# Design a digital capacitance meter

Discussion in 'Homework Help' started by mwobobia, Jul 6, 2010.

1. ### mwobobia Thread Starter New Member

Jan 5, 2010
1
0
parameters
capacity range is between 100pf and 100uf
at least to measuring range
the result to be displayed with at least 4-bits LED

Apr 5, 2008
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3. ### beenthere Retired Moderator

Apr 20, 2004
15,815
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What is your idea about how to make the meter sense capacitance?

4. ### debjit625 Well-Known Member

Apr 17, 2010
790
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How? let the result be displayed with binary number system,then with 4 bits (the max num you can display is 15 and min number you can display is 0 ,its far from your range 100pf to 100mf ,I think thet in other number system or in any other symbolic system this cant be displayed.

An idea
Ok how these are measured in simple way first charge the unknown valued capacitor now with a known valued resistor discharge it.What values you will get? the time taken to discharge the capacitor and the value of the known resistor ,using RC formula you can find out the value of the capacitor.
You can also do it another way ,using a known resistor charge the unknown capacitor and check the time it needs to be fully charged then using the same RC formula you can get the value of your capacitor.

Good Luck

5. ### tom66 Senior Member

May 9, 2009
2,613
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With 4 bits binary output, you have only 16 steps, so you would have an effective resolution in capacitance of ~6.24 µF per step, no way you would get any result for pF.

Unless you mean a 4 bit LCD display, like a HD44780 based display...

Or unless you were considering autoranging, though still 4 bits would not be much.

Consider using the LM2907, a frequency to voltage converter, there is an application diagram on using it for capacitance measurement. This chip outputs an analog output voltage depending on the input frequency. Measuring capacitance only requires an adjustable oscillator then (so you can accommodate a range of capacitances.)

6. ### Wendy Moderator

Mar 24, 2008
21,438
2,958
A common technique is a constant current source and a timer. In many ways a 555 does this (there are 555 circuits that have constant current sources), from there it is a simple frequency counter.

I point to the 555 as an example, it is a very simple RC oscillator. There are many examples of this type of oscillators, all of them would work well in this application.