Since coming home in January from my most recent contract, I’ve been crazy busy working through all the myriad details of getting TechnoTomato up and running with a full set of 9 plants as well as completing TechnoArugula and all the other things that had to be done after a year long absence. I had hoped to have plants in the system and everything debugged and running by mid May but getting a full scale system in place in that short a time was more than I could manage. Consequently my aeroponic tomatoes are a couple of months behind this season. Fortunately the mother plants are in grow bags and doing fine so I should have a decent harvest this year regardless.
Unlike TechnoArugula where the environmental conditions are very predictable because it’s indoors under lights in a grow room that has fairly uniform temperatures and humidity, TechnoTomato is outdoors in Northern Colorado. Being outdoors leads to wildly variable conditions which make providing the right amount of water and nutrients at the right time a real challenge. Further complicating my design is that I want to do a drain to waste aeroponic system with little or no waste. This means that spray timing needs to be controlled very precisely. The big hurdle to doing this right is knowing when and how much nutrient solution to spray. I approached this from three directions. The first involved better controlling the environment and the other two were computational.
In my first attempt at this a couple of summers ago, I did one plant in a five gallon bucket with spray nozzles arranged in a circle aimed inward at the root bundle. The system was pretty simple, making use of an Arduino to control spray timing based on the output from a leaf sensor. That plant did remarkably well despite a number of problems and produced an exceptional yield of tasty tomatoes. High on the list of problems was that the interior of the bucket got up well over a hundred ℉ on hot days. It doesn’t take a degree in biology, which I have, to know that this can’t possibly be good for the roots of a plant but my plucky tomato plant soldiered on and produced a considerable crop of tomatoes that summer.
This time around I was determined to control the temperatures in the root chamber without spraying huge amounts of water for cooling. My approach was simple, dig a deep trench for the roots to hang in. So, I spent a good bit of time this spring digging a two foot deep, foot and a half wide twenty foot long trench to take advantage of a big, relatively cool thermal mass of soil. Of course the trench needed support so it wouldn’t collapse and it needed to be lined with plastic to prevent contamination of the roots. This has worked quite well; despite outside temperatures over a hundred ℉ on some days, the temperature of the root chamber has never gotten much over 70 ℉. A large amount of dirt and some insulation does wonders.
The original attempt at this used a very simple Arduino sketch that sampled the sensor and then triggered a three second spray at intervals determined by how much the leaf thickness had changed. That sketch was very rudimentary but worked fine for a single plant. Handling three or four plants at the same time required me to put on my software architect hat and actually design a small application. In my professional life I design and build iOS and MacOS apps using Swift; C++ on a little Arduino Uno is a long long way from what I’m used to so designing a solid, object oriented, asynchronous state machine for this system was an interesting challenge.
I will go into more detail in later posts but TechnoTomato is a fully asynchronous, object oriented, state machine that is handling three plants per controller quite nicely. TechnoTomato is reading multiple leaf sensors, temperature and humidity, and controlling multiple valves with fraction of a second precision all while sending data for each plant to my ThingSpeak channels once a minute. While it remains to be seen how well this system will do over the next couple of months it is gratifying to hear the spray valves thumping away in the garden.
I’ve got veggies to harvest and I think that since I’ve got ripe tomatoes and fresh spices, this weekend is going to be the first stone oven pizza of the summer. Until next time – happy techno gardening.