>
> >I am trying to design a PIC-programmable pwer supply. I'd like to select by
> >software any voltage between 1.5V and 15V. PCB space is VERY small.
>
> Hi Andres, what is the resolution required? 0.05V per step? 0.50V?
>
>
> >So I decided to use a Dallas DS1804 8-pin programmable potentiometer managed
> >by a PIC 12C509 or similar to drive a LM317L (100mA is enough). The problem
> >here is that the DS 1804 can't stand more than 5V on ANY pin, including the
> >potentiometer pins. So, I can't use it to directly drive the LM317L.... It
> >follows that I have to use a transistor or FET to convert levels. While I
> >have no problems with digital circuits, I have little experience with Analog
> >ones, so I don't exactly how to do this.

Circuit deleted for brevity.

One last thing. I use 7406/7407 Open collector hex inverters/buffers for
selecting the resistor for the LM317 circuit. Compact and can stand input
voltages up to 30V. Just a touch easier to deal with than raw transistors.

Be aware that a LM317 cannot do 0V because the output is always 1.25V above
the input. The first time I saw the circuit, in Steve Ciarcia's Intelligent
Serial EPROM programmer in Byte Magazine, he used a transistor with a negative
bias to pull the line to 0V. I've found for PICs MCLR/Vpp that it's sufficient
to limit current from the LM317 output pin through a 100 ohm resistor then
use another 7406/7 buffer to pull the output of the resistor to near 0V.

This is circuit I have on my current home grown 16C84 programmer. Works like
a champ.

BAJ