I think you could do it over 2 wires, with nothing
needed in the remote but the led and switch.
With 2 PIC pins? Is that what you're after? Or must
it be 3 wires?
-Roman

Josh Koffman wrote:
>
> Hi Roman. Actually, it's to get two colours and one switch on only three
> wires. My goal is to remote mount these, and three wires is all I'd like
> to use. As well, I'd like to keep the remote unit as low parts as
> possible as I'd like to try and squeeze it in the shell of a connector.
> Looking over some of the other posts, I could use three pins to
> accomplish this, but I wonder if I could do it with two (two pins, three
> wires). The addition of a third colour would be cool, but not really
> needed. I'd like to keep it to two pins if possible, but it'll be
> cheaper and easier to go to a larger PIC than to add more wires. Of
> course, if there is no way to get this to work over a bunch of wire
> other than to start to add gates to the remote ends, then I have a
> problem.
>
> Josh
>
> --
> A common mistake that people make when trying to design something
> completely foolproof is to underestimate the ingenuity of complete
> fools.
>         -Douglas Adams
>
> Roman Black wrote:
> >
> > 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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