I'm sure we can drive a bi-colour led from one
pic pin without having to resort to transistors.
:o)

Did you want to use the 3-pin led or the 2-pin
(dual reversed) type?

If you're talking volume production I can see
advantages to using the 2-pin type, ie soldering,
board cost, footprint etc.

If you can spare some timeslice (2% ??) i'm sure
it can be done with an RC system of some type.
Is there a desired current limit for when the leds
are off?
-Roman


Lawrence Lile wrote:
>
> Roman,
>
> Josh and I are in the same boat, so to speak.  We are trying to drive the
> dual LED with just one pin, on the cheap.
>
> Most dual LED's come in a common cathode arrangement, and the only way I
> could figure out to drive both with one pin was to add a bunch of extra
> compnents, specifically an NPN and PNP.
>
> I had another arrangement for driving 8 common cathode LEDs with 9 pins, the
> one extra pin driving a PNP/NPN that turns on the green or the red
> respectively.
>
> THis is another one of those impossible projects where every dime counts,
> board space is limited, and if I were sane I'd just throw more pins and
> money at it.
>
> With the kind of volumes my company does, if I spend a month's pay figuring
> out how to save $0.10US in product cost, it is still worth it.
>
> --Lawrence
>
> ----- Original Message -----
> From: "Roman Black" <fastvid@EZY.NET.AU>
> To: <PICLIST@MITVMA.MIT.EDU>
> Sent: Saturday, February 02, 2002 5:53 PM
> Subject: Re: [PIC]: Driving a dual LED
>
> > Josh Koffman wrote:
> > >
> > > Hi all. I had an idea for driving a bi colour LED using only one pin. My
> > > basic idea is to get one switch and one bi colour LED (basically 2 LEDs
> > > back to back) to work over just 3 wires. Here is a pic of the remote end
> > > of this (drawn by hand as Roman has taken his page down):
> > >
> > >          (-|>-)bi colour LED   _=_ Switch
> > >       ,--(    )--,------------o   o--,
> > >       |  (-<|-)  |                   |
> > >       |          |                   |
> > >       |          |                   |
> > >       |          |                   |
> > >       O          O                   O
> > >      1          2                   3
> > >
> > > 1= PIC Output
> > > 2= +2.5 Volt
> > > 3= PIC Input
> > >
> > > Basically the idea is to float one end of the LED in the middle of the
> > > PIC output range so that when I output a low, one LED is forward biased,
> > > and when I output a high, the other LED is forward biased. If I want the
> > > LEDs off, I'd tristate the pin.
> > >
> > > So...will this work? I'd have to make sure the LEDs have a forward
> > > voltage of under 2.5V I guess. What will happen with the input pin? What
> > > will the PIC do with 2.5V on an input? Is that a high (because it is
> > > greater that .8V)? The other question is will this work over a long
> > > wire? Since the currents are so low, will line drop become a factor,
> > > making the voltage too low to forward bias the LEDs? I guess the other
> > > question is how to generate the 2.5V. Could I use a zener, or is there
> > > such a thing as a 2.5V zener?
> >
> > Is there a specific challenge? Like how to
> > drive the bicolour led

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