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Homework Help: Comparator op.amp circuit

  1. Sep 19, 2007 #1
    A botanical laboratory requires an op.amp. alarm circuit which will indicate when the soil in a plant pot is not moist enough. With the assistance of the in information below design such a circuit remembering to include suitable values for any resistors used.
    when two copper probe wires are inserted into the soil 15cm and the soil is wet, the resistancw between them is 1.3kohms. However as the soil dries out the resistance increases and reaches a value of 85kohms when the soil is too dry.

    All i know is to use a wheatstone bridge. from here i am lost can someone please help.
  2. jcsd
  3. Sep 19, 2007 #2


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    Staff: Mentor

    Hint1: Form a voltage divider with that soil resistance and some other fixed, appropriate value resistor. Then sense the mid-point voltage of this voltage divider, using an opamp in the comparator configuration (just guessing this is what you are supposed to do, based on your thread title).

    Do you know how to hook up an opamp to serve as a comparator? Hint2: positive feedback is the key (why?). Hint3, use google or wikipedia or your textbook if you don't know what a voltage divider or comparator configuration are.

    Then show your work here if you are still having problems, and well see if we can offer more hints.
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