Blog - Sketching with Hardware

Starting tomatoes from seed and automating the process

Published on: | Author: Fabian Braun | Categories: 2018a, Tutorials

Our project for the course „Sketching with Hardware“ this semester, was the “Seedling Sitter”, an automated seed starting station for tomatoes and other vegetables. Hand in hand to our project documentation, I want to give a small introduction on how to start tomatoes from seed, what factors have influence on the plants growth and how we’ve tried to optimize these in our project.

  1. Starting tomatoes from seed

There are many sorts of tomatoes to choose from. Most Seeds you can buy at your local hardware store are F1 hybrids, which are crossovers from two “pure line” plants combining beneficial features from each of the parental plants. While bringing advantages in quality and yields these hybrids cannot be used for seed saving, the reusing of seeds from your own harvest for next years plants. Alternatively you can get old seed varieties that are good for seed saving from various online shops like Arche Noah. (Shhh! Don’t tell Monsanto)

Once you have your seed of choice you need the right soil to start growing your seedlings.

It is important to use a rather lean soil, since nutrient rich mixes can damage the young seedlings roots. You can either buy prepared seed starting soil or mix it yourself, for example using 1 part of peat moss, 1 part of sand and 1 part of regular flower soil. There are many different soil mixtures, so picking the “right” one can almost turn into a religious question.

After mixing the soil it can be sterilized by putting it in a microwave oven for 5 to 10 or a regular oven for 20 minutes at 200° C. Alternatively, if you don’t want to mess up your kitchen, you can water the soil with boiling water. Doing this kills off pests like the dark-winged fungus gnat, that can afflict your seedlings.

After you’ve prepared your seeds and soil you can start planting, using either coconut fiber/pear based grow pots and pellets or, if you want a cheaper alternative, plastic cups with a hole punched into the bottom. Grow pots have the advantage that you can just plant them into the ground once the plant is big enough, since it can just root through them.

Once the seeds are planted there are 3 important parameters to keep an eye on. Temperature, moisture and, generally as soon as the seeds have germinated, lighting.

For tomatoes the optimal germination temperature is between 18°C and 25°C. You can look up the values for other plants in growing spreadsheets. (These also tell you how deep you should plant your seeds and how dense the plants can be grown)

In terms of soil moisture you should never let the soil dry out completely while still allowing it to breath, since the roots need oxygen or they will start rotting.

After 5 to 10 days, as soon as the seeds have germinated and you can see the first sprouts, light becomes an important factor. While you can just place the seedling on a sunny window still, natural sunlight can be insufficient, especially if you’re starting in early spring. To supplement this, there is a range of grow lights you can use, for example power saving LED based lights. Insufficient light can lead to leggy seedlings, with stretched skinny stems, since the plants try to reach brighter spots.

When using a grow light, it’s important to look up the light cycle of the plant you’re growing, since some plants need a few hours of darkness for a healthy growth. For tomatoes an 18 hour light cycle with 6 hours of darkness is often advised.

  1. What can be automated?

Looking at awesome projects like the FarmBot Genesis, the answer is pretty much everything.

These ambitious concepts completely automate mechanically challenging tasks like planting the seeds and individually watering the plants.

For our homemade Seedling Sitter that we built within a week, we decided to build a simple system that automatically controls important parameters for our seedlings growth. Our plan was to regulate the temperature, soil moisture and light levels within a mini greenhouse.

In the limited time we had, we managed to implement light and soil moisture control using an Arduino micro controller.

  • Soil moisture:

There are a few commercial soil sensors that are compatible with the Arduino, but we decided to build our own, which worked surprisingly well.

We used a laser cutter to cut out the general shape, then simply glued two strips of copper duct tape to it and soldered the positive and negative connector wires to them. We hooked this homemade sensor up to a voltage divider behind a 100k Ω resistor grounding the sensor and using the other side of the divider as an input for the Arduinos analog pins.

This yielded values between 5V and 1023V. We measured around 160V for moist soil, 1000V in air and 560V for dry soil. Its important to use one of the Arduinos digital pins to output voltage instead of simply connecting the sensor to the 5V output. By doing this you can measure the soil moisture in short intervals every few minutes instead of constantly having the sensor running. This prevents fast corrosion of the copper duct tape.

As soon as we had the sensor running, we used it to control an old printer ink pump (that we cleaned thoroughly), connected to an H-bridge. The associated code is pretty simple, as soon as the moisture drops below our boundary value, we activate the pump for a fix amount of time, filling the bottom plate of our greenhouse. Afterwards we completely lock the pump for at least one minute, to give the soil time to absorb the water and prevent flooding our living room.

  • Lighting

To supplement the lighting, we decided to use an LED grow light. Now there are two ways to do this. One could simply hook the light up to a timer switch, activating it 18 hours a day, but where’s the fun in that.

We decided to attach the light to a homemade ball bearing pulley and connect it to a stepper motor. This allowed us moving the light up and down, controlling how much extra light the plants get. We used a photoresistor with a setup that is similar to the one we used for the moisture sensor. The light sensor was connected to a voltage divider behind a 7500 Ω resistor grounding the sensor and using the other side of the divider as an input for the Arduino analog pins. Using this setup, we measured around 900V in regular daylight and 960V with the plant light turned on in the lowest position.

Again the supplementing code was pretty simple. If the measured light level drops below our bottom threshold the stepper motor will move the plant light down until the wanted lighting or the lowest possible position is reached. If the light levels go over our top threshold, the motor would move the light up accordingly.

It’s important to note that the light should still be connected to either a timer switch or a remote controlled socket to turn it off for 8 hours at night time. A more efficient way to optimally control the usage of the plant light would be to implement an internal clock in the Arduino and connect the light sensor value to a controller for remote controlled wall sockets. This way the light can be turned on and off depending on the light cycle of the plant and the current amount of natural light.

  • Temperature

Since we ran out of time for the project we couldn’t realize this part. Our plan was to build a system that could rise or lower the internal temperature of the mini greenhouse. When you’re starting your seedlings in the early spring lowering the internal Temperature isn’t hard, just open the lid of your greenhouse. To automate this we wanted to connect a temperature sensor in the greenhouse to another stepper motor with an attachment similar to a crankshaft. This device could rise and lower the lid, allowing cooler air to enter the greenhouse.

Heating it when the temperature drops below our threshold of 18°C for tomatoes is a bit trickier, since you need a heating element that’s small enough to fit into the greenhouse, without burning the plants. One possible solution are wire-wound resistors connected to around 5W input, depending on the resistor, and a small heat sink attached, to consistently dissipate the heat. One could connect this small heating element to the temperature sensor, turning it on when the internal temperature drops below the bottom threshold and off once it’s warm enough in the greenhouse.

A simpler, but less efficient method would be to use a regular electric heater controlled by another remote controlled socket, to just rise the room temperature.

linked categories 2018a, Tutorials

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[…] to grow are of course water and light. You can read more about this topic in Fabians blog entry here. Therefor our main features should be an automated water and light […]