Introduction: Zero Gravity Grow Box

The following is a design for a micro-gravity grow box. I had no thought to make such a thing before I saw the Growing Beyond Earth contest. Space travel however always catches my attention, and this sounded like a fun, challenging, and perhaps useful project. I can sympathize with space travelers wanting something other than "camping food". This is in the Professional contest category.

My design adheres to the stated contest limit of a 50 cm cube. It assumes ambient air and temperature is suitable for plants and that 12 v (8 amps) power will be available. It is also assumed that water can be manually added every few days. The grow cube should not require monitoring more than once a day. Most monitoring can be done via the smartphone app connected to the grow box via Bluetooth.

What may be unusual about this design is that the plants grow out of a rotating tube at the center of the cube. Lights are on adjustable panels in corners surrounding the grow tube. Four walls of the cube are mirrored to maximize light distribution to the plants. To simulate zero gravity on Earth and as a proof of concept, the tube is constantly turned. In this way each plant spends as much time “upside down” as “right side up” (1G -1G = 0G). This also shows that the grow box design elements are not gravity dependent (e.g. containment of water). The grow tube is filled with a growth medium and the plan is to keep it quite moist all the time. Water is automatically pumped in from a reservoir as needed. Since roots will be kept quite damp, air is also injected into the tube to aerate the plant roots.

The system is controlled by microprocessor within the cube. Air temperature and humidity are monitored. Soil moisture level is monitored and automatically adjusted. The balance between blue and red light can be controlled and varied to suit the type of plant.

Step 1: Grow Box

The entire system is contained within the cube (except for the rotation motor and light locator adjustment pegs.

  • The grow box is constructed of acrylic sheet.

  • Four sides are 1/8 inch mirrored acrylic, and two sides are 3/16 inch clear acrylic.
  • The mirrored acrylic pieces are painted on the back to prevent moisture damage to the mirror coating.
  • One the edge of the door the electronics and pumps are mounted. Adjacent to this area is a 5 sided small box to contain the water reservoir bag..
  • ½ inch square strips of acrylic are glued to the side edges and the box is assembled with stainless 4x40 screws, tapped into the acrylic though the box sides. 4x40 stainless screws are used throughout the build.
  • The intent is that the unit can be taken down and stored flat (more or less) by removing the screws.
  • One of the mirrored sides is hinged and the "door" can be swung open for manual access. The door is secured in closed position by Velcro tabs.
  • At the bottom of one of the clear sides a fan is mounted to provide input air and to regulate temperature. At the top of the opposite clear side is a cluster of holes for output air. The box itself is not airtight.
  • Aluminized (mirrored) Mylar sheets may be draped (affixed by tape) to the clear sides to further reflect/seal in the light.

Step 2: Grow Tube

The grow tube is made of 4” corrugated drainage tubing, mounted with a PVC cap on each end, with plants to be grown on all sides of the tube. Fourteen, one inch diameter holes are drilled in the tube and “rapid rooter” plugs are inserted in the holes. The plugs hold the seeds, provide a medium for initial growth, and keep the growth medium contained.

The grow tube is rotated continuously, back and forth, giving all plants an equal exposure to lights and equal time with the various angles of Earth gravity. Rotation of the tube back and forth is done automatically via a DPDT (double pole, double throw) toggle switch. As the rotation reaches its maximum, a projection on the 3d printed adaptor pushes the switch, reversing the current to the DC motor and causing rotation back the other way. Continuous rotation in one direction is not possible because of the tubing and wires connected to the grow tube.

The grow tube has within it a ½” ID (inside diameter) soaker hose held in place by 3d printed ends screwed into the end caps. This larger soaker hose is surrounded by potting soil. Within the soaker hose is a smaller ¼” ID soaker hose which introduces water to the system. Within it also is a ¼” poly tube which introduces air to be forced through the soil (injected).

Components of the grow tube are:

  • 4” drainage tube corrugated, solid, 18 inches
  • End caps for 4” tube – x 2
  • ½” ID Soaker hose - 18 inches (held in center of 4” tube and containing smaller soaker and air hose)
  • ¼” ID Soaker hose – 16 inches (watertight connection to water poly tube, plug at end. Small holes drilled throughout tube to relieve backpressure)
  • 3d printed ends (to hold larger soaker hose in center) x 2
  • 3d printed adaptor for motor (screwed onto end cap. Engages motor shaft and hits reversing switch)
  • ¼ inch OD poly tubing
  • Collar cut from sheet plastic to keep poly tubes away from plants
  • Misc. fasteners

Step 3: Lights

Everyone has an opinion of what intensity and what color lights are best for plant growth. The approach of this design is to allow for experimentation. Intensity is managed by changing the distance between plants and lights. The approach to the color conundrum is to provide options of red with white, and/or blue with white. Red and blue are on separately controlled timers.

