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.lists@gmail.com>:
> On Mon, Dec 7, 2009 at 7:22 PM, Cristiano Cesaretto
> <cristiano@cesaretto.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.
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