Living in the SF Bay Area, land is infamously at a premium... but that doesn't have to mean we can't grow our own fresh and healthy food.
Hydroponics is a popular method to dramatically increase the growth of plants (up to 50%), and reduce water use when compared to soil-based methods. Using hydroponics should allow us to produce an appreciable amount of fruits and vegetables in a footprint compatible to a windowsill or a small backyard. Although it's more intimidating than dirt in a pot, hydroponics can be made easy given the right technology.
Main goals of the project:
1. Low cost options for sensing of pH, nutrients, water level.
2. Low cost control of water pump and additive injection.
3. Intelligent control algorithms to maintain parameter levels and alert inconsistencies.
4. WiFi connection for browser interface to charts, controls, etc.
This is my first foray into hydroponics, so there will be a lot of learning opportunity for myself, and I will try to log as much of my learnings as possible to make this a richer project log.
My general strategy to get this project done is to first identify the major components or design decisions that need to be made and address them separately, before lastly tying them all together. I imagine a project like this is at risk of feature-creep so I want to stay focused on the subtasks at hand.
Below are the major components I’ve identified and a short description of the work that will need to be done for each:
** Grow bed design **
At the very least, a hydroponics system needs a container + medium to hold plants, a container to hold water, and a method of getting the water/nutrients to the plants.
For plant containers, there are a lot of ideas out there from a very simple tray, to PVC pipes with holes in the top, even to vertical towers (e.g. the “Tower Garden”) which are more space-efficient. My consideration for this part of the design is to choose something fairly inexpensive but also somewhat aesthetically pleasing. I suspect I will be able to find a fairly attractive solution using food or storage containers, or simple planters from the home store. I will need to be sure the materials are food safe though.
For the water container, I also would like to choose something more attractive than the popular big blue 55-gallon drum. I would draw inspiration from rain-water barrel design since that is becoming more popular these days and sellers are wise to the fact that homeowners don’t want an ugly barrel in their yard. Partially my set of choices would be dictated by the size of the container needed. A large container is good because you’d have to refill it less and it has a greater dilution factor for balancing chemicals. A smaller container is more aesthetic and also would have more fresh water turnover to avoid it getting scummy inside.
For getting water to the plants, there are several strategies- I will use one of the most popular which is an ebb and flood system. Essentially, the system works by flooding the roots of the plants with water for ~15 minutes, a few times a day, during the other times, the water is allowed to drain out back into the reservoir. An electric fountain pump provides the motive force needed to move water from the reservoir to the plants. By placing the reservoir under the plant bed, we let gravity drain water out when it comes time. There are two openings into the grow bed- one is for the pump to push water in, and for water to drain out when the pump is turned off, and the other is an overflow pipe (e.g. similar to what bathtubs and sinks have). Both will need to have some sort of filter mechanism to keep solids from entering the water reservoir.
** pH Sensor Design **
Apparently plants thrive best when they are grown in a certain range of pH which is conventionally provided by soil. In general, a slightly acidic environment of around pH 6.0 is preferred. Outside of a certain range, nutrients will begin to precipitate out of solution or otherwise become unavailable for the plants to take in. Therefore, it is important to monitor the pH of the solution continually and make adjustments as necessary.
The least expensive method for pH measurement is to use test strips, like the ones in high-school chemistry. However, these are not adequate for our purposes since we would like to monitor and adjust pH automatically, so something with an electrical interface is needed.
Traditional pH meters rely on a glass pH probe where the probe consists of a treated-glass electrode and a reference electrode and measures the difference of electrical potential between the two. In addition to being fragile and expensive, they also require constant calibration to maintain accuracy.
A more recent invention has been the ion-sensitive field-effect transistor (ISFET) which has found use as a semiconductor device...