Mini Green House

Green house is misleading, it's a seed starter pod for the basement. I have the light, I just need an insulated box.

Ok, so I did that and I don't like it. I want to do this with LEDs.

Build

1. Get measurements 50"x18"x32"
2. Layout design, get cut list
3. Find spare timer
4. Buy insulation
5. Cut insulation
6. Assemble
7. Add blower
8. Gather temp data

Heater Circuit

Only if necessary. Or if I really want to. But don't let it be an impediment to something good-enough

Use an RTC module and an Arduino. The RTC module will wake the Arduino every 10 minutes. If the temp is too low, it will turn on a small heater. It will also check the time and turn the light on or off as needed.

The heater can be pretty simple, just run a few volts through a power resistor and blow a small fan over it. At 5v and 0.75A I can use 8x 50 Ohm 1/2W in parallel That will handle 4 watts.

The light is a bit more of a challenge because I need a latching power relay. I also need a way to check the state of the light, not hard, but there's a few ways to do it. It might actually be easier to hook a servo up to a light switch.

Components

• Blower to circulate air
• Heater
• Light control

Build

• Start by just building the box - the light might be enough to keep the plants warm
• Add a fan - the air movement helps build stronger stems
• If the temperature is getting too low, add a circuit.

V2

Buy

• LEDS
• 2xPower resistor
• Power supply
• 12xLM317 current regulator

Owned

• Arduino
• Fan
• RTC
• Power mosfet

All electronics go inside the box. I don't need to worry too much about efficiency because heat is an issue.

Structure

Just foam insulation. I should probably cover the walls with aluminum foil to reflect the light.

The grow area is about 14"x14", though I need to check if that's enough. The insulated box will be larger with a section off to one side for the electronics. There will be a thin, non insulating wall of aluminum foil separating this section.

And since this is not in use most of the year, it should have a way to break down for storage.

Power

I'll clearly need 100W. 120w @$12, 240w@12v:$18, 2402@24v$12

I'll need a buck to drive the arduino.

LEDs

I can get an LED panel for about $25, but I'm not sure I trust those.

Plants have A and B chlorophyll see here

• A: 430nm and 662nm
• B: 453nm and 642

https://www2.mcdaniel.edu/Biology/botf99/photo/p3igments.html

So I want LEDs with peaks around those values. They should use about 50-80 watt/sqft

• 440nm blue led 1w : $4.00/50
• 660nm red 1w : $10/50

$14 for LEDs, not bad. And that 100w So about 14"x14"

The forward voltage is different, 3.2v for blue and 2.1v for red. But the current is the same 280-350mA.

So if we're at 24V, we can use 4 of each for 21.2V and then the regulator will need to drop about 3V. At 350mA that's just over 1W, I don't think that will be hard for an lm317. And we'll have 12 of those. We can use one mosfet to turn the whole thing on.

Water

I'd like to setup a water reservoir that operates like a pet water bowl. Probably with something like a 2L as the tank. This not only makes sure that the plants have water, but it will also serve as a thermal reservoir.

Heat

I need a sensor, but I think I have some.

This depends on if the lights are on. If they are off, I should use more power and I may need it because the lights will generate a lot of heat. But if they are on, I need to limit the power.

I should also add an emergency cutoff - if the temperature gets too high, turn off the lights.

Heat is generated with a power resistor. If the resistor is on, the fan is on. The fan will also turn on periodically because it's good for plants.

Let's say we want 1 resistor for 100W. At 24V that's about 5Ω about $1.50.

Note: pc fan only draws a few watts, should be less than 5.

Arduino

IO (AI: analog in, DO: digital out)

• Temperature : AI
• Lights on : DO
• Fan on : DO
• heater 1 : DO
• heater 2 : DO
• RTC: 2xDIO

Interface

I'd like an LCD interface to display the data.

Total Cost

• LEDs : $14
• Power : $12
• Heaters: $3

Additional parts might be worth $15, so less than $50. Not bad.