At 15:16 22/12/95 -0600, you wrote:
>>I am looking for a way to use the pic to control a resistor in a circuit
>>(to use as a microcontroller controlled variable power supply).  Would
>>anyone have any ideas how?  Thanks in advance.
>>
>You could pulse-width modulate an RC integrator to develop the reference
>voltage for a regulator, use a CMOS analog switch pulse modulated with an
>integrateing capacitor to form a variable voltage divider, an R-2R network
>to source current to a current-mirror controlled regulator or pulse modulate
>a MOSFet and filter its output.

...and then we'll just reverse the polarity of the neutron flow and connect
the input of the high-frequency tachyon generator to the subatomic particle
spin mirror and..blam! Antimatter!

On a more serious note, you can, provided the voltages around the rest of
the circuit can be made to fit, use a resistor and a CMOS switch in series.
Turn the switch on/off with a variable duty cycle at a sufficiently high
frequency, and the resistor value will appear to change from R (with the
switch on all the time) to infinity, with it off all the time.  I've used
this technique for digitally controlled pots in a guitar amplifier, where
it worked a treat with the switches going at 40kHz or so.  A 'sufficiently
high frequency' is one where the rest of the circuitry doesn't notice
what you're doing too much.

Dave
-----------------
David Knell M.A.
Tel: 01843 846558
Fax: 01843 846608
E-mail: dave@dave-ltd.co.uk