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Subject: Re: LED flickering problem
Date: Wed, 25 Dec 2002 03:12:20 +0800
Organization: Another Netscape Collabra Server User
X-Newsreader: Microsoft Outlook Express 6.00.2720.3000
the design is as follows, given each group of leds has a common anode
connected to transistor's collector
i use a logic low to turn on a PNP 4403 transistor(uP I/O pin <-> resistor
<-> 4403 base) so that the anode is at 5V
i also use ULN2804A to connect to the cathodes one by one to control which
LED to be on.
a problem is if i give a logic high to 4403, there is still leaky current
that the common anode is not at 0V as i expect
how can i solve it??
"John Jardine" ¼¶¼g©ó¶l¥ó·s»D
> Spehro Pefhany wrote in message
> > On Mon, 23 Dec 2002 14:33:19 -0000, the renowned "John Jardine"
> > wrote:
> > >Unless you have an incredibly high visual response speed, then a
> > >rate of even 20Hz would be sufficient.
> > NO.. 20Hz is WAY too slow. Try it (with a numeric display) and see.
> > But 80Hz is acceptably fast, with double that being pretty darn good.
> > >Sounds like the LEDs when in the 'on state, are not being given enough
> > >current to produce the intense, short, bright, flash needed to give the
> > >human eye a perception of continuity.
> > Drive current doesn't matter other than to establish brightness level,
> > there is no effect that is as you describe. The only flicker effect in
> > the human eye that I'm aware of is the better perception of flicker
> > in the periperal vision. Not a factor here.
> > >I.e. As you say, when static, the current is as it should be. But ...
> > >being scanned, the current needs to be a lot higher to give more light
> > >to balance the shorter time they are 'on'. Essentially the current
> > >high enough to burn out the LEDs in the static unscanned condition.
> > It shouldn't be that high with conservative design practices- under
> > typical room-temperature conditions.
> > Best regards,
> > Spehro Pefhany
> > --
> > "it's the network..." "The Journey is the
> > email@example.com Info for manufacturers:
> > Embedded software/hardware/analog Info for designers:
> Yes, 20Hz is *very* slow but look at the early calculators where they used
> every trick in the book to get the power consumption down. A lot of them
> visibly flickered even when viewed straight on and these operated in the
> 20Hz area. The interupt servicing is a good idea but what drove me was
> "I am sure this is not insufficient current problem coz if it is static,
> LEDs are fine".
> I would have expected him to mention a very high static brightness as if
> was a problem but he didn't, which is why I pointed up the 'burn out'
> I reckon Timptt is seeing "normal" brighness under static conditions.
> Hopefully he may report back with his solution but from past observation
> say we're all plaiting fog ;-)
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