Automated terrarium cooling

[schmiggle]

I recently acquired a mini fridge for free, and would like to break it down and recycle the functional parts to cool a terrarium (for highland plants). However, I don't want it to just stay on the whole time, since I think it will get to cold and draw an absurd amount of power. I figure it would be a pretty simple arduino program to hook it up to a timer and have one temperature threshold during the day and another at night. However, does anyone know how hard it is to hook up an arduino to that kind of set up instead of the controller built into the fridge? Before anyone gets snarky--yes, I did Google it, and no, I couldn't find a straightforward answer. People have all sorts of solutions, so it's definitely possible, but they usually leave the fridge intact.

 

[Introvertebrate]

I think the frog guys dendroboard have experimented with Arduino based climate controls.

 

[Hardus nameous]

I would imagine you could just replace the thermostat with an arduino controlled switch, but I've never tried this so I can't say for sure.

You could probably bypass any defroster circuit too.

Do you have a wiring diagram or a make/ model of the fridge?

 

[l4nsky]

Hola,

I dont think you necessarily have to go the computer controlled route if you dont want to. They sell freezer temperature control kits, commonly used to turn chest style freezers into keg refrigerators. It's basically a reverse rheostat.

https://www.amazon.com/Johnson-Cont...eg+freezer&dpPl=1&dpID=41zSzIcUocL&ref=plSrch

How big of an enclosure are you cooling by the way?

Thanks,
--Matt

 

[schmiggle]

I think the frog guys dendroboard have experimented with Arduino based climate controls.

Cool, I'll check that out.

I would imagine you could just replace the thermostat with an arduino controlled switch, but I've never tried this so I can't say for sure.

You could probably bypass any defroster circuit too.

Do you have a wiring diagram or a make/ model of the fridge?

This is good information--thank you. I don't have the fridge by me right now, but I'll post the make and model later tonight.

Hola,

I dont think you necessarily have to go the computer controlled route if you dont want to. They sell freezer temperature control kits, commonly used to turn chest style freezers into keg refrigerators. It's basically a reverse rheostat.

https://www.amazon.com/Johnson-Cont...eg+freezer&dpPl=1&dpID=41zSzIcUocL&ref=plSrch

How big of an enclosure are you cooling by the way?

Thanks,
--Matt

I didn't say in the OP, but I would like to automate humidity and lights as well. However, I have a better idea of where to start with those, since right now they're totally manual. The arduino should be able to handle all three with appropriate sensors and timers, since it's three simple input-output, threshold based programs.

The enclosure at the moment is an 18" cube, around 30 gallons, but eventually I might use something shorter and wider, since that's a fairly inefficient shape for what I'm trying to grow (with the exception of Nepenthes tenuis and, with some luck and many years, Nepenthes aristolochioides).

 

[l4nsky]

Hola,

Are you dead set on the arduino or would you consider a Raspberry Pi? There is someone who has a step by step guide for this project with Nepenthes here: https://www.carnivorousplants.co.uk/resources/raspberry-pi-terrarium-controller/

The one thing I didnt see was the mention of refrigeration as cooling. I haven't dug into it too far, but I would think a simple relay controlled outlet tied to a temp sensor would do the trick. Directions here (also RBP dependent): https://web.archive.org/web/2018030...v.com/raspberry-pi-vivarium-controller/relays

 

[schmiggle]

Hola,

Are you dead set on the arduino or would you consider a Raspberry Pi? There is someone who has a step by step guide for this project with Nepenthes here: https://www.carnivorousplants.co.uk/resources/raspberry-pi-terrarium-controller/

The one thing I didnt see was the mention of refrigeration as cooling. I haven't dug into it too far, but I would think a simple relay controlled outlet tied to a temp sensor would do the trick. Directions here (also RBP dependent): https://web.archive.org/web/2018030...v.com/raspberry-pi-vivarium-controller/relays

This is excellent information, thank you! Don't care that much about arduino vs RP, but I've read that RP has an operating system and whatnot, which seemed like overkill. Should still be fine though.

 

[l4nsky]

This is excellent information, thank you! Don't care that much about arduino vs RP, but I've read that RP has an operating system and whatnot, which seemed like overkill. Should still be fine though.

It is overkill in most situations, but it allows for an easy potential expansion of the system should you ever wish to include watering, data logging, cameras, soil ph, etc in the future. Arduino's can handle that just fine, but I always prefer the power of a small computer vs just a controller.

Thanks,
--Matt

 

[schmiggle]

Well, I took apart a fridge today. I decided to replace my old one with the nicer newer one and break down the older one for use in the terrarium.

Well, six hours of sweat and a couple drops of my blood later (80% of the work was done by a friend who was helping, but he owed me--that's another story lol), I had a pile of insulative foam and torn plastic, and a completely intact set of refrigeration equipment. Here's the new plan.

The way the mini-fridge worked was that there was a very cold unit that also functioned as a tiny freezer. All of the heat transferred into this unit was radiated on back and sides. The compressor was at the bottom of the back (which is, I think, where they commonly are). This is not the setup I was expecting, but I think I can work with it. The temperature control survived, and I think that, instead of replacing it, I'll just set it to a low setting and tell the arduino/RP to only supply power to the refrigeration when the temperature rises above a certain threshold and to keep going until it drops down about 5 degrees lower than that.

I'm planning to put bubble wrap between the terrarium and the coils that radiate heat, but this sets up a new problem. I'm concerned that the freezer bit, which has to stay inside the terrarium for the whole system to work, will set up a fairly steep temperature gradient inside the terrarium and might freeze whatever is put near it. Should I use fans to circulate air, or just not worry about it? It used to keep the refrigerator cool, but sometimes stuff near it would freeze; on the other hand, the coolest this will ever get is probably 15 degrees warmer than the refrigerator ever got.

A separate question--I would like to automate humidity as well, as I said above. I'm thinking I'll use a spray nozzle that I use a pump to spray water through, but does anyone have an idea how strong the pump will have to be for that?

 

[l4nsky]

I would definently do fans. Even if you have the fridge innards on their lowest setting, I dont believe that changes how cold the output air is, only how long the compressor runs. You dont want one specimen to get the brunt of the cold.

Just spitballing here, but maybe you could place a 2" thick piece of foam (top to bottom with a 2 inch gap between the walls and the foam either side) between the plants and the cold coils to shield them from direct exposure and allow the ambient temperature to change slower? As far as I know, these are uncharted waters so it might just come down to trial and error.

As far as the pump, google is probably your best friend. I know for my future automation project, I'm probably just going to buy a MistKing system and use a RBP for the controller instead of the provided one. Maybe you can check the specs for one of those systems to see the GPH on their pumps? Hope that helps.

Thanks,
--Matt

 

[Hardus nameous]

I would use fans to keep warm air blowing over the coils so they don't ice up.

 

[l4nsky]

On second thought, what about just using an ultrasonic humidifier instead of a misting system? You could get one of those small, hockey puck shaped ones and set it in a container of water inside the vivarium or you could get an external one and pump in the fog.

Thanks,
--Matt

 

[schmiggle]

On second thought, what about just using an ultrasonic humidifier instead of a misting system? You could get one of those small, hockey puck shaped ones and set it in a container of water inside the vivarium or you could get an external one and pump in the fog.

Thanks,
--Matt

I thought about that, but most of them are battery powered and I wanted to hook the whole thing up to the same system.

What I decided to do instead is get misting nozzles intended for use outdoors, hook them up to a pump (that, after about an hour and a half of trying to use the Hagen-Poiseuile equation, I eventually was able to work out should be strong enough), and put tubing through the whole setup. Then I'll leave the pump inside a reservoir outside the terrarium (it's an underwater pump). This has a couple of advantages: It takes up less space inside the terrarium, and I'm easily able to hook it up to a relay module. In addition, a few of them should have the added benefit of functionally watering my plants. I'm thinking I'll put the tubing around the sides of the terrarium not occupied by lights, but I might instead snake it around the lights, which I have to redo anyway. My current thought is to take maybe a 1 gallon container of water, cut off the top, put the terrarium in, fill it up, and then tape the top back on to reduce evaporation. I can even put a whole in the lid to put the tubing through and then put that on so there's almost no evaporative loss. Then when you need to add water, you remove the lid and pour it in through the top. It would only take distilled water.

When I get this working (fingers crossed!!!!) I'll post pictures and specs, along with the entire process (including disassembling the refrigerator). I'm almost positive you can't get this particular make of refrigerator anymore, but the basic design of mini-fridges probably hasn't changed much since it was manufactured.

Speaking of which, the make and model is the SPT RF-17W. I can't find it on the SPT website, because it's a very old refrigerator. I'm hoping that its infrequent use to cool the terrarium will extend its shelf life somewhat, and it was pretty solidly made, so hopefully that bodes well also.

I ordered all the materials I expect to need last night, so hopefully this will be finished within a couple of weeks.

 

[l4nsky]

I thought about that, but most of them are battery powered and I wanted to hook the whole thing up to the same system.

I've never seen a battery operated ultrasonic humidifier . I guess it's possible since all they are is basically little speakers. I use an external one with a 1g reservoir that I picked up from Walmart and modified to pump moisture rich air through a tube to my mushroom fruiting chambers. I have a humidistat that it plugs into, which is basically the same as a RBP tied to a relay outlet with a sensor, just less customizable.

Personally, I'd probably go the ultrasonic method for humidity, and save the pump for an auto watering system (relay outlet tied to a soil moisture sensor), but that's just my opinion, based on my success with them in the past.

Thanks,
--Matt

 

[Hardus nameous]

I couldn't find a wiring diagram of that model online, but I think you've already got a handle on it.

 

[schmiggle]

I've never seen a battery operated ultrasonic humidifier

LOL I think we must use different sources. I still have one lying around in a university building that was intended to be used for research.

I use an external one with a 1g reservoir that I picked up from Walmart and modified to pump moisture rich air through a tube to my mushroom fruiting chambers. I have a humidistat that it plugs into, which is basically the same as a RBP tied to a relay outlet with a sensor, just less customizable.

This didn't occur to me, and it's a good idea. The one advantage spray nozzles have for me is that if I can set the whole thing up such that it doesn't have to be watered for a couple of months, I'll feel better about leaving it with someone over the summer, which currently seems like my only option (I'm likely to be away at a job over the summer, one way or another, and transporting the terrarium temporarily would be extremely difficult). I'm imagining that the plants will be watered much like the plants near a waterfall, even though I would want to water them deeply before and after (before to make sure the media are wet, after for that reason and also to get rid of salts, which even distilled water has, I believe?) I would like to set up automatic watering eventually, but I figure one thing at a time is a good idea and automatic watering is more complicated than the other things (because I'm likely to end up with excess water with nowhere to go). Once you add in fertilizer the whole thing becomes even more painful. So for now I'm assuming a self-sustaining time frame of a couple of months max (with, of course, someone filling the pump reservoir).

I couldn't find a wiring diagram of that model online, but I think you've already got a handle on it.

Wiring was pretty simple: the temperature controller had three plugs and a temperature sensor, and the wall plug went to the compressor. The three plugs from the controller were spliced and went to the compressor as well. My plan is to hook up the main plug to a female end, stick the wiring in between into a relay, stick a male end on the other side and plug the whole thing into a power strip. That's what I'm basically planning to do with all of the automated units.

 

[The Snark]

Sounds like a fun project. I assume you know you are re-inventing the wheel, home brew and on the cheap. Infant incubators/controlled environment chambers/ICUs come in all flavors of the rainbow. Just replicate and toss the evaporator of the refrig unit in the air recirc duct.
All the environment chambers now days use arduino like controllers. So versatile they also incorporate O2 monitor, ECG and respiration sensors. All sorts of configurations are available for remote monitoring and alarms as well.
It wouldn't be too hard to contact the infant ICU companies and get their specs and schematics.
http://www.medicalexpo.com/medical-manufacturer/neonatal-incubator-2963.html

 

[schmiggle]

Sounds like a fun project. I assume you know you are re-inventing the wheel, home brew and on the cheap. Infant incubators/controlled environment chambers/ICUs come in all flavors of the rainbow. Just replicate and toss the evaporator of the refrig unit in the air recirc duct.
All the environment chambers now days use arduino like controllers. So versatile they also incorporate O2 monitor, ECG and respiration sensors. All sorts of configurations are available for remote monitoring and alarms as well.
It wouldn't be too hard to contact the infant ICU companies and get their specs and schematics.
http://www.medicalexpo.com/medical-manufacturer/neonatal-incubator-2963.html

They would just give me the designs? Those aren't trade secrets? Though I guess maybe they're patented.

People usually modify regrigerators straight up, but a. I would like to be able to see my plants without opening the terrarium and I don't want to deal with installing a glass door, b. I don't have a refrigerator with enough capacity (this whole thing is because I was given a free refrigerator), and c. installing wiring in a refrigerator seems even harder than taking it apart, which was bad enough as is.

 

[The Snark]

They would just give me the designs? Those aren't trade secrets? Though I guess maybe they're patented.

Search engine: infant incubator circuit diagram
Many many PDFs going into great detail and ways to accomplish things. Somebody has to repair the puppies. Not like it's top secret stuff.

 

[Gnarled Gnome]

How cool do you need the terrarium? Have you considered using thermoelectric cooling with a Peltier device? They are pretty cheap and I think they can be stacked. I've seen setups with heatsinks and small fans, all fairly easy. Peltier devices are solid state and use less power. Of course they also dont cool as efficiently as a mechanical refrigerator, but depending on your insulation and temp needs it may be worth it.