Measure soil moisture and control irrigation

The OpenSprinkler It has two digital inputs, which are essentially an on/off switch. This makes it easy to use a rain sensor to turn off the irrigation system when it rains.

Soil moisture, however, is a different story. This isn't a value that can be defined as 0/1. Different soils have different densities for different plants with different needs. Therefore, the value is measured in 0-100%, with 0% being bone dry and 100% being pure water (or a little air). So, even 50% is an equal ratio of water to soil.

So what is the correct value?

There are various tables from experts who have compiled these values, for example here:

https://www.acurite.com/blogs/acurite-in-your-home/soil-moisture-guide-for-plants-and-vegetables
or use one of the well-known plant apps, e.g. FYTA or PlantNet, to find out the care instructions for your plants.

In order for this to work, you have to be able to determine a value that is not digital, but analog. This is where our analog sensor board with the SMT50 comes in. The SMT50 can measure humidity from 0-50%, making it sufficient for most plants. However, if you want to measure up to 80%, for example, for cranberries, you need the SMT100, which can measure humidity from 0-100%.

The Analog Sensor Board has eight connections for analog sensors. All data is measured in a time series and is available for download.

And how can you now control irrigation with it? Using the "Program Adjustments" program function, the irrigation time can now be adjusted dynamically. You specify a humidity range, e.g. 3-20% and OpenSprinkler The watering duration is then scaled according to the humidity! So, if the humidity reaches 20% or more, watering is deactivated; if it reaches 3% or less, watering is performed to the maximum. In between, the scaling is linear (or various options can be selected).

This example shows an irrigation adjustment to 28,8% due to soil moisture of 14%

For further information please see the product pages of the Analog sensor board and the SMT50

For professional users

For the professional Use, i.e. if a large number of sensors are to be used, then we recommend using our Truebner RS485 adapters, with which up to 64 SMT100 can be connected via Modbus.

The SMT100 in RS485 Modbus mode can provide 3 values:

– Soil moisture

– Soil temperature

– Dielectric permittivity