EcoIrrigation is a small electronic module which applies an AC current to the two electrodes of the Watermark sensor, and returns a frequency as a output. The library will manage the EcoIrrigation module and convert the output to a humidity value in kPa.
The PCB will be available in the next few months.
Watermark™ sensors are available from Irrometer Company. These soil moisture sensors are to be buried in intimate contact with soil at root depth, where they will reach equilibrium with the local soil moisture. The measurement correlates well with soil water potential. This is fine for agriculture, because water potential (in units of kilo-Pascal or centibar) best quantifies the work plant roots do to extract moisture from the soil.
The sensor contains a perforated stainless steel cylinder which supports a permeable membrane. Inside, there is a tightly packed sand aggregate, called “granular matrix”, and at one end there is a wafer of gypsum, and concentric electrodes. The electrodes are attached to wires that emerge to the soil surface.
The gypsum wafer serves as a buffer against differences in soil acidity and salinity, so that the electrical resistance between the electrodes depends on moisture and temperature only
The sensoor return an electrical resistance, which decreases with increasing soil moisture. DC currents must not be allowed to flow through the wet part of the circuit, or else irreversible reactions will occur and spoil the readings. However, a AC current avoids these problems, by reversing the polarity of the current many times per second, so that no reaction takes place at either electrode.
EcoIrrigation board provides an AC current at the electrodes. The output signal is a square wave, and its frequency varies (as for the the electrical resistance) from 50 hz when the sensor is bone dry, up to 10000+ hertz when the sensor is soaking wet.
The library helps you to convert the output to an humidity value in Kpa.
It's necessary to measure the temperature of the soil with a DS18B20 to have an accurate measure. Here is an example, how to use a DS18B20
The switch allow you to choose how you want to power the WATERMARK sensor
| Mode | Powered by |
|---|---|
| MOSFET | Powered by the 3.3V while the MOSFET is triggered (Default and strongly recommended) (1) |
| µC | Powered by the pin of the microcontroller (2) |
(1) Applying a LOW value will trigger the MOSFET and power the sensors
(2) If you are using the EcoBoard, keep the switch to µC position because the EcoBoard already has a MOFSET.
(Ignore RD0. RDO is not soldered on that board)
(EcoBoard or any Arduino board)
| Pin | Output |
|---|---|
| 1 | GND |
| 2 | Ctrl (on/off) |
| 3 | Outpout (Analog pin). It need to be pulled up with a 4.7K if you do not connect the pin 4 to 3.3V |
| 4 | 3.3V. If you connect it to 3.3V, the output is puleld out with 4.7K. Otherwise, leave it disconnected. |
The Ctrl must be LOW to power the sensor while you use the MOSFET mode. Change to HIGH between two measures.
The Ctrl must be HIGH to power the sensor while you use the µC mode. Change to LOW between two measures.
Important: The Watermark sensor must be powered only while reading a measure. The rest of the time, it must not be powered, to avoid AC current in the electrodes.
Watermark sensor wires
| Pin | Output |
|---|---|
| 1 | Wire 1 |
| 2 | Wire 2 |
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