  • There are strips of LEDs in each corner.
  • In the corner with the electronics and water reservoir, shorter strips of lights are in a fixed position.
  • In three other corners, LEDs are mounted on adjustable mirrored platforms. This allows lighting distance to be varied as needed.
  • Adjustable light platforms are held in varied positions by pegs shoved in from side panels and secured by Velcro tabs.
  • Light platforms are 9" by 16" 1/8" mirrored acrylic, painted on the back to prevent moisture from damaging mirror coating. Packing tape on both sides form hinges for the platforms.
  • Acrylic pegs that secure the light platforms are 3/8" diameter rod glued into 3/16" acrylic pieces (about 3/4 by 3").
  • How long the lights are on is user controlled by color. A preponderance of red light is said to encourage flowering, while blue light encourages growth of leaves.
  • The red circuit controls "full spectrum grow lights". These are 4 reds to one blue. The blue circuit controls blue and white lights
  • The mirrored walls on all sides help distribute light and make light available are all angles. It remains to be seen if light from all angles is a help to the plants.
  • The mirrors also reduce absorption and leakage of light, making the system more efficient
  • The LED light strips consume most of the power of the system. An 8 Amp 12v source is used initially.

Step 4: Water

  • A liter of water is held in a reservoir bag (catheter bags are perfect for this).
  • The water reservoir bag is located within the grow box inside a 5 sided acrylic holder and secured with elastic straps.
  • The bags have a port with one-way valve on one side where water could potentially be added. This port however seems to leak a little and I've sealed these with a stopper.
  • As an alternative to refilling the reservoir in place at either valve, it's much simpler to substitute another full bag.
  • There is a port on the other side with a closable valve. This valve can be closed for refilling or when substituting a full bag, but is kept open to the pump when the grow box is in operation.
  • The pump is a 12 volt dosing pump controlled by relay via micro-controller.
  • A length of poly tubing leads to the grow tube where a small soaker tube is placed in the larger porous tube in the center of the grow tube.
  • The soaker tubes (one inside the other) distribute the water evenly through the length of the grow tube.
  • A moisture sensors are pushed into slots cut into the tube on opposite sides.
  • When moisture falls below level set by the user, water is pumped from the reservoir into the grow tube.

The procedure for changing the water bag is as follows:

    • 1. Examine controls to ensure moisture level will not be calling for more water during this change (temporarily reduce desired moisture level if necessary.
    • 2. Unfasten elastic straps.
    • 3. Close valve (turn blue lever so it is parallel with white nozzle)
    • 4. Remove black poly tube from white nozzle
    • 5. Replace empty bag with full one
    • 6. Replace black poly tube shoving it in as far as it will go while valve is closed
    • 7. Open valve
    • 8. Re-fasten elastic straps. Tuck stoppered port out of the way.
    • 9. Set desired moisture level if necessary.

    Step 5: Air

    • As stated in the overview, it is assumed that ambient air quality and temperature will be maintained. Specifically, this means that O2 & CO2 levels will be at levels suitable for plant growth and need not be monitored by this system.
    • A 12 volt fan brings in outside air. In Automatic mode, it turns on when air temperature is greater than 80F and shuts off when temperature is less than 75F. Via the Android app, the user can select if the fan is On, Off, or Auto mode, the default.
    • If necessary, a filter could be mounted on the fan so all incoming air would pass through it. This would allow for filtering of ethylene or other contaminants harmful to the plants.
    • Air is continually pumped by a 12 volt pump into a length of poly tubing inserted halfway into the larger porous tube. This air pump is always On.
    • Air bubbles are distributed via the porous tube and forced through the surrounding wet growth medium.
    • The intent of the air injection is to oxygenate the growth medium to stimulate growth and to prevent rot in roots that are kept very wet.

    Step 6: Soil

    My initial approach was to use potting soil inside the grow tube. I thought it would have the advantage of providing nutrients without the need for nutrients added to the water. But I discovered the peat moss grow plugs required nutrients to be added to the water in any case. Also, the soil was messy and very heavy when wet. So after the first batch was completed (baby lettuce), I replaced the potting soil with "stone wool" cubes.

    • Stone wool cubes - Gro-dan brand "grow cubes".
      • These little cubes (about 8mm cube) are very light when dry,
      • Retain their size when wet (though they may be compacted),
      • Seem to transmit water much better than potting soil.
      • Don't fall out of the holes when grow plugs are removed
    • Hydroponic “rapid rooter” plugs.
      • Seeds are poked down into holes about ¼ inch. The grow plugs hold the seeds in place, providing a sprouting environment with nutrients and moisture.
      • They also plug the holes in the grow tube, containing the growth medium within.

    Step 7: Electrics and Electronics

      Electronics (except for soil moisture sensors) are mounted near the hinge edge of the "door" panel. Air pump and water pump are mounted there also. Electronics and electrics are:

      • Arduino Mega
      • Temperature/humidity sensor - DHT21
      • Soil Moisture sensors – CYT1033 x 2. Readings are averaged.
      • Relays – bank of 6 – 5v (Need more power on the 5v side than the Mega has available, hence..)
      • 12v to 5v converter (to power 5v side of relay bank)
      • LED Strips –
      • Bluetooth module – HC06 to connect to Android
      • Android smartphone
      • Geared motor -12v DC – 7 RPM. (Slower would be OK, but this was in my "inventory")
      • DPDT toggle switch (hit by motor adapter to reverse rotation)
      • 12v DC power supply (8Amp)
      • Dosing pump – 12v DC
      • Small pancake fan – 12v DC
      • Air pump – 12v DC

      So far there has been no problem with water interfering but it's possible that the design could include a plastic cover to isolate from the grow box interior. This would also require ventilation holes through the door.

      Step 8: Software

      Arduino Program

      The following describes the functions performed in the Arduino Mega:

      • At startup, retrieves system hour and user set values from non-volatile memory.
      • Activates relay to keep tube rotation turned on unless phone connection turns it off.
      • Activates relay to keep air pump turned on.
      • Checks soil moisture level.
      • If moisture level is below user specified criterion, turns on water pump for xx seconds, then waits for yy seconds before allowing pump to come on again (so water has time to soak in and get to sensor). xx has evolved to 10 seconds and yy is 5 minutes.
      • Keeps track of elapsed time (not clock time).
      • At each hour boundary updates System Hour and checks to see if lights need to come on or off.
      • At each day boundary, when System Hour rolls over from 23 to 0, it adds to day counters.
      • Checks the air temperature and humidity
      • Turns on and off outside air fan to keep temp between 75F and 80F,
      • Every two seconds looks to see if any data has come in from phone.
        • If rotation is toggled, activates relay accordingly
        • If System Hour is reset, sets it to zero.
        • If Days in Phase is reset, sets it to zero.
        • If Days from Start is reset, sets it to zero.
        • If light hours values are changed, checks to see if lights need to go on or off.
        • Changed data is written to non-volatile memory
      • Every two seconds writes via Bluetooth connection to update phone data:
        • Grow Box ID
        • System Hour
        • Red On/Off indicator
        • Blue On/Off indicator
        • Grow Tube Moisture Percent
        • Air Temperature in F
        • Air Humidity Percent

      Android Program

      I used App Inventor 2 for this.

      • Requires the user to select a Bluetooth connection and reports unique Grow Box ID when connected.
      • When connected, retrieves user-set values from non-volatile memory.
      • Allows user to disconnect from Bluetooth connection (to connect to different Grow Box).
      • Every second, if data is available from the Arduino it reads the data.
      • Displays System Hour. This is not related to any standard clock time. It shows internal time used for lights on or off and advancing to the next day. Allows reset to zero.
      • Displays a “heartbeat” of alternating colors to indicate communication with the Grow Box is active.
      • Allows the user to specify the number of hours per day grow lights are on. Red and blue have separate controls. (increments of 4 hours).
      • Indicates whether red or blue lights are currently On or Off
      • Reports Tube Moisture percent and allows the user to set the threshold below which the pump will come on to deliver more water to the tube (increments of 5%. 0 keeps the pump off)
      • Reports Air Temperature inside the grow box.
      • Reports Air Humidity inside the grow box.
      • Allows user to toggle rotation of grow tube on and off (for harvesting, close inspection, etc). Rotation will resume automatically on disconnect or exit.
      • Allows user to override the air fan default mode of Automatic to turn it On or Off.
      • Reports number of days since a grow phase began (e.g. to keep track of sprouting time or when water bag was changed) and allows reset to zero.
      • Reports number of days from start (e.g. to keep track of time of entire grow cycle) and allows reset to zero.

      Step 9: Reloading the Grow Tube

      The following steps are used to re-load the grow tube to begin a new growing cycle

      Clearing out the old

        • 1. Remove power from the system by unplugging the 12v connector
        • 2. Remove moisture sensors.
        • 3. Turn off water reservoir valve and disconnect air and water poly tubes (easiest way I’ve found is to use diagonal cutters and carefully clip away the tube end from the barbed connectors, sacrificing ¼” of tubing).
        • 4. Helpful hint here is to leave the barbed connector on the grow tube side on one of the poly tubes. That way it's easy to know which poly tube is which when you go to re-attach.
        • 5. Flex the motor side away from the tube to disengage the adapter, pull to the side, then pull the tube out of its pivot on the opposing side
        • 6. Remove both end caps and make sure all old growth medium, roots, and grow plugs are removed.

        Loading the new

          • 1. Replace the end cap with poly tubing ensuring that the inner soaker hose and air tube are inserted in the larger soaker hose and that the larger soaker hose is inserted securely in the 3d printed end screwed into the end cap
          • 2. Put a small cork into the end of the larger soaker hose to prevent growth medium from getting in.
          • 3. Introduce growth medium (rock wool) into the grow tube. On Earth, gravity is helpful, but in zero-G, this step could require pushing it in manually. Ensure the tube is completely full.
          • 4. It may be helpful to pre-wet the growth medium as it is introduced to the tube. I didn't find it necessary, but if the growth medium wants to escape from the holes, you might temporarily cover the holes with tape.
          • 5. Remove the cork and put on the end cap to cover the soil, making sure that the soaker tube is held in the center.
          • 6. It may be easier to plant seeds at this point since the tube is free to manipulate. Insert 2 or 3 seeds into each grow plug hole and poke them in about ¼ inch. Too deep and they can't see the light and gravity can't help them. Wet the grow plugs so the seeds will stick in place.
          • 7. Insert a new set of grow plugs into grow tube holes. Push and twist the grow plugs into the grow tube so only about ¼” shows.
          • 8. Replace the grow tube in the box by first reengaging the pivot, then flexing the motor side out so the adaptor can fit over the motor shaft. Turn the tube by hand until the motor shaft engages with its flat spot.
          • 9. Inset moisture sensors in their slots, taking care that wire will not be pulled tight during rotation.
          • 10. Re-connect the air and water poly tubes pushing the connectors in tightly. Take care here also that poly tubes will stay behind plastic collar and not be pulled tight.
          • 11. Re-connect 12v power and ensure that it comes to life.
          • 12. Use the Android app to set the desired amount of red and/or blue light.

          Step 10: Results to Date

          My intent is to update this section periodically, reporting on what worked, what did not, and changes made.

          1. After 2 days of keeping the box warm (mid 70'sF using red lights only.) baby lettuce sprouts have begun showing on 7 of the 14 plugs. With the sprouts showing I've switched to red and blue.. I'll try 16 hours a day.
          2. After 3 days, sprouts are still showing on only 7 of the 14 grow plugs. I put more seeds in the "empty" ones, not going beyond 1/4 deep.
          3. After 4 days, I see signs of life on some of the laggards. The original 7 are leggy though, making me think there isn't enough light. I'll try 20 hours a day. I added two strips of UV LEDs to the blue light circuit to make the light more intense.
          4. 9 days in there are shoots in almost all the plugs. But they're all rather leggy and not growing leaves. I added 4 strips of white LEDs to the red light circuit. The whole system now pulls 4.8 Amps, 0.6 with all the lights off. Back to 16 hours a day.
          5. After 13 days, the white lights seem to have helped. The shoots that survived are growing green leaves!
          6. Water usage: I've been changing the reservoir bag every two days since it's looked saggy and I didn't want to run out. But the "empty" ones I remove still have between 400 and 500 ml remaining. I suspect at this stage of growth it could go for three days before a filled bag is needed.
          7. Day 14 & 15 - I've updated the overall design to have adjustable panels in three of the corners so light distance can be varied. I added a strip of white LEDs to 3 of the panels and got rid of the UV LEDs The white LEDs are 6000k white (daylight, not "warm"). The system with lights on now pulls 5.5 Amps.
          8. Day 19 and the survivors don't look big enough to qualify as lettuce. I doubt the roots have gone past the grow plugs into the soil, and therefore are not getting any nutrients. The grow plugs are only peat moss plus a little sponge binder. So - I've added some hydroponic fertilizer to the mix. According to the directions it's one teaspoon to a gallon of water which translates to 1.32 ml per liter. I put that into a refilled reservoir and I also dribbled some on the survivor grow plugs.
          9. Day 23. The survivors are beginning to grow nicely so I left them in. I planted red lettuce seeds in the "empty" plugs. I also put three plugs with red lettuce seeds in a flooded tray on the bottom of the grow box as a sort of control, to see if they sprout any differently from the rotating ones. They are not facing the lights but have the same concentration of liquid plant food. I reset Days from Start to zero.
          10. Day 10. The original baby lettuce greens aren't growing any more. My assumption is that this is as big as they get. So it's harvest time. I snipped them off, leaving stubs to see if they will grow from there. I had a small salad.
          11. Day 0. I cleaned out the grow tube and replaced potting soil with rock wool (Gro-dan grow cubes). Planted red-lettuce seeds in new grow plugs, and started a whole new batch.
          12. Day 8. All 14 grow plugs have viable sprouts. I moved the lights in closer in hopes of speeding up growth. Water reservoir only needs changing after 3 days. It strikes me that if I were do build the grow tube again, I'd drill 6 rows instead of 4, increasing from 14 to 21 plants. We'll see how crowded the lettuce gets with 14.
          13. Day 9. The lights are too close, drying out the plants and toasting them. I replaced two of them and moved the lights back out to medium.
          14. Several days later...
            1. The plants were not growing and looking very poorly. I speculated that continuing to put water WITH fertilizer into the medium was probably resulting in an unhealthy high concentration of fertilizer. I pulled em all out and started over again.
            2. This time I used a dry mixture (about 1/2 and 1/2) of potting soil and rock wool chunks. I soaked the grow plugs in a water fertilizer mixture so the sprouts would have something to go on. I reloaded the water reservoir with straight tap water.
            3. Out of curiosity, I measured PPM (parts per million) dissolved salts of the liquid I squeezed from the used rock wool, then from the fertilizer mixture I had been using, and then tap water. The numbers were 1150, 875, and 65 respectively.
            4. According to a chart on the web, seedlings want 350-450 PPM, early growth 800-1050, and later growth 1000-1400. So maybe over-fertilizing was NOT the problem. I had turned the lights back to 8 hours a day and maybe that wasn't enough.
          15. Several weeks later...
            1. Lettuce plants got more and more leggy (all stem and little leaf) indicating not enough light.
            2. I noticed LEDs were dim. Voltage was fine so I compared to unused strips and saw the old ones were "worn out". The white strips were the most noticeably bad.
            3. I tore out all LED strips and replaced each corner with 2 strips of "full spectrum" grow lights (4 red to 1 blue), 1 strip of white and 1 strip of blue. Full spectrum is wired to the red circuit. Blue and white wired to blue.
            4. Started over with rock wool cubes inside the tube, grow plugs with 2 red lettuce seeds each, and each liter of water with 1 1/2 ml of hydroponic nutrient.
            5. Reset counters to zero.
            6. Replaced moisture sensors with Vegetronix sensor. Old sensors were not reliable. I'm hoping that spending $40 for a sensor will help.
          16. Weeks later...
            1. I was one of the "winners" in the contest.
            2. I started over with a grow tube with 21 holes instead of 14. And I made the holes 1 1/8 instead of 1" to allow the grow plugs to sit lower and have less medium exposed to air.
            3. I inserted the Vegetronix moisture sensor in one of the grow plugs instead of in a special slot at the end of the tube. This is an effort to get a more representative reading. This reduces capacity to 20 plants.
            4. Sprouts showed after 3 days in most grow plugs.
            5. Utility power failure while we were out of town coupled with poor programming caused plants to go without light for 3 days. When we returned they were very leggy so I pulled them out and started over using the same grow plugs, putting two seeds in each.
            6. After 3 days, there were sprouts in all.
            7. After 10 days there were good leaves on all 20.
            8. At day 13 the Arduino Mega power circuit died due to corrosion. This would have to be prevented somehow for actual use. I replaced it with another Mega.
            9. At day 17 I received a link to the spreadsheet we were to use for Phase II. A little late for my use. Plants are growing nicely but don't look like lettuce yet.
            10. At day 23 the plants are marginally bigger, but still don't look like lettuce. I have a few control plants in a dish at the bottom of the box, and they are perhaps even smaller than the "zero-G" plants. My theory is that there is still not enough light, even at 16 hours a day. Harvest weight of 18 surviving plants (not including control plants) is a measly 134 grams. This is as far as I go with this project. Disassembly of the grow box is scheduled for today.
          Growing Beyond Earth Maker Contest

          First Prize in the
          Growing Beyond Earth Maker Contest