Introduction: Lettuce Growing Module
This is a project for entry in the Beyond Earth contest, aiming to put a new lettuce growing chamber on the International Space Station; that being said, anyone can make this or use it as inspiration for their own growing programs on Earth as well. It has been designed in such a way that experiments can be run with some of the modules as controls, or testing different parameters without having growing conditions leak from one cell to another. It was also designed to be lightweight, by replacing the need for aluminum or other dense materials with mostly hollow 3D printed PLA parts. Light, water, air circulation, and growing medium are all provided in the module, with microcontroller systems to ease the workload for astronauts and Earthbound hobbyists. Finally, it is designed so that if a component is broken, worn out or damaged it can be easily replaced, and individual modules can be transported without disturbing other plants growing in the space. As per competition rules, it abides by the requirements to provide all needs for the plant, make innovative use of space in microgravity, and constrain itself to a half meter cube.
- PLA Filament
- 3D Printer
- Arduino Uno
- Arduino .96" OLED Screen
- Capacitative Soil Moisture Sensor
- 5mm LEDs in White, RGB, UV, and IR
- 5v Motor
- Root Riot Hydroponic Plugs
- #22 Wire
- Soldering Equipment
- 2N222 Transistors
- Perfboard (For Permanent Installation)
- Silicone Water Repellent Spray
- Conformal Coating Spray
Step 1: Making the Housing
First, the PLA housing must be printed and assembled. The housing is built with a small tray at the bottom, so that growing medium can be easily held in place with straps or hooks. The base has four attachment points, so that additional modules can be clipped on. The bars going up from the base are shaped rails, and can have any number of things clipped to them. Some examples are:
- Fans to circulate air around the plants and encourage growth
- Lights to test the affect of light on the underside of lettuce leaves
- Panels to block light from other modules
- Wiring harnesses and water hoses
- Clips to other modules
- Mounting structure on the ISS
The rails have a clip joint at the center, because the roof of the structure is impossible to 3D print as an overhang and support structure would fill the entire module. These clips are sturdy enough to hold the parts in place, but for added strength the top and bottom halves should be welded together using additional filament and a soldering iron on low heat.
The roof of the container serves to block light from outside the module and maintain control over conditions, and block light from reaching a potentially sleeping astronaut. For Earthlings, this will protect the lettuce from falling things and clumsiness. It also has mounting points for a 3D printed light source, which will be discussed in further detail later. Finally, the roof has a set of four mounting points for clipping other modules in place.
Start by printing out the top and bottom parts with their largest surface down. After printing, the two pieces should fit together as shown in the picture. After testing the fit, spray the parts with the water repellent spray. Should water get pressed out of the medium, the hydrophobic surface of the silicone will keep it from spreading across the plastic. Rejoin the two halves, and for added strength, press filament down into the joints with a soldering iron to weld the top and bottom together. Painting is not required.
Step 2: Light Source
After building the housing, the light source comes next. There has been a lot of study into possible light recipes for Outredgeous Lettuce  and other red lettuces , so rather than providing a set light recipe this Instructable ensures light recipes can be made unique for each module. For the prototype I used only white LEDs to show the module better.
Start by 3D printing the LED holder. This has a pair of clips that go into the top of the module and can be removed at any time. If needed, the STL can be modified to increase the emitted light.
Place the LED holder in a pair of jaws with the tabs pointing down, and place 14 of the LED's into the ring so that the positive leads are on the outside of the ring, and the negative leads are on the inside. One of the LED positions will be vacant; this should be at a 45 degree angle to the clips so that the leads point toward the rail structure. Bend the leads so that the positive leads form a ring and the negative leads form a ring pointing the opposite direction. Double check that none of the LED's are backwards and that none of the positive and negative leads are in contact, and solder the rings where leads are in contact.
Now, place a red and black #22 wire into the remaining two slots. These will be the leads to the arduino/power supply. Solder the ends of the wires to the leads of the LED ring, with the red on the positive leads and black on the negative leads.
Finally, test the assembly with a power supply and a resistor to make sure all LED's turn on. If it works, a conformal coating should be sprayed on all of the exposed wire surfaces to prevent corrosion and accidental short circuiting.
Step 3: Fan Assembly
The module includes a fan to circulate air. In microgravity, convection is less effective at moving air. Without a fan to move the air, the plants would slowly use up all of the CO2 in the area and starve. By the same token, it is important for plants to have air flow to regulate moisture and improve growth. The fan clips onto the rails of the module, so it is out of the way of growing plants.
Start by 3D printing the fan blades and the fan attachment. After printing, slide the fan blades onto the motor shaft and slide the motor into the hole on the attachment. Some glue may be needed to hold the motor in place.
To install the fan, bend the joints and clip it into position on a rail close to the top of the module.
Step 4: Growing Medium
The growing medium, where the roots of the plant grow, is not entirely decided. In microgravity, a medium pillow is typically used that is waterproof and contains a soil like material.
The module is designed to hold a 10x10 cm pillow, but I did not have the equipment to make a pillow. Instead, I looked for a commercial solution. I found a product called Root Riot, a hydroponic medium that is sponge-like, but unlike rockwool still allows beneficial bacteria to grow. The Root Riot is very hydrophylic, and when damp I can hit it with a water gun without droplets of water splashing off. On the space station, this means that even when overwatered, water will be repelled by the silicone sprayed surfaces and attracted to the sponge-like material of the medium keeping the fluid controlled.
To put the plugs in the growing tray, I first cut them in half lengthwise with a groove toward the midpoint of the semicircle. Then, I place them in the tray where they line up end to end.
To plant a seed in the Root Riot, simply cut a small slit in the plug and push a seed in. Watering can be done either with a syringe and hose to reach tight spaces, or using needles and hoses managed with hose clips as shown later.
Step 5: Clip Attachments
One of the benefits of the rails, is that anything you need can be connected to a rail and held out of the way. Using a 3D model of a generic clip template, anything can be added.
I have designed several clips; one to hold wires routed through the module, one to hold 5mm OD rubber hoses for water, one to reinforce a weak rail weld, and one to connect a pair of rails together. Simply print the ones you need and clip them to the rails.
Step 6: Planting
To plant the seeds, simply take a seed and place it in the hole on the Root Riot or pillow. Make sure the sponge is moist. The seed will be held in place by the compression of the sponge, and will not be shaken out when planted properly. If the root riot hasn't been fertilized yet, mix some fertilizer into the sponge with the recommended dose. I like to cut a slit into the tab end of the plug so I can tell it has been planted already.
Step 7: Sensors
This is where the wires come in. This module can be fit with a number of sensors using customized clips. Right now, I have used a soil moisture sensor to make sure the plants do not need water. The sensors I bought needed some modification when they arrived, since I didn't expect them to be so long. Simply cut the board at the point before the round trace to leave the capacitor traces intact. Since the sensor does not have to penetrate into soil, the point is not needed.
Once this is done, lay the sensor into the bottom of the tray and place the sponges on top to hold it down. In zero g, the sponges can be strapped down easily holding the sensor in place, or the base can be modified to have a slot for the sensor. Run the wire through a clip to hold it in place.
A thermistor will be connected at the main board, so that the local temperature is recorded.
Another sensor you can use would be a light sensor, as a sanity check for the LED bulbs and a data point for experimentation. This would be clipped onto a rail as well.
Step 8: Circuits
Since this is supposed to be scalable, I have not decided to include a complete circuit schematic. Instead, things will have to be added individually. For a second prototype, I would standardize the sensors for each module and run a standardized connection to the microcontroller.
Once all of the sensors are in place, run the cables to a breadboard and connect them so that the ground pins go to ground, the 5V pins go to 5V, and all of the signal/data pins go to connections on the arduino. Edit the code so that the pin connections match up with what is in the code. As an example, the attached image shows a schematic that takes all of the sensors into account for a single module. Note that the thermistor does not need to be replicated for each of the modules, since they should stay at ambient temperature.
Additionally, there is a schematic for the fan motor. This can be run using the arduino so that the fan doesn't run continuously, or with pulses to conserve power.
Step 9: Growing
Once the full module is put together with the sensors, circuitry, and attachments, it is time to grow!
Use the arduino to monitor the moisture of the sponges and stay on top of watering, and use the light sensor to control the energy received by the plants. Run water to the plants in hard to reach places using hoses and needles. Fertilizer can be mixed into the water before watering.
Harvest the microgreens when you want. Modules can be pulled and the microgreens removed without disturbing the other modules, so harvests can be staggered over time to give continuous greens.
Step 10: Citations
This was done using information from a number of sources, so I will be posting some of them here.
Mickens, Ph.D., Matthew & Skoog, E.J. & Reese, Laura & Barnwell, P.L. & Spencer, L.E. & Massa, Gioia & Wheeler, R.M.. (2018). A Strategic Approach for Investigating Light Recipes for ‘Outredgeous’ Red Romaine Lettuce Using White and Monochromatic LEDs. Life Sciences in Space Research. 10.1016/j.lssr.2018.09.003.
Paz, Maria & Fisher, Paul & Gómez, Celina. (2019). Minimum Light Requirements for Indoor Gardening of Lettuce. ua. 4. 10.2134/urbanag2019.03.0001.