Monitor and detect small voltage change

Discussion in 'The Projects Forum' started by pilko, Jan 6, 2009.

  1. pilko

    Thread Starter Active Member

    Dec 8, 2008
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    I am trying to detect when a voltage increase of around of 100 mV per min reduces to less than 5 mV per min.I have tried a differentiater without success. I think I need to sample the voltage maybe every 5 min,save the value and compare it with the next sample.Could you help me? My digital knowledge is limited to combinational.The circuit is operating around the 6 volt range.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    Perhaps you could expand a bit on the voltages? What causes the different rates of change, and what kind of electronics gets the signals?
     
  3. pilko

    Thread Starter Active Member

    Dec 8, 2008
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    The voltage is at the junction of a 100K thermister and a 100K resistor,forming a potential devider accross a 12V regulated supply.As the temperature increases,the voltage falls at the junction.I need to operate a relay when the rate of temperature increase reduces to 1 degree C per hour or in other words when the rate of voltage decrease falls to about 5 mV per minute.
     
  4. mik3

    Senior Member

    Feb 4, 2008
    4,846
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    Use a microcontroller to read the voltage and to process it to measure the rate of change.
     
  5. Alberto

    Active Member

    Nov 7, 2008
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    It is not clear if you need to plot this rate of change or you need simply one value over and one below a fixed point.

    Alberto
     
  6. pilko

    Thread Starter Active Member

    Dec 8, 2008
    213
    20
    I need to monitor the rate of change and operate a relay when the rate of increase falls below 5 mV per minute.I think I may have to sample the voltage every 10 minutes and compare it with the previous sample.If the new sample is less than 50 mV greater than the previous sample,the relay operates.
    mic3 suggested using a microcontroller, and I appreciate his suggestion,but my knowledge is limited to non-processor chips and discreets.
     
  7. Alberto

    Active Member

    Nov 7, 2008
    169
    36
    Pilko, since you have problem with microprocessor the solution is somehow more complicated but not impossible. See my block diagram for a start.

    Timer 1 load the analog value to a sample & hold. Timer 2 one minute later will compare sample and hold with analog in and logic will let comparator value to pass to the up & down counter in such a way that if value of analog in is higher than sample & hold, count will increment on the contrary will decrement if equal nothing happen. If counter you choose is decimal, then as soon the carry will go high it will mean that ten reading have yield values higher then the prior one, hence a positive slope. On the contrary if the borrow will go high, you are measuring a negative slope.

    Alberto
     
    Last edited: Jan 7, 2009
  8. pilko

    Thread Starter Active Member

    Dec 8, 2008
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    Great salution---Really appreciate the help from you guys.---Also great to find out there is such a thing as a Sample and Hold chip.
     
  9. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    I would suggest to use an differential amplifier to remove the bulk of voltage that is of no interest to your measurement and focus on the changes.

    Then connects this conditioned signal to the MCU ADC input. If you can limit your input range of voltage changes to about 2V, then an usual MCU 10-bit ADC will resolve down to 2mV.

    @Alberto,

    A one minute sample and hold is not an easy circuit to build.
     
  10. Alberto

    Active Member

    Nov 7, 2008
    169
    36
    LChang, with the last generation of no leakage capacitor this is no more an issue. But in case it is, then you could add a counter and reduce the sampling time to few seconds.

    Of course using a micro with adc you will be a more elegant and cheap solution, relegating all the timing problem to software, but if you read post # 6 this is not possible.

    Have a nice day, I am under half a meter of snow!

    Alberto

    Edit:

    Another suggestion even simpler:

    Use a voltage to frequency converter. with a timer feed at regular interval (few milliseconds) the counter once up counting and once down counting. If no change in the analog in then the count will not increase. If analog will increase count will increase and the circuit will detect the derivative of the function triggering the carry or the borrow.
    See block diagramm.

    Alberto
     
    Last edited: Jan 7, 2009
  11. Alberto

    Active Member

    Nov 7, 2008
    169
    36
    Another Idea to investigate.

    Using a voltage to frequency converter. sample the frequency at regular interval (to be choosen) and feed clock out to a series of up/down counter.

    If properly tuned a positive slope in the analog in will trigger the carry of the last counter while a negative slope will triger the borrow. Binary counter can be used to have a higher resolution.

    Alberto
     
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