Infrared Soil Moisture Sensor

Design by Node257 | Licensed Under CERN-OHL v2

Summary:

This sensor combines an IR LED with a photoresistor and a transistor to deliver a reliable analog signal indicating an approximate soil moisture level. This setup avoids interacting with the soil electrically, and therefore avoids corrosion-related drift. It is also very simple and can be made with cheap, passive components.

A separate (or on-board) microcontroller is attached to the sensors' LDR, LED, and Signal lines, and does the following:
1) Powers the IR LED.
2) Measures the corresponding voltages on the signal lines.
3) Estimates approximate soil saturation based on readings and prior calibrations.
4) Outputs a value in a desired unit, for example: 5% (Dry) – 95% (Saturated), or actions needed: "Needs Water", etc.

Theory of Operation:

Water absorbs IR light very strongly and that physical property can be used to roughly measure the amount of water saturated in the soil surrounding the sensor.

For a proof of concept you can point an infrared remote control at a night-light's ambient light sensor, and hold down one of the buttons. Most CdS light sensors are sensitive to IR light in addition to thier standard visible spectrum sensitivity; so the night light's LED will flicker as the IR LED transmits IR pulses at it.

About Me:

Hello! I'm an Electrical Engineering student from Colorado. This has been a pet project of mine for years, but it's time to just release it. I developed another design using different physics that you can find at the following link: Thermal Soil Moisture Sensor. I have listed both projects as 'work in progress', since the PCB designs still needs to be validated. I will continue to work on these, though work and school keep me very busy. Any donations will go directly towards building more prototypes, and are greatly appreciated! I am publishing the designs under an Open Hardware License to encourage others to refine and diversify the technology. I think it could be very useful for water conservation to have accurate, affordable sensors that don't drift or corrode. Thank you for checking out my project!