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  • How to convert change in capacitance into a digital signal

    adi21288

    Member

    Updated: Oct 26, 2024
    Views: 1.3K
    Hi CEans, i needed a small help regarding my electronics project

    how can i convert a change in capacitance into a digital signal.. or even a change in voltage or current which can be later fed to an ADC

    my capcitance change is expected to be in nf or even pf.
    i know of two methods but i am not pretty sure

    Method 1)
    Wheatstone bridge :- what values of the capacitance change can this measure. i am not sure if such low capacitance changes will be detected, and if they will be what should be the other capacitor values and the excitation frequency.

    Method 2)
    Oscillator :- this method does work for pf changes in capacitance but i do not know how to design this and also how would i change the change in frequency obtained at the oscillator output to a change in digital signal

    kindly help guys!!!!
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Replies

  • shonali55

    Member • Jan 20, 2010

    Hey! you seems to be very confused and making also,capacitor... is itself used to store the charge, it basically block DC and by pass AC. And you are using ADC...I hope you know the output.. that's a digital signal(square wave).
    As resistor capacitors are also fixed and variable....fixed capacitors are mostly used in robotics.
    Variable capacitance used in analog communication.

    Can you ever be able to change a resistance into voltage..than how you are going to change a capacitance into digital signal.This one is interesting, please it may be that i am not getting your point....Let me know and make me under stand.

    Oscillators are used to produce variable frequency(Such as in case of modulation).
    Take the example of a radio circuit....

    If you are really in need than go for the books

    Electronics Instruments and Instrumentation Technology – by Anand , PHI..
    Modern Electronics Instrumentation & Measurement Techniques , by Albert D. Helstrick and William D. Cooper. Pearson Education.

    Advance analog electronic circuit book
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  • adi21288

    Member • Jan 20, 2010

    hey shonali,

    u dont seem to have got my doubt

    i am making a capacitor which acts as a weight (pressure to be precise) sensor and i want to feed this output to a mu p

    its like a strain gauge changes resistance when under stress.. this resistance change can be converted into voltage change by using a wheat stone bridge or a simple voltage divider circuit.. understood
    in my case the pressure changes the distance of separation and hence the capacitance
    i know what oscillators are. i just dont know how to convert that varying frequency into a digital signal
    i hope u got my point
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  • reachrkata

    Member • Jan 20, 2010

    I got this nice idea from a colleague of mine, so quoting him here.

    Use the capacitor as part of an RC Low pass filter circuit. Connect the R to one of your microprocessor pins and feed a high signal.

    Then measure the voltage at the C using an ADC and calculate the time needed to reach some specific voltage. Basically implying rise time measurement.

    When the capacitance changes, the rise time also changes proportionally.

    -Karthik
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  • shonali55

    Member • Jan 21, 2010

    adi21288
    hey shonali,

    u dont seem to have got my doubt

    i am making a capacitor which acts as a weight (pressure to be precise) sensor and i want to feed this output to a mu p

    its like a strain gauge changes resistance when under stress.. this resistance change can be converted into voltage change by using a wheat stone bridge or a simple voltage divider circuit.. understood
    in my case the pressure changes the distance of separation and hence the capacitance
    i know what oscillators are. i just dont know how to convert that varying frequency into a digital signal
    i hope u got my point
    Hello, Adi
    Ok, Know I got your Point you want to build a pressure sensor....Using capacitive whistone bridge by using voltage divider circuit, by using OPamp (MCP6XXX), now you you will get a analog out put, to convert it into digital signal use microcontroller integrated with analog to digital converter (MSC12xx and ADS125x), which is further connected to a multiplexure, providing a single digital output.
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  • piyushh

    Member • Jan 22, 2010

    well..i dun knoo m rite or wrong..for this stupid(may be) answer..but if u just want to only consider .using capacitance..then u may use envelope detector circuit followed.by subtractor and adc...but for that u need to first give the dc shift to your sgnal....one more idea in turms of capacitor..u can use the concept of "methord of variation of parameters" in detection of amplitude modulated signals..in which ..they use a "varicap" which varies its capacitance value in acordance with voltage..i mean the carrier voltage
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  • adi21288

    Member • Jan 23, 2010

    @karthik
    dude i dint understand your concept, i mean u r feeding a dc voltage to a low pass filter (high voltage frm the mu p rite? ). how will it act??

    @shonali
    i knew a wheatstone bridge can be used, but my doubt is that my capacitance is in nf and the change in capacitance is in pf.. will a wheatstone bridge work at these values. and if yes then what shud the excitation frequency be?
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  • reachrkata

    Member • Jan 24, 2010

    adi21288
    @karthik
    dude i dint understand your concept, i mean u r feeding a dc voltage to a low pass filter (high voltage frm the mu p rite? ). how will it act??
    This is like seeing the Step response of an RC circuit. When you apply a step voltage to a RC, the Capacitor voltage charges towards the Step with a time proportional to the R & C. We use this to measure the capacitance value.

    -Karthik
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  • adi21288

    Member • Jan 24, 2010

    @karthik

    will it work for a pF change in capacitance?
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  • reachrkata

    Member • Jan 26, 2010

    I think so yes !😕

    -Karthik
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