DIY Moisture Sensors

Moisture sensors are commonly used with automated grow systems to determine when plants require water. They work by using two metal probes in close proximity pushed into the soil or grow material. When the soil is wet, because of the electrical conductivity of the water, the electrical resistance between the probes decreases. As the soil dries, the resistance between the probes increases again. By measuring the resistance between the probes, the grow system can determine whether the plants need watering. Low-cost probes can be made at home using galvanized nails and plaster of paris.

Things You'll Need

• 1/2-inch diameter rubber tubing
• Utility knife
• 2 galvanized nails
• Block of scrap wood
• Plastic bowl
• Plaster of paris
• Drill
• Wax paper
• Multimeter

Instructions

1. Cut off a piece of the rubber tubing slightly longer than the galvanized nails, using the utility knife. The smaller piece of tube should then be cut with a diagonal line from top to bottom, making it easier to peel from the plaster of paris at a later step.

2. Drill a hole in the block of wood slightly wider than the piece of plastic tube, and about half as deep. This serves as a stand for the plastic tube while the plaster of paris is drying. Place the piece of plastic tube in the hole in the wood. In a plastic bowl, mix up a small amount of plaster of paris according to the instructions on the packet. When you have an even consistency, pour the mixture into the plastic tube filling it to the top.

3. Push the two galvanized nails through a small square of wax paper, making sure there is a small gap between the nails so they do not touch each other at any point. Lower the nails into the middle of the plaster of paris, so the wax paper rests on top of the plastic tube. Leave the mixture to cool and set for a couple of hours.

4. Remove the plastic tube from the hole in the wood. Using the slit cut from top to bottom earlier, carefully peel the plastic tube from the plaster of paris. Leave for 24 hours to dry out fully.

5. Connect wires to the nails. You can either solder the wires to the nails, or wrap the wire around the nails. The connection can be insulated and strengthened if required, by covering the joint with some hot glue.

6. Read the resistance manually using a multimeter. Do this by turning the multimeter to its resistance scale and connecting the two probes to the two wires from the nails. The resistance will be shown on the multimeter's display. The probe can also be connected to an existing grow system, according to the system's instructions for adding sensors.

Tips & Warnings

• Calibrate the sensor by taking a reading of its resistance when the sensor is totally dry and again when immersed in water. This determines the maximum and minimum resistance of the sensor, and provides reference points to compare readings against.

• Check the probes regularly for any damage. If signs of wear are seen, the sensor should be replaced with a new one.

References

• The Cheap Vegetable Gardener: How to make a cheap soil mosture sensor