Monitor rate of change

Discussion in 'The Projects Forum' started by mmerritt, Oct 30, 2012.

  1. mmerritt

    Thread Starter New Member

    Oct 30, 2012
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    0
    Hey guys,

    This is my first time posting and I am definitely a newbie at electronics, but could really use some ideas or advice.
    I am trying to design a small circuit that will monitor a modulating input from a small humidity sensor (either 4-20ma or 0-10v) and energize a relay if the humidity rises 20% over a 1 or 2 minute period of time.

    Any suggestions would be very much appreciated.

    Thank you
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    So you don't really care about the level so much as detecting a rapid change?

    What sort of power supply do you have for running the circuit and providing power to the relay?

    There are several ways you might accomplish this. Some will suggest using a microprocessor, but this could be a challenge if you've never worked with one. Do you have any experience with programming such a thing?

    A more traditional approach would be to use a comparator. The reference voltage would be a slow-moving voltage and the test voltage would be the "live" sensor. If a large delta develops, the comparator trips a MOSFET which in turn sends power to your relay. I'm not 100% sure how to establish the slow-moving reference voltage but something with an RC tank might work.
     
  3. JohnInTX

    Moderator

    Jun 26, 2012
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    R12/C5 will adjust the reference voltage as it moves slowly. R10/R29 have a faster time constant. If the voltage rises faster than the reference can keep up, the comparator's output goes low. You would have to adjust the time constants and probably use a better comparator for 1-2 mins. Also, this was a shutdown circuit so no hysteresis is present. Probably want to add some..

    I use a PIC with ADC for this kind of thing now..
     
    Last edited: Oct 30, 2012
  4. wayneh

    Expert

    Sep 9, 2010
    12,156
    3,063
    Exactly what I was thinking of, thanks for posting it.
     
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