Soil Moisture Sensor Circuit Explained

Update 2011-09-05: I have created a new blog about electronics only – check it out here: Electronic measurements, NSLU2 and soil moisture sensor

The 1-wire soil moisture sensor circuit I use in my garden is designed by Eric Vickery from http://www.hobby-boards.com.

(c) 2006 Hobby Boards  Designed by Eric Vickery
Title: Moisture Meter 3
Rev: 3
Date: 10/27/2006 06:11:30p

(Click the picture for a larger version.)

Here’s a bit of explanation on how the humidity sensor circuit works.

The circuit is connected like this:

and the circuit works like this:

  1. Watermark sensor gets wet
  2. The time constant of oscillator IC 555 changes
  3. 555 IC oscillates faster and its supply current goes up
  4. Current in resistor R2 goes up causing the voltage across R2 to go up too
  5. The value of the current register of IC DS2760 changes
  6. Current register of IC DS2760 is read via 1-wire network

In my case I got the following values during calibration. I let the Watermark sensor dry in the wind and recorded the current register value, and afterwards the sensor was soaked in a bucket of water:

Dry = -0.2368
Wet = -1.400

These numbers form the 0 and 100 % limits of my soil moisture readings.

17 comments on “Soil Moisture Sensor Circuit Explained

  1. -

    Quick question. What type of data is provided for the output? Is it serial or a square wave? Please let me know, I want to send the output to a transducer or a pizzo for a simple alarm when the saturation point is reached. Thank you for your time and consideration in reviewing my request.

    Sincerely,

    Dennis Hall
    technician1965@hotmail.com

  2. -

    @Dennis: As I see it the “output” of the circuit is a software register.
    I think what you need is an OpAmp Schmitt-trigger measuring the voltage across R2, and removing U1, R1, C3 and D1, but I’m not sure if that’s what you want?…

  3. -

    Hi, I want to do this circuit, but in dallas website, the ds2760 is no loger available, I could change the ds2760 to ds2762? thanks, best regards

  4. -

    @Braulio: I think you can use DS2762 instead as the manufacturer Maxim states, “DS2762 is an update to DS2760”.

  5. -

    I require to measure water content at root zone of plants. What material shall be used as a probe. whether still wires will work.

  6. -

    I want measure soil moisture n give its output to controller.I had made one circuit but it is giving me output even at low humidity.I want to compare present moisture value with threshold value.how can I do that?.thank you for giving your time and consideration.

    sincerely,

    Riteeka

  7. -

    @Riteeka: What type of controller do you want to use, and what does your circuit look like? Do you have a schematic? What sensor are you using?

  8. -

    hi Tom,its Mpho in Botswana,one country in southern Africa..i need some assistance from you,i want to design a circuit which control the moisture content,pliz send me some possible circuit diagrams

  9. -

    @Mpho: What device / component will you be using to control the water flow? Some kind of pump or valve? I plan on using an electric drill pump (Gardena, 1,700 l/h @ Ø 13 mm hose, 3 bar) connected to a sprinkler or soaker hose depending on the crop (some crops are sensitive to water on their leaves due to blight.)

  10. -

    sir..how i want to control the soil moisture sensor?can i use the microcontroller?can u recommended what type of microcontroller?my project is about solar system for irrigation that using soil moisture sensor..

  11. -

    sir, can relay switch and timer can be added to this circuit?

  12. -

    Sir I want to analyze the content of soil. (Phosphorous, Calcium etc) so can you suggest any kind of sensors for the same? If I could make some on my own would be great for me to learn. Thanks.

  13. -

    hello,can I have the email address of Thomas W I need help with my soil moisture design.
    thank you

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