What does it do?
| The moisture sensor is used to detect changes in the wetness of its surroundings. |
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How does it operate?
The usual moisture sensor circuit gives a low voltage when it is dry and a high voltage when the sensor is wet. The dryness sensor gives a low voltage when it is wet and a high voltage when the sensor is dry.
If the changes in the moisture level are small then the moisture sensor needs to be followed by a comparator.
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Normal circuit (moisture sensor) |
Inverted circuit (dry sensor)
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The moisture sensor circuit uses a moisture probe to test moisture levels.
A simple low cost moisture probe can be made up from a small piece of strip board – with leads connected to a pair of the copper strips.
When the probe is dry there is a very high resistance between the strips. When it is wet the water reduces the probe’s resistance. |
| Click on the circuit diagram to download a Livewire file of the circuit that you can investigate and add to your own circuit. |
The moisture sensor forms a potential divider whose output voltage is determined by the upper and lower parts of the circuit. |
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A PCB-mounting terminal block |
The moisture sensor is connected to the PCB by wires connected to a terminal block mounted on the PCB. |
Possible applications
- Check if plants need watering
- Flood warning system
Making
How part of the PCB might look
Connect the moisture probe to the PCB with wires and a PCB-mounted terminal block.
Testing
Turn the variable resistor to its mid point. Make sure that the signal going out (on the green PCB track) changes from high to low.
Fault finding
If there is a fault, check that:
- The resistance of the moisture probe is high when it is dry and falls to less than 100k when it is wet.
- The resistance of the variable resistor changes when it is turned.
If there is a fault, check the tracks and solder joints.
Alternatives
Web links
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