marcelb.listsKILLspamspam@spam@gmail.com>:
{Quote hidden}

> -

2009\12\07@141619 by Paul Hutchinson

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> -----Original Message-----
> From: .....piclist-bouncesKILLspamspam.....mit.edu On Behalf Of Cristiano Cesaretto
> Sent: Monday, December 07, 2009 1:22 PM
>
> Hi all.
> I was wondering if someone has an advice for a driver for a 7 segments
> display. I need to be able to adjust the brightness of the display "in
> software", with a command. The display drivers I have found so
> far expect an external resistor to set the brightness level.

Maxim's MAX7221 has both hardware and software brightness control.
<http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1339>

Paul Hutch

> Other approaches are ok, if I can get the same result. I also have enough
> pins to connect directly avery single segment of the display, if
> this helps.
>
> Thank you.

2009\12\07@142104 by Marcel Birthelmer

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> Maxim's MAX7221 has both hardware and software brightness control.
> <http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1339>
>

costs about $10 a pop in low quantities, though, unfortunately.

2009\12\07@165020 by Jinx

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> I was wondering if someone has an advice for a driver for a 7
> segments display. I need to be able to adjust the brightness of the
> display "in software", with a command. The display drivers I have
> found so far expect an external resistor to set the brightness level

Cristiano, I occassionally use the CMOS 4543

http://www.futurlec.com/4000Series/CD4543.shtml

http://home.clear.net.nz/pages/joecolquitt/counter.gif

(sorry about the inverted colours, I'd change it but my ftp is playing up)

In that circuit there would be a couple of options. Strobe the Blank
pin (which enables the output drivers) or strobe the common cathode
(the ground connection to the right of the LSD) via a FET. Either could
be done with a PWM output

For ICs that expect a resistor to set the current, that resistor could be
selected from a group with a transistor or 4066

2009\12\08@063038 by Cristiano Cesaretto

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Thanks to everybody.
I get very usefully ideas here.
I think I am going to try to add a transistor to the common cathode, driven
by a PWM.

Cristiano


2009/12/7 Jinx <EraseMEjoecolquittspam_OUTspamTakeThisOuTclear.net.nz>

{Quote hidden}

> -

2009\12\08@084737 by Harold Hallikainen

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I've used the Allegro A6282
(http://www.allegromicro.com/en/Products/Part_Numbers/6282/index.asp). It
has a shift register input with parallel latch, then constant current
sinks for the LEDs. The currents are all set with a single resistor. You
can vary brightness by either PWM on the OE pin or putting a variable
voltage on the bottom of the current set resistor (instead of grounding
it).

Harold


--
FCC Rules Updated Daily at http://www.hallikainen.com - Advertising
opportunities available!

2009\12\11@161624 by Bob Axtell

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You might consider using a 74HC595 on an SPI output/clock module. Then
you can put your PWM signal on the OUTPUT ENABLE and you are set.
Costs 0.25$USD for each 8 signals,
with up to 50mA output.

--Bob A

On Tue, Dec 8, 2009 at 7:01 AM, Harold Hallikainen
<haroldspamspam_OUThallikainen.org> wrote:
{Quote hidden}

> -

2009\12\11@164802 by Spehro Pefhany

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At 04:16 PM 11/12/2009, you wrote:
>You might consider using a 74HC595 on an SPI output/clock module. Then
>you can put your PWM signal on the OUTPUT ENABLE and you are set.
>Costs 0.25$USD for each 8 signals,
>with up to 50mA output.
>
>--Bob A

More like 35mA, and subject to absolute maximum sink or max source of
70 or 75mA TOTAL, so more like 9mA/output (absolute maximum rating
with all outputs allowed to be active).

TI makes some with internal open-drain DMOS transistors connected to 8
outputs which can handle considerably more current, but they are not 0.25 ea.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
@spam@speffKILLspamspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2009\12\18@180315 by Bob Axtell

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Gee, after all these years, and all those units shipped with HC595s
and not a single failure,
now I discover that I'd screwed up all along. I'm gonna have to study
the specs closer, I guess...

--Bob A

On Fri, Dec 11, 2009 at 2:55 PM, Spehro Pefhany <KILLspamspeffKILLspamspaminterlog.com> wrote:
{Quote hidden}

>

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