Maker Matheus Bitencourt has shown off an impressive first project: An Arduino-based wireless and solar-powered soil moisture and temperature sensing system, as part of an automated irrigation system, using an entirely homebrew sensor.
"I messed around with cheap capacitive and resistive sensors. They [are] worth nothing and can't work after one week," Bitencourt explains of the reason for developing a custom sensor. "I've created this sensor. It can read the soil moisture through this thick rubber enclosure, so no contact with water whatsoever. Plus, I even applied a layer of urethane coating in the board to make it even more resistant. It's so sensitive it can tell 3mL of water added into 1L of water."
The sensor is connected to a weather-sealed driving system located beneath a solar panel, which provides for the system's power needs in-the-field with no need for a mains supply. "There's a hidden subroutine in the receiver, which requires you to put the sensor inside dry soil and later, wet soil," Bitencourt explains.
"So,[it] get[s] these two extremes and does the
map() to 0-100%. In order to save power, the data is collected and sent only once per 5 minutes. The sensor is power hungry (60mA), so a MOSFET turns it ON just for a couple of seconds each 5 minutes. I'm using an Arduino Nano, with LED and linear regulator removed. Instead, I'm powering it with a LM2936Z-5.0v linear regulator which has low quiescent current - [the] LM7805 draws a lot of energy. I'm sleeping my Arduino with the help of the watchdog."
The system has proven both sensitive and stable, operating for four months so far without error. Sadly, Bitencourt isn't looking to share his work fully: "I won't publish this design," he explains, "since it's so complex and requires so much efforts and tools to make, nobody would do it. Also, I'm trying to make a product out of this. Instead, I'm working an a simpler, easier to produce unit that I'll make open source so anyone can use of the design and source code. Stay tuned!"
More information is available on Bitencourt's Reddit thread.