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newb: I've got my 1st circuit working, but it can be better..

Discussion in 'Electronic Basics' started by SklettTheNewb, Feb 17, 2005.

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  1. OK, I made a simple and far-from-perfect moisture sensing circuit,
    basically I've broken the base feed to my transistor into two leads
    that I stick on the dirt of a potted plant, the wetter the dirt, the
    brighter my LED is.

    I'm learning.

    I would now like to modify the circuit so that the LED is either ON or
    OFF, but I'm not sure how to accomplish this... I need to fully
    saturate a transistor I'm thinking, but don't know how to do this
    correctly.

    I've been reading up on http://www.AllAboutCircuits.com as well as
    http://www.kpsec.freeuk.com/index.htm but haven't found the answer yet.

    If anyone has a couple seconds and could give me some suggestions or
    maybe a link, that would be very much appreciated.

    Thanks!
    Steve
     
  2. If your intention is to get something working easily, a
    comparator with open collector output would be your
    best bet for this. Just setup a pair of voltage dividers
    on the input that set the trip level at the conductivity
    that you want the LED to be barely on or barely off.
     
  3. Thanks for the reply Larry!

    Uhmm... I don't follow you too well on your response, I'm sure it's
    simple to most everyone here, but it lost me a bit.

    I found this site: http://home.cogeco.ca/~rpaisley4/Comparators.html
    and will be reading it to try and grasp this whole thing.

    Thanks again!
     
  4. That looks like a good introduction to comparitors.
    The circuits shown in the 3rd schematic block from
    the top closely resemble what you will need. With
    your LED in series with RL and your soil probe
    substituted for R1 or R2, it's the right topology.

    The important thing is to understand what the
    comparitor actually does. Then, seeing how to use
    it for your application will be easier.
    You're welcome.
     
  5. John Fields

    John Fields Guest

     
  6. Study that until you understand, its a very commonly used device and you
    will be able to design many circuits around it.
    Lots of luck with your new interest !!
     
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