Nest Conversion for Minisplit

Google nest devices are only compatible with central units of air conditioning systems. I decided to make a conversion to be able to use them with my minisplit AC. This conversion is a journey since the idea all the way to the installation procedure.

If we do some research on automating air conditioning systems for home, most of the available products are focused on central units. These units are mainly used in countries where the standard is to have this big giant device in the roof or in the basement and that device is responsible of injecting cool air or hot air during winter.

So central units are controlled from inside the house using other devices called thermostats, these thermostats are the ones doing the magic trick, they tell the central unit if they need to inject more cool air or reduce the air injection or even stop it completely, some of the most famous devices are the Google Nest and the Ecobee, these are great neat devices whose technology is only applicable to the central units.

Now, on the other hand, some of the most modern ways of cooling and warming a room is by the usage of minisplits, minisplit technology makes in a more affordable way to cool down a house because it only focuses on 1 room and not the entire house, however, the problem here is that if you use a minisplit, the temperature selected is not really the real temperature and sometimes if you are sleeping, you just get too cold and so most of the people have the same comments about using the minisplit technology, you cannot know exactly what is the current temperature, how much time it takes for the appliance to reach to the desired temperature and finally how to enable the minisplit to be able to be more intelligent and wifi enabled.

• Chisel
• 24v transformer, like the one in the picture, with central deviation
• Extra cables
• A switch to turn on/off the nest installation
• Additional screws for fixing the nest with the nest-backplate

It all started when I naively thought a nest could be connected to the Minisplit appliance, most of the people also think that way because in the ads you can see that it controls the temperature and lets the house be at the most comfortable temperature you feel is right for you.

However I quickly realized that the nest is designed for another type of Air Conditioning system technology. So I went hands on, my main point was to avoid in any way modifying the nest, avoid openining and avoid altering it in any way, the original idea is to let it run out of the box by just connecting it just as the original ads but with the big difference of using it with the minisplits.

Once I knew that I needed to create a bridge, the only way of make them communicate between themselves is via IR, so I opened eagle and started adding the necesary IR components. After this I quickly realized I needed to start making some prototypes in order to see how to turn ON the Nest thermostat. Once I got it turn on I then continued with selecting some of the other pieces needed to interface with my now in the works, board.

Some of the most difficult challenges is that this is a 24vac supplied device, it's not something common and small glitches can break the entire prototype, actually it happened at least 6 times that I ordered a total of 30 prototypes, with the help of Autocad Eagle I was able to take the prototype to a stable production stage.

Some of the challenges I had were:

• Size matters, I wanted to be able to attach nice and simple the adapter, so i went for a matching size for Nest 3rd generation and then on a second round for the Nest Snow size, which is smaller. The size is important because you want to make sure the board is running, is accesible for reset/restart and possible maintenance, that's how the circle shaped came in, with enough space to be "hidden" or not entirely visible and also to make it "shaped-compatible" with the original nests shapes.

• Components, If you see the image I used a ESP8266, this is a device that let's you run a ton of code with WIFI capabilities, however for my case I noticed that I was over charging the Microcontroller with more tasks so I took some days to decide if I was in the correct path, after a couple of days of refactoring the code, debug and minimizing routines I noticed I achivied my intention, make it solid for running the communication with the nest interface and keep the wifi connection also ON.

• Power supply, I really tried to make a propper power supply conversion as stable as possible but I noticed some components were missing some additionally quality, I had to find a propper supplier to be able to test the power supply and be as stable as possible, that's how the power supply stage is taking more space in the board.

• IR communication, managing the data transmission from the board to the minisplit is hard, that looks like I came up quickly with the solution but in realiy I had to find the proper balance between SