> I have used this approach for years.  I have never become aware of a
> failure.  It is a very reliable circuit, providing the parameters are
> properly determined.

I assume you mean his two resistor circuit.
But, unless I am missing some fundamental principle completely, it doesn't
work in the example he gave!
At 14 volts, either -

- The LED gets minimal current and doesn't light in a worthwhile manner or
- The LED always receives enough current to turn on enough to glimmer or
worse or
- The PIC pin sees more than Vdd when turned off.

I would genuinely welcome an explanation and/or analysis that shows how such
a circuit can sesnibly achieve the objective.


        Russell McMahon


> > > I noticed the following circuit in a friend design of a very cost
> > > sensitive product and am curious what the people think about it.
> >
> > >   [14V]----[A LED C]----[R1]---(A)----[R2]----[GND]
> >
> > > Where R1 is the current limiting resistor for the LED, point A is
> > > connected to a PIC digital output and R2 is a relativly large resistor
> > > (about 50K or so). The idea is that to turn the LED off, the PIC
output
> > > goes to HIGH or TRI_STATE and because of the 'knee' of the LED curve
and
> > > the resistor R2 that draws a minimal forward current, the voltage at
the
> > > PIC will not exceed its VDD.
> >

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