Weather station soil probe
Weather station soil probe
Weather station soil probe

Measuring the temperature and humidity or moisture content of the soil is really useful when deciding whether the ground needs watering or a particular crop can be sown or not. Here, I managed to improve one of the auxiliary sensors from a Sinus TFA weather station to work in the soil.

Difficulty: . . . . . . . . . . Cost: . . . . . . . . . . Satisfaction: . . . . . . . . . . Hazards: . . . . . . . . . . Poisonous lead fumes when soldering

Carrots are particularly temperature sensitive and when I was a youngster, my dad used to stick his finger in the ground and announce:
"Son, it's time to plant the carrots".
At that time I thought it was all some kind of random disconnected magical event that people do every now and again and it took me several years to work out that the act of putting one's finger in the soil and planting carrots were in fact seemingly connected. My dad did not explain that part of it to me!
Now that I am old and wise I could pretend that my finger could tell when it was time to plant carrots or I could get a high tech digital device with a fail proof mathematical carrot planting algorithm bolted on. Obviously, the starting point would be to monitor the temperature and moisture content of the soil, so I looked up such devices on the interweb and found to my horror that they were so expensive that they did not even display the prices! Sod that, I said to myself, I must be able to hack into one of those cheapish weather stations and do it from there, and, additionally, I would be able to monitor the wind speed and rainfall.
Having done a fair amount of research, I eventually decided on the Sinus TFA 35.1095 weather stationas it was the only affordable device with 'add on' remote sensors available. I guessed that I would probably be able to modify these external plastic units by adding a stainless steel probe to go into the soil itself. Fingers crossed!

Step 1: Tools and Parts required

Weather station soil probe
  • Sinus TFA 35.1095 weather station
  • Dremel multitool or equivalent
  • Small soldering iron and solder
  • Jeweler's cross point screw drivers
  • magnifying glass and clamps
  • Epoxy resin and hardener
  • Stainless steel tube, outside diameter 20 mm, 100 mm long.
  • Old USB cable
  • Wine bottle corks x 2 of
  • Rubber bands, size 63, and plastic bag

    Step 2: Unscrew the temperature/humidity circuit board

    Weather station soil probe
    Weather station soil probe

    These external units are really well designed and have extra space at the bottom as if they were designed to be improved in the future. The little gadget in the foreground is the temperature and humidity sensor combined on one small circuit board - perfect - it can be disconnected from the main board and reconnected with a much longer cable.

    Step 3: Mill a slot in the inner plastic casing

    Weather station soil probe
    Weather station soil probe

    Use a dremel with a milling attachment to create a slot just bigger than the USB cable to be used. This is for the USB cable routing through the plastic housing of the remote sensor.

    Step 4: Solder in the USB cable

    Weather station soil probe
    Weather station soil probe
    Weather station soil probe

    I decided to have a go at soldering the USB cable directly on the main board, but I must say it was slightly tricky and just cutting the wires would have been easier, if not so professional. But I needed to feel like I was doing a professional job even though nobody would ever see my handicraft. The colours on my USB cable matched the colours used in the sensor, which made it all a bit easier.

    Now this is the time to test the wiring. Screw everything back together again and replace the batteries - BINGO - it still works!

    I have to say that at this stage I was feeling extraordinarily pleased with myself. There was some weird, primeval satisfaction about being able to out do my father with my intricate soldering techniques and clever knowledge of
    electronics. The more the project continued, the more I looked forward to presenting it to my dad and saying:
    "Hey dad, what are you going to stick your finger in now then?" Or
    "Dad, you don't have to stick your finger in the soil anymore, it's all electronic these days you know!"

    Step 5: Routing the USB cable

    Weather station soil probe
    Weather station soil probe
    Weather station soil probe
    Weather station soil probe
    Weather station soil probe

    The first photo shows how the USB cable is routed within the sensor unit. The next one shows the cable, soldered onto the other smaller board, going through a piece of cork stuck in the end of the stainless tube. The next thing to do it turn the tube upright and very carefully pour some resin + hardener solution into the bottom so that it acts as a strain relief device on the cable and seals it from any water. The other end of the tube is plugged with another cork which will allow water vapour to be sensed within the soil but stop liquid water from passing through. Clever eh?

    Whilst we've got the resin solution going, we can pour a bit into the bottom of the sensor unit to act as cable strain relief there as well. Just tilt the plastic unit to one side a little and prop it up for 10 minutes or so.

    As soon as the resin is hard the plastic unit can be screwed back together again.

    Step 6: Siting the probe

    Weather station soil probe

    Weather station soil probe
    Plant your probe in the shade.

    At first I dug a hole out in the field as shown in the photo and mounted the plastic unit on a stick with a rubber band. After a few days, I had a better idea and moved it to a position in full shade as I noticed that the sun had a big impact on the soil temperature, even when the probe was buried 200 mm down. This time I dug down to 200 mm in a full shade location where it never gets direct sunlight and noticed that it was colder and much less variation through day and night.

    Step 7: Carrotid Illumination

    Weather station soil probe

    Finally I got to demonstrate the finished system to my dad, but his response was not quite as I anticipated. The conversation went something like this:

    "Hey dad, have a look at this - no need to put your finger in the soil anymore!"
    "Err, yes that's fantastic, but I never put my finger in the ground to measure temperature or anything. I did it only cos my father did it himself."

    "I really don't understand", I replied "So why did you do it then?"

    "It's just something that reminds me of my own dad and kind of stuck with me".

    Well that stopped me dead in my tracks, like I had been run over by a Massey fergussen 185 tractor. Never before had I felt so stupid and so humbled at the same time. There I was trying to 'out do' my father when all along he never even knew when to plant carrots with his finger. There was, however, one last question:

    "So dad, just to clarify the situation, how did you tell when it was time to plant carrots?"

    "Oh, that was really simple, they always mentioned when the time was right on the farming program on the radio!"

    "Well they don't do that any more ..... do they?"

    "No cos one year they got it wrong and all the farmers sued them. Now they have to pay an expensive consultant a lot of money to do it all for you, they're called agrononomists or something. They'll even work out your European subsidies too."

    "That's just totally crazy!" I replied, rather relieved that my hard work might have some benefit after all.

    "Well it's a crazy world son .... And good luck with the carrots!"

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