Re: how to control LED array?


"Anthony Fremont" <spam@xxxxxxxxxxxx> wrote in message
news:KKsbe.26582$AE6.19084@xxxxxxxxxxxxxxxxxxxxxxx
>
> "Lord Garth" <LGarth@xxxxxxxxxxxx> wrote in message
> news:9psbe.423$gd5.117@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> >
> > "Anthony Fremont" <spam@xxxxxxxxxxxx> wrote in message
> > news:f1qbe.26530$AE6.26160@xxxxxxxxxxxxxxxxxxxxxxx
> > >
> > > "John Fields" <jfields@xxxxxxxxxxxxxxxxxxxxx> wrote in message
> > >
> > > <snip for brevity>
> > >
> > > > The display will need to be multiplexed, and the most efficient
> way to
> > > > do that would be to broadside data, a byte at a time, into the row
> > > > drivers and then to scan the columns. If you want to make a
> > > > non-flickering display, then the pixel update rate should be
> somewhat
> > > > higher than 30Hz, say 50Hz just to make sure. That means that
> since
> > >
> > > I disagree on the refresh rate. Having just experimented with PWM
> on
> > > some RGB LEDs, I can attest to the fact that 40Hz refresh is easily
> > > visible as flicker at very low duty cycles. You _will_ see the
> annoying
> > > flickering. I suggest using at least 80Hz for completely flicker
> free
> > > operation. Before you get all upset, I know it seems awfully high
> > > compared to what's acceptible for TV and movies, but I assure you
> that
> > > the flicker is much more visible on the LEDs. There is plenty of
> advice
> > > on the net about using refresh rates around 100Hz for this very
> reason.
> > > Just commenting for the benefit of the OP.
> > >
> > > Of course if the OP doesn't need low intensity levels, then a slower
> > > refresh may be fine.
> > >
> >
> > I was just thinking that a column multiplexing scheme as opposed to
> > a row multiplexing scheme would be more likely to show strobing
> > as one drove by. Is this indeed the case? (not that this is at all a
> > concern
> > to the OP and his 8x8 array)
>
> I don't know if that would make any difference. My experiments were
> with only one LED and flickering was clearly visible at >40Hz refresh
> combined with low duty cycles. [I was using a software based 8 bit PWM
> routine (~25mS period with ~100uS resolution)] When I upped the clock
> speed to 8Mhz so that the period was ~12.5mS with ~50uS resolution the
> flicker was not visible to me even with a 1/256 duty cycle. I was also
> surprised by the brightness of the LED at 1/256 duty cycle, it was
> clearly visible.
>
> Disclaimer: I know that a linear duty cycle is not the most appropriate
> method of PWM driving an LED, but it was easy to implement. With the
> ISR running every 50uS, you don't have allot of time to jack around.
> ;-)
>
> Using an 8Mhz internally clocked 16F88, I was able to achieve my goal of
> imperceptibly varying the colors of an RGB LED, in a rainbow like
> fashion, without generating shades of grey, or taking a week to cycle
> thru all 16 million combinations. No biggy, but it looks allot better
> than those cheesy things with the built in color sequencer. ;-) My 10
> month old daughter really likes the effect and IMO it's going to look
> really good on my scooter as full-color, adjustable accent lighting.
> 8-)
>

Remember that certain color lights are illegal on personal
vehicles...regulations
vary however.



.

Relevant Pages

  • Re: how to control LED array?
    ... >> I disagree on the refresh rate. ... I suggest using at least 80Hz for completely flicker ... >> the flicker is much more visible on the LEDs. ... flicker was not visible to me even with a 1/256 duty cycle. ...
    (sci.electronics.basics)
  • Re: Issues with LED grid driving
    ... I have seen 24-ch led drivers along ... The chips I've used do use PWM and other 'nice features.' ... not LEDs were on all the time or at full brightness, ... same current and same duty cycle. ...
    (sci.electronics.design)
  • Re: Issues with LED grid driving
    ... I have seen 24-ch led drivers along ... the maximum current value and the PWM was used to reduce the intensity ... not LEDs were on all the time or at full brightness, ... Most are 2-10 times the cost of the chip I gave below. ...
    (sci.electronics.design)
  • Re: PWM in a digital control loop
    ... like yielding what a sigma-delta would, however being forced to do it ... so I was thinking of using 16-bit PWM from a microcontroller. ... For example, if full power is at 50% duty cycle, and I want to ... PWM-powered things are generally quite linear. ...
    (sci.electronics.design)
  • Re: LED brightness control
    ... If you want to write just to the PWM of one particular LED then address it directly. ... If you read somewhere further down you see an example of using the 'auto-increment' function, which just traverses the registers you see listed. ... the chip needs series resistors for the LEDs which ain't so nice. ...
    (sci.electronics.design)