Build an oscillator using the sensor as the capacitance.
Use a PIC to measure the period. Convert period to Humidity using
math. For greater accuracy, measure multiple periods (say 10).

My CMETER PIC controlled capacitance meter is overkill for what
you need, but you might find some useful ideas contained therein
that you can use. CMETER will measure from .01 pf to over
16,000 microfarads and has auto-zeroing and the ability to
cancel out the effects of stray capacitance.

With a differential of 40 pf, CMETER will display 40.00

Fr. Thomas McGahee


----- Original Message -----
From: "Diego Sierra" <dsierra@TELEFONICA.NET>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Tuesday, April 02, 2002 5:07 PM
Subject: [EE]: How to measure low cap (120pF)


> Hi!
>
> I have a humidity sensor from Philips (H1
> ftp://ftp.iac.es/out/dsg/H1_Humidity.pdf), it's a device that changes
> its capacity value from 110pF to 140pF. I found its datasheet not on
> Philips web, where I cannot find either an application note about it. On
> its electrical characteristics is talks about a frequency range which
> have no meaning for me.
>
> How could be this device conected into a circuit?
>
> How to measure such a low capacity value?, with an oscilator (555)?, if
> the capacity is so low, could the parasitic capacities easily changes
> the measured value?
>
> Thanks in advance,
> Diego.
>
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>

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