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Using my Twine to tell me when to water a plant

This is a multipart question...

A.) If I stick the moisture sensor into the moist dirt in a potted plant, it reads "wet." I'm just curious how dry it will need to be for it to read "dry." This sensor seems very sensitive. Just holding it in my hand reads "wet." I'm guessing anything that conducts electricity is going to cause it to read "wet," right? So will dry dirt still read "wet" because it probably still conducts electricity a tiny bit? Or is that not how it works?

B.) Will leaving the sensor tip in damp soil for an extended period of time (days, months, decades) be bad for it? Do I need to worry about it corroding or anything like that?

Thanks in advance for any advice!
And thanks to Supermechanical guys for making this awesome gizmo.


  • 8 Comments sorted by Votes Date Added
  • Just wanted to update anyone who may be curious about this idea. My twine just texted me letting me know I needed to water my plant! It was very very dry at this point as I had not watered it since I started this thread (6 days ago). I wanted to see just how dry it needed to be for it to register as "dry." My plant looks fine, but this plant is very hard to kill (trust me I know). More sensitive plants would probably not like waiting so long for water.

    So yes, it does work.

    But I am still curious about whether or not I should be worried about corrosion, or rust, or anything like that. I'm thinking of attaching some leads to the sensor so that if it does cause corrosion, it will just be on some replaceable wires, and not the sensor itself. What do you guys think?
  • I'm thinking if you tape up a portion of the electrodes with electrical tape, you'd reduce the surface area exposed to the dirt, and thus the measured conductivity would decrease. Adjust to taste.
  • Update:
    my plant is dying and my twine still says its wet. :-(
    Looks like this sensor should probably not be used for plant monitoring.
  • Not sure if this would somehow cause problems with the data or sensing but what about extending the contact points with some plain copper wire or tape then securing it to the sensor via solder, conductive tape or clips.
  • Here's a suggestion - a two wire cable, with alligator clips on one end, and a couple of (preferably stainless steel) probes on the other end. Attach alligator clips to the moisture sensor electrodes, and insert the probes into your plant pot. Distance between the probes controls sensitivity to moisture, close together, high, far apart, low (some experimentation required to get to the right level for your plant).
    This idea also solves my problem of using my Twine on my Winter holiday problem; needing to alert someone in town while I am away somewhere where it is warm; if one or both of two things happen; the level in my sump rises beyond where the sump pump should kick in, and/or the temperature in my house somewhere near the thermostat location goes below some safe temperature, and there is perhaps 30 feet between the thermostat and the sump, and I have a 4 foot sensor cable.
    One more problem with this, however, if the sump pump failure and furnace failure are due to a long-time power failure, none of this will work. Which is why we need a "sign-of-life" trigger from the Twine server if our Twine has not reported in (because the WiFi has gone down) for some time, eg. at 1 hour, again at 2 hours, etc., which would tell someone that they need to go check for power at the house.
  • Because different soils will have different levels of conductivity, it's hard to definitively say what will register as "wet". Some of the ideas to adjust this through experimentation are worth trying.

    Don't worry about the prongs corroding - that's why we used gold. (Also makes a great pendant.)

    A "network up/down" virtual sensor would be a great addition. It's on the to-do list.
  • edited December 2012 Vote Up0Vote Down
    I assume it works by the water bridging the prongs, does the sensor hardware just say to Twine "yep, that's connected" or does it send accurate readings of voltage to the Twine which then converts to "Wet" and "Dry"? If the latter, a firmware update could add very useful functionality to the people (including me) in this thread!

    EDIT: Just added this as a feature request, As it would be useful in other situations too.
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