> If so, would appreciate any info you might want to share.  AADE has a
> nice one for about $100, but it does not seem to me that it would be
> all that difficult to build one.  Trouble is, my expertise is more in
> the area of programming and less in the area of electronics, so am
> having trouble visualizing the circuit.   An example to go by would be
> helpful -- just to stimulate thinking if nothing else.

A simple method would be to measure the rise time from 1/3 to 2/3 Vdd with
a known pullup.  Discharge the cap thru a FET and low value resistor for
long enough to ensure it is at 0V within the required accuracy.  Then turn
off the discharge FET and raise the other end of a pullup resistor tied to
the cap.  This can be a PIC pin with a 10Kohm resistor in series, for
example.  Two comparators produce digital signals when the capacitor
voltage crosses 1/3 and 2/3 Vdd.  The PIC measures the time between these
two signals, and the rest is a little math.  You could even have different
values of pullups to switch ranges to keep the time delta within a
reasonable range.


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