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 in all 3 colours using
> nothing more than one PIC pin? With variable
> brightness on both the red and green it would
> give a vary-colour display, cool effect for
> a "limiting" light on an amp or a one-led
> audio level meter.
>
> Or is it to get 2 colours and a switch all
> on one PIC pin?
> -Roman
>
> --
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>

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