If you don't mind losing a little "headroom" on the pot and want the
follower to more accurately track the pot voltage then connect a resistor
from +supply to a diode anode and diode cathode to pot wiper. Follower base
goes to resistor/diode-anode  junction. The resistor needs to be
significantly higher than the pt value. The diode anode is 1 diode drop
above the pot wiper voltage compensating for the Vbe drop in the follower.
If available an opamp follower has much better performance.



      Russell McMahon
_____________________________

> Scott Dattalo wrote:
> >
> > On Fri, 19 Oct 2001, Thomas McGahee wrote:
> >
> > > You mentioned Alice's problem with the changing potentiometer
> > > impedance as the wiper is moved over it's range. You can
> > > easily eliminate this problem using a single NPN transistor
> > > as a voltage follower. Connect the collector to +5 or higher
> >
> > Now that's a good, simple idea! A simple $0.05 voltage buffer turns a
> > digital I/O into an analog I/O. I like it.
>
>
> But remember this requires the additional 0.7v
> drop between the input voltage and the sensed
> voltage. :o)
> -Roman
>
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