Soil moisture sensor

!!This article is out of date. Use that Analog sensor board or at least that one A2Dto be able to connect analog sensors!!

Here my solution for soil moisture sensor and OpenSprinkler 3.0AC/3.2AC/OSPI. At the moment I don't see a solution for the DC versions, unless you use your own power supply for the threshold switch.

Note: For OSPI if there is an alternative solution via the ​PCF8591T 8-bit ​ADC converter ​will come soon.

So you need: humidity sensor, threshold switch, Power Supplies, OpenSprinkler 3.0/3.2, Housing, Cable

  • The threshold switch is supplied with 24VAC power via the power pack (A1+A2)
  • The humidity sensor is powered by the OpenSprinkler 5V VIN output powered (VIN+GND to humidity sensor)
  • The signal cable of the humidity sensor (yellow) comes to the threshold switch input.
  • Switching takes place on 11/12/14: if the voltage at YR of the humidity switch is above the set value, 11-14 is switched, otherwise 11-12.
  • 12 is GND and 14 is VIN. 11 is SN1
  • Now use a screwdriver to turn the threshold switch Hys% to 5 (to the left until stop) and VDC to 1 (to the left until stop). Now adjust the humidity circuit from here.
  • The LED on the threshold switch comes on when moisture is detected and goes out when dry. In the OpenSprinkler-App then appears “Rain detected” and running programs (except those with “Ignore rain”) are interrupted.

Wiring Diagram:

OpenSprinkler-Humidity sensor switch plan


Related Links:

Connection of a radio sensor:

Information about the ​PCF8591T 8-bit ​ADC converter des OSPI:

General functioning of the capacitive sensor:

Edit 23.01.2021:

With the A2D adapter, a threshold switch is no longer necessary. The value can be set directly with the potentiometer.

Edit 05.06.2019:

Looks quite good, the lowest setting of the threshold switch fits quite well. However, we still have to make some adjustments to the software:

  • OpenSprinkler 3.0/3.2 actually only has one connection for a rain sensor. This is intended for when it rains, then no irrigation or cancellation of an ongoing irrigation.
  • However, a moisture sensor triggers while you are watering. This leads to the interruption of the irrigation because the sensor just got wet. But what if it started raining at the same time? After all, the sensor cannot know whether the moisture is rain or watering!
  • Likewise, the weather report is trusted more than the humidity sensor. If the weather report says it has rained, but the moisture sensor says everything is dry, there will be no watering. But the solution is simple, there is the option “Ignore rain”, then the humidity sensor is trusted.

So we now need a solution to the problem of the triggering humidity sensor.

Edit 30.06.2019:

I've started development on an improved software. I also immediately activated the 2nd input. Problem: The app development lags behind or cannot show me the options. I'm working on a solution...

Edit 01.09.2019:

Software is now in test mode. Stand 2.1.9 now supports BOTH sensor inputs and in addition there is support for humidity sensors.

Edit 15.09.2019:

Instructions for setting:

  1. Set Hys to smallest value (turned to the left)
  2. Set Vdc to the lowest value
  3. Install a Soil Moisture Sensor in the irrigated area, but not in a location directly irrigated.
  4. Now it gets difficult: you have to find the right value.
  5. Water the area. Now if the area is wet, turn on VDC until the LED comes on.
  6. Now it has to be dry again. If the LED is still on, then turn VDC very slightly until the LED goes out again.
  7. Then water the area again. If the LEDs now light up, then everything is set correctly. Otherwise repeat 5-7.

Unfortunately, I have to admit that the threshold switch is not the optimal solution, since it has a range of 0-10V, while the soil moisture sensor has 0-3V. Truebner is working on a solution here

The way food is OpenSprinkler-Software is not optimal for a soil moisture sensor. If the sensor gets wet during watering, watering will stop. We recognized this and solved it with software version 2.1.9: There is an adjustable delay value here. The software for this will be released soon. If you want, I can send them to you now, but the APP does not show the new parameters, only the web interface (on the PC)

Sensor 2 will then also work with this version.

Oh yes, recently there was a discussion about the best way to connect a sensor. Ray (from thought the best would be to use SN1 open (dry) and GND-SN1 connected (wet) and NOT GND-SN1 (dry) and VIN-SN1 (wet) as shown in the wiring diagram. Here is the current wiring diagram

OpenSprinkler-Humidity sensor switch plan

8 opinions on “Soil moisture sensor"

  1. Joachim
    Joachim says:

    Am I right – there are eight “zones” for irrigation (and more with expansion), but I can connect a maximum of two moisture sensors? Find the mistake 🙂

    • admin
      admin says:

      That's right, after all we have a weather service that sets the watering duration depending on the temperature, humidity and amount of rain.
      Especially with large areas such as lawns or other open land areas, it makes no sense to measure just one point with a moisture sensor.

      • Joachim
        Joachim says:

        That may be the case with large open spaces. However, this is important in a greenhouse, where plants with very different water requirements are in different sun-exposed positions.

      • Uwe
        Uwe says:

        I have several zones around my house that need to be watered very differently.
        Is there a possibility to use more than 2 humidity sensors and assign the zones to one sensor?

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