Measuring soil humidity to control the garden irrigation

Vegetronix VH400

I have ordered Vegetronix VH400 – Moisture Sensor Probe. This is very interesting device. It is amazing in particular by the speed of measurement and immediate reaction to the moisture level change. Its power consumption is less than 13mA. However, the supply voltage has to be between 3.5V and 20 VDC which in case of Loxone integration represents additional voltage regulation.

Voltage regulator to create 12V for the humidity probe
Fixed output regulator – Standard wiring diagram

Simple voltage regulator and two capacitors will make the trick and allow you to make 12V to feed the probe. As an output this gives you output range from 0 to 3V related to moisture content with 2% accuracy. Capacitors I used are based on the datasheet recommendations CI=0.1uF, CO=100uF.

I have connected this back to the Loxone Extension to one of the analogue inputs and set the proper correction curves in Loxone config to map 0V to 0% and 3V to 100% soil humidity.

I have buried the sensor under the grass lawn. Since the sensor takes an average across the blade, for a precise reading at a particular depth, it is recommended to bury it horizontally such that water will not pool up on the blade.

Now with the live soil moisture reading it is very easy to block the watering system every time the soil moisture exceeds given limit. You have to play a bit with the values to set the optimum humidity levels.

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