Hardware Design

There were many considerations when it came to the physical structure of WASP. How does the water get to the plant? What kind of sensors do we use? What kind of microcontroller? How do we power it? The list goes on and on. The most important considerations and solutions are summarized below.

• Low-pressure solenoid valve

This valve allowed us to use a gravity-feed system for the water, that is, once the valve is opened, the weight of the water is sufficient to overcome the minimum pressure requirement. Many other valves require high pressure like that of a garden hose. The low-pressure valve prevents the need for connecting directly to a faucet or adding a pump.

• Capacitive sensors

We used the Vegetronix VH400 sensors in this project. They are a clearly superior sensor to many others we encountered while developing this project. Most sensors operate by measuring conductivity of the soil, which can be inconsistent. Not only that, but these sensors tend to degrade after only a couple weeks in the soil. The VH400 sensors effectively measure the dielectric constant of the soil, providing a much more accurate reading. They are also insensitive to soil salinity, making them far more robust for long-term use.

• Multiple sensing locations

Sensing at a single point in the soil does not give accurate results as water percolates through soil in many ways and distributions are seldom uniform. By placing sensors in 2 locations we were able to get a much better overall view of the soil moisture. More sensors, of course, would further increase accuracy, but cost was a factor.

• Raspberry Pi microcontroller

We were originally going to use a Portable System on a Chip (PSoC) to run the system, as it was inexpensive, easy to use, and we already had some familiarity with it from our coursework. However, the Raspberry Pi allowed for more seamless integration with our touchscreen GUI and had built-in wi-fi.

• Analog to Digital Converter (ADC)

The major drawback to the Raspberry Pi is that it has no analog input, which was required for the moisture sensors. To remedy this, we purchased the MCP3008 10-bit ADC. It converted the analog output of our sensors to a digital format readable by the Pi.

• Touchscreen

We wanted WASP to be a standalone system, with no need to connect to a computer. We considered using a button pad for the user to adjust moisture threshold and to test the system. Instead, we went with a more modern approach with the touchscreen. It ended up being a large part of our budget, but was ultimately worth it. Our particular screen connected directly to the Raspberry Pi with no additional hardware, allowing us to begin using it right away.