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Moisture Sensor

Discussion in 'Electronic Basics' started by Sparks, Jul 19, 2004.

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  1. Sparks

    Sparks Guest

    I am in the middle of installing a whole garden irrigation system.

    I want to build a controller for this, but need some kind of moisture sensor

    It will be interfaced to a PIC with a simple high/low input

    Any ideas on where I can either buy some, or plans on building them?

    Sparks...
     
  2. You might start by reviewing how other people do it:
    http://www.microirrigationforum.com/new/sensors/
    http://www.ext.vt.edu/pubs/rowcrop/442-024/442-024.html
    http://www.rainbird.com/landscape/products/controllers/moisturesensor.htm
    http://www.geneq.com/catalog/en/ismsdm.htm
     
  3. andy

    andy Guest

    i've been trying to do something similar - i had a go a while back making
    a soil moisture sensor using a pair of electrodes cast in a block of
    plaster of paris, and measuring the resistance between. (the plaster works
    as a buffer to stop soil salinity from affecting the resistance)

    This didn't work so well - i was using galvanised wire, and i think it
    corroded slightly so that there was a battery type circuit, and i was
    getting a voltage off the sensor even when it was disconnected. The other
    problem was there was a long time delay between the sensor going in water
    and the reading stabilising.

    But this is one of the well known ways of measuring soil moisture, so you
    could try the same approach but with quite a small block of plaster
    (maybe 2-3 cm on each side?), and some electrodes that aren't going to
    corrode or react with the ions in the plaster.

    The other way i thought of is to put some glass fibre tape through the
    middle of a thin plastic pipe, bury one exposed end, and then work out
    moisture by how far the water wicks up the tape to the other end of the
    pipe.

    Another possible approach would be to bury an unglazed ceramic pot in the
    soil, arranged so that it is just filled whenever you water, and whenever
    it rains. Then whenever the water in the pot has completely drained into
    the soil, it's time for another watering.

    for the moment i'm just sticking with a simple daily watering system with
    a fixed amount of water.
     
  4. You could use pencil mines (graphit and clay) as electrodes, as they are
    quite inert. The other problem to consider is polarisation of electrodes
    (i.e. the formation of insulating gas bubbles due to hydrolysis of
    water) if DC is used between them. Use AC instead. Make the pair of
    electrodes part of a Wheatstone bridge, with an OpAmp to pick up the
    signal.
     
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