Soil Moisture Sensor (Resistive)

A peripheral for use with the Kookaberry

This sensor is an INPUT device. It measures the moisture in a ground medium such as soil. It is an analogue device.

The principles of Electrolysis and Electroplating can also be demonstrated by observing that the metal coating of one of the prongs deteriorates over time.

Specification

  • Power supply: 3.3v or 5v

  • Output voltage signal: 0~4.2v

  • Current: 35mA

  • Size: 60x20x5mm

  • Value range:

    • 0 ~300 : dry soil

    • 300~700 : humid soil

    • 700~950 : in water

How does it work?

The sensor is plugged into the Kookaberry in accordance with the app instructions.

When the prongs of the sensor are plugged into a ground medium, a current flows between them that is proportional to the resistance of the medium.

A transistor embedded on the sensor converts this varying resistance into an analogue voltage that is sent back to the Kookaberry on the signal wire. The electrical circuit can be found behind the Schematic button in the Sidebar

When there is just an air barrier between the prongs, or very dry soil, the resistance is very large; no current will pass across the gap; and there will be very little voltage on the signal wire. When the sensor is immersed in water the resistance will be very low, and the voltage on the signal pin will be close to its maximum value.

Because water is not a perfectly conducting medium (try adding salt to improve its conductivity) the moisture value will never be 100%.

Electrolysis and Electroplating

After a long period of use, one of the prongs will start to corrode and lose its metal coating and the whole sensor will need replacing. This is caused by electrolysis, which is the chemical reaction that makes, amongst other things, batteries work.

Go to this video link from FuseSchool for more information on electrolysis.

This is because the current flow breaks down the metal coating on the prong sending the current across to the other prong (this is the anode – or positive Vcc prong), and takes minute bits of metal with it.

These tiny bits of metal increase the thickness of the metal on the receiving prong (this is the cathode or Ground prong) and decrease its thickness on the anode prong. This process is called electroplating.

Go to this video link from FuseSchool for more information on electroplating.

Where is this used in real life?

This is a relatively¬† low tech sensor, but ideal for monitoring an urban garden or your pet plant’s water level.

Sensors such as this provide the feedback data to automatically water plants in aquaculture projects

How can they be used to enhance Lesson Plans outcomes?

  • Sustainable food production and agriculture
  • Sustainable micro-climate (Terrarium)
  • Manufacturing processes (electroplating)
  • Chemistry (electrolysis)

Learning Plans that use this sensor

Apps that use this sensor

Image

SKU:SEN0114


Output Type: Analogue


Supply Voltage:3.3 to 5v


Brand:DF Robot


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Schematic
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