> On Mon, Dec 7, 2009 at 7:22 PM, Cristiano Cesaretto
> <cristianoKILLspamcesaretto.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.

> > 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

> 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!

> 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"
> RemoveMEspeffTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
> Embedded software/hardware/analog  Info for designers:  http://www.speff.com
>
>
>