Aaron wrote...

>First thanks to all that helped so far. Especially Brian Taylor for that
>great idea.  I actually fabricated a little pressure transducer today on
>pcb worked great!! (approx 35pf 28-30pf on applied pressure)
>Used a cut up chip bag for the diaphram :P
>
>I was wondering though what's the best way to read the valuee of a
>capacitor with a pic? I know I could use a 555 and simply measure the
>freq, but I seem to remember a URL (no idea what it was) where someone
>used two pins on a pic to measure an rc time constant, one pin simply
>went high and the other waited for a 1 on another IO pin and then you
>knew how long it took to charge the cap to the IO's Vthresh, seems
>easiest to me.  Any ideas?

This technique works great for larger capacitance values, where
you can get a decent time delay with reasonable-size resistors
(i.e., not so big that pin leakage currents mess things up).

But for capacitances in the low tens of picofarads, the 555
oscillator approach is much better.

>Also I've discovered that they also do this commercially for pressure
>transducers, anyone know a rough cost?  (low pressure as before of
>course)  Can't seem to find pricing on capacitance transducers, only
>piezo.

I've never seen capacitance transducers sold alone, though I
admit I haven't looked very hard.  Some of the industrial
pressure transmitter manufacturers (Rosemount, Inc., for example)
use their own proprietary differential capacitance transducer in
their products; but these are specially built.

One of the problems with capacitance transducers is that if you
need high resolution, repeatability and stability, they require a
significant amount of analog circuitry to convert the capacitance
change into a DC signal that can be measured with an A/D
converter.  This tends to make them a challenge to use.

Dave D.

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