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capacitance

 
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chris
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capacitance
PostPosted: Tue Apr 06, 2004 2:56 pm     Reply with quote
I am a new to using pics.

Does anyone know how I can detect a change
in a sensor from 112pF - 142pF to a PIC
I was thinking of using it in a clock circuit and using the CCP to
capture the frequency and convert.

THIS MEANS THE PIC WOULD HAVE TO RUN FASTER THAN THE CLOCK??

Any easier simpler way??

Thanko
SherpaDoug



Joined: 07 Sep 2003
Posts: 1640
Location: Cape Cod Mass USA

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Re: capacitance
PostPosted: Tue Apr 06, 2004 3:48 pm     Reply with quote
chris wrote:
I am a new to using pics.

Does anyone know how I can detect a change
in a sensor from 112pF - 142pF to a PIC
I was thinking of using it in a clock circuit and using the CCP to
capture the frequency and convert.

THIS MEANS THE PIC WOULD HAVE TO RUN FASTER THAN THE CLOCK??

Any easier simpler way??

Thanko


The simplest would probably be someting based on Microchip's App note AN512 where you measure the change in the capacitor instead of change in the resistor.
For more accuracy I would make an oscillator with the sensor and measure the frequency or the period. If the oscillator is fast measure the frequency. If that is too slow for the resolution you need then measure the oscillator period. If the oscillator is very fast you could add an external divider which cascades into the PIC counter.
If you want to get really fancy you could have a bridge type oscillator such that the bridge is balanced near 112pF and goes unbalanced as you increase capacitance. You would have to be a real analog guy to try that though (like me but with more time!).
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PostPosted: Tue Apr 06, 2004 4:59 pm     Reply with quote
The best way I know of is to use the sensor in a 555 oscillator circuit then rectify the oscillator output which will give you a voltage proportional to the frequency. Thats what I do for my capacitive humidity sensors..
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