Dill on the stones
Buying microgreens and cognac at the same time, you can catch an evaluating look. Do not explain to everyone that the growing kit is purchased for scientific purposes. For 98 rubles we get a plastic container, mineral wool, a bag of seeds and instructions. “Keep in the dark for 2 days…remove the lid…put it in the sun…water it periodically.” Such simple actions will help to grow a small number of useful plants.
About two years ago I purchased one and successfully grew a small number of radishes. At the same time, I got the book Hydroponics for All by William Texier. In addition to plant-related information, the book contains many simplified pictures of hydroponic setups. It is Texier who is responsible for the idea to turn a plastic box with mineral wool into something more.
I will give a few quotes (section “System of periodic flooding”).
– I also like another, really simple old system of periodic flooding. It does not originate in the greenhouse industry, but was developed by the inhabitants of the Californian outback in huts that did not know electricity.
– These days, give people pumps and timers …
– … people are getting creative and inventing a lot of ingenious home-grown systems.
– It is important to find the optimal irrigation cycle, and this can vary depending on the season (even indoors!) and the size of the plants.
The last premise to start the development story is that I had the necessary electronics and software for hydroponics. In a previous publication, I talked about a device for watering room violets. The task of growing microgreens is no different, you need to water the plants according to a given schedule.
Idea
The device really needs a pump and a timer. USB powered – convenient, adapters are sold at every turn. If you add a display, the controls will become more understandable. Instead of setting the modes, we select plants: “Dill”, “Wheat”, “Basil”, the device must itself set the necessary irrigation mode based on the water level, time of day, temperature, type of plant.
We will track the water with a magnetic float. Plants are in a substrate that dries fairly quickly. It is necessary to inform the user when the container is empty with a sound signal.
Design
The first question of the task is the design of the device. Several evenings in a row looking for a good solution. On the one hand, the container must have sufficient volume to “flood” the substrate, on the other hand, it must be compact enough to place the unit on a windowsill or under a lamp. Water and plants should be at different levels or in different tanks. Aesthetic appearance and ease of use also matter. Having worked through several different cases, the choice was made in favor of the presented option.
The case consists of three parts: a pallet, a base and a cover. The design is collapsible, so the device can be washed. The main trick of the design lies in the principle of water supply: an air pump is used instead of a pump, water is pushed out by air pressure. The body is divided into two communicating reservoirs, the volume of the lower reservoir is almost equal to the volume of the upper one. After completing the irrigation cycle, turn off the pump and open the air valve, the water returns to the lower tank by gravity.
Electronics
The circuit is designed on the STM32G070RBT6 microcontroller. A sufficient number of legs, rich peripherals for a series and an attractive price. I ordered several chips on Aliexpress from different sellers. In the first package came output resistors and a set of transistors, in the second – what you need. Duplication of orders has already become part of the development practice and has helped out more than once.
The device turned out to be very simple, here is its block diagram. At a low cost, the controller gives everything you need:
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two SPI ports – it is convenient to read data from Flash and immediately transfer it to the display;
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DMA – for sound reproduction;
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64 GPIOs – 28 are used;
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RTC, internal voltage reference, temperature sensor.
Program
The program had to tinker a bit. The first stop was on the WAV playback algorithm. The data is read from the memory and transferred to the timer, which in turn operates in the PWM generator mode. The scheme of the working version is shown in the figure.
Let’s organize a ring buffer with a size of 8 kB. We set up a bunch of DMA and TIM with two signs of interrupts (Half, Full). The processor will be busy reading data from Flash and filling the buffer during the interrupt. The operating frequency of the microcontroller is 44 MHz, at which it is possible to obtain a PWM period of 42.9 kHz (as close as possible to 44 kHz WAV). An external crystal must be used to obtain an accurate sampling rate.
An invaluable find was the program “WAVToCode Converter”, all download links at the end of the publication. The program converts the downloaded file into C – code that can be written directly to memory. In this way, the device acquired several useful sounds.
The second stop is working with the TFT display. The ST7735_TFT_RPI library was taken as a basis, for which many thanks to the author. I needed to add a couple of fonts, but otherwise the library started up without problems. The fonts were generated using a program from @riva-lab, for which we also thank him. Bitmap Converter(ST) for emWin was used to prepare the images.
The algorithm of the microcontroller is quite trivial and is built on the basis of a simple dispatcher. A task called “Schedule” periodically turns on the pump (the period depends on the type of plant), the “WaterLevel” task checks the liquid level in the tank, the “Display” task updates the time on the screen, the “Sound” task gives the necessary sound signals, etc.
A float with a magnet is used to determine the water level. The Hall sensor monitors the position of the float, the obtained ADC values are converted into percentages. Depending on the current water level, we make adjustments to the irrigation time, because when the tank is low, it is necessary to pump air “to idle” so that the level of the ejected liquid rises to the substrate.
Cost and production
The cost of manufacturing a hydroponic setup is the saddest part of the story. Electronics, pump, display (892 rubles) + case (3795 rubles) + solution for hydroponics Simplex Hydro Vega A + B (1799 rubles) + mineral wool for growing microgreens (248 rubles) + Soil for the aquarium, “Amber” (269 rubles) ). Total: 7003 rubles.
The installation turned out to be very capricious in terms of production. It was not immediately possible to find a contractor for printing the case, not everyone has a printer of the required dimensions. The base was printed the second time, dividing the part into two parts with subsequent gluing. PETG plastic was used, followed by a solution treatment to prevent liquid from leaking between the fibers. The hull required further “trimming, sanding” and painting.
A bunch of hydroponic systems are currently being sold on marketplaces. For relatively little money, you can buy installations for growing lettuce, strawberries, onions, fodder plants, and anything else, almost on an industrial scale. Developing your own is always difficult, long, expensive, but interesting.
results
Whether the initial idea corresponds to reality is what prototypes are created for. The brilliant, exciting, commercially successful image of the future device fades as we move forward, when more and more difficulties, nuances and features in work appear.
The first tests – planting dill and parsley on mineral wool. And immediately a change of course, since the substrate turned out to be the main variable of the equation being solved. It is the substrate that determines how often plants need to be watered and how they will subsequently develop. Mineral wool retains moisture too much, does not allow the liquid to return to the lower tank.
We correct the software: we choose the substrate (and not the type of plant), depending on which the watering cycle changes. We do re-sowing on stones.
The second equally important component of the successful development of plants is lighting. Despite the fact that the apartment is located on the sunny side, there was not enough light. In combination with waterlogging, the plants stretch out and take on an unnatural appearance.
And the most unpleasant thing is the mold that forms on the seeds. Humidity and temperature provide good conditions not only for microgreens. The mold problem is not solved by the substrate, the experiment with the use of aquarium stones confirmed this.
conclusions
Each area of activity has many details. Gaining experience is always associated with errors, clarifications and adjustments. Even such a simple device as a system of periodic flooding was not as simple as it seemed. Let me recap the main questions.
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Choice of substrate. It is necessary to understand the degree of moisture retention, depending on which the irrigation regime is built. The physical holding capacity of the substrate (filling depth) is also important for large plants.
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Mold and other infections. Mold practically does not spread on stones, but this does not solve the problem of its presence on some seeds.
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Irrigation mode. It depends not only on external conditions, but also on the degree of development of plants. Grown dill and parsley consume water much faster.
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The volume level when the air pump is running. Watering will not be heard, only if you put the installation in another room.
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The problem of cheap Chinese equipment. Attractive for the constructive ultra-thin speaker is called so nominally. We had to forget about low frequencies, as well as about an acceptable volume level. But the dynamics like a clean tone. The choice of sound effects for button clicks “Clicks” had to be smoothly moved to “Piano Sounds”.
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The problem of defective equipment. Declared 160 x 80 pixels of the display turned into 140 x 80, underweight 20 x 80. The problem was eliminated by the purchase of a new display, but the program had to be corrected (change the Red and Blue color coding).
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Calibration difficulties. I had to literally finish the board in order to correct the position of the Hall sensor. It was necessary to change the design of the float to get a sufficient signal level.
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The safety of the resulting product. It must be understood that the resulting biomass is the result of the processing by plants of the sun, water and substances that make up Simplex Hydro Vega. The stones perform the function of retaining moisture and mechanical support.
Perhaps the reader has the impression that this whole experiment is described in gloomy terms. But this is far from true. In general, the created hydroponic installation of periodic flooding coped with its task, and the first crop was obtained. The taste of dill and parsley, according to my feelings, is ordinary. But what a process! The product was originally developed for the sake of entertainment, the problem to be solved is the object of research, it has not been possible to grow greens on the windowsill. I think many will agree that work tasks are limited by time, technical requirements and cost, outsourcing tasks are limited by time, cost and risks. Engineers have no room for creativity at all. Therefore, in hobby projects, all restrictions are removed.
Links to used materials
2. Program for creating fonts;
3. Bitmap Converter(ST) for emWin display program.
