please help me to design a digital capacitance meter
parameters
capacity range is between 100pf and 100uf
at least to measuring range
the result to be displayed with at least 4-bits LED

 

Hello,

What you just did is hijacking an exisiting thread:
http://forum.allaboutcircuits.com/showthread.php?t=40149

Now you have a thread of your own with all attention for it.

Bertus

 

What is your idea about how to make the meter sense capacitance?

 

the result to be displayed with at least 4-bits LED

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

 

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.)

 

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.