Tutorial: Waterproofing your Capacitive Moisture Sensors for the Smart Garden System

This is a tutorial showing how to waterproof inexpensive capacitive soil moisture sensors for the Smart Garden System.

What is the the Smart Garden System?

The Smart Garden System allows you to build your own remote monitoring and management system for your indoor or outdoor garden.   Do you want to share your garden and the weather world wide?  You can learn the Raspberry Pi and how to connect to the real world through this easy to build no-soldering kit.   You can measure soil moisture and then use that as feedback to provide your plant or garden just the right about of water.

Want to hook up to a Raspberry Pi or Arduino? Check out this article.

The Capacitive Moisture Sensor

The Smart Garden System (or SGS for short) is an introductory, easy to build Raspberry Pi based environmental monitoring and plant watering system using advanced sensors to monitor the soil moisture, monitor the sunlight, watch the air quality and monitor temperature and humidity.

Our SDL Capacitive Moisture Sensor with Grove Cable is available here.

 

Waterproofing your Capacitive Moisture Sensor

While the lower part of the capacitive moisture sensor is waterproof, for outdoor use and applications that involve overhead watering or splashing (or over active 5 year olds according to one customer),  you will need to waterproof your moisture sensor.   While a number of people have used heat shrink tape, our preferred method is to use silicon caulking to seal the electronics on the front and back of the sensor.

Step 1 – Examine the electronics on the front of the sensor and the thru holes on the back

 

 

 

 

 

 

 

 

 

 

 

Step 2 – Plug in the included Grove Cable and cover the bottom of the plug and the top of the plug (including the wires) with silicon caulking.  Put silicon caulking on the top of the device, covering all the electronics.

 

 

 

 

 

 

 

Step 3 – Put silicon caulking on the upper part of the back of the device, covering the small through holes in the board.

 

You are now ready to go out and measure soil moisture!

If you need a longer Grove cable, check out our Grove cable extension system.

Other ways of Waterproofing

You can also use:

1. Clear Nail Polish (but you still need to cover and seal the plug and wires using something like silicon caulking)
2. Heat shrink wrap (used for sealing wires) over the electronics and wires.
3. Conformal coating for PC Boards (designed for this!) on Amazon.  Still need to cover the wires and plug with something like silicon caulking.

How Does a Capacitive Moisture Sensor Work?

Simply stated, a capacitor stores electrical charge.

The electrical component known as a capacitor consist of three pieces.  A positive plate, a negative plate and the space in-between the plates, known as the dielectric.  The physical form and construction of practical capacitors vary widely and many capacitor types are in common use. Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium.

A capacitive moisture sensor works by measuring the changes in capacitance caused by the changes in the dielectric. It does  not measure moisture directly (pure water does not conduct electricity well),  instead it measures the ions that are dissolved in the moisture  These ions and their concentration can be affected by a number of factors, for example adding fertilizer for instance will decrease the resistance of the soil.  Capacitive measuring basically measures the dielectric that is formed by the soil and the water is the most important factor that affects the dielectric.

Capacitive measuring has some advantages, It not only avoids corrosion of the probe but also gives a better reading of the moisture content of the soil as opposed to using a resistive soil moisture sensor.   Since the contacts (the plus plate and the minus plate of the capacitor) are not exposed to the soil, there is no corrosion of the sensor itself.

The capacitance of the sensor is measured by means of a 555 based circuit that produces a voltage proportional to the capacitor inserted in the soil.   We then measure this voltage by use of an Analog to Digital Converter which produces a number that we can then interpret as soil moisture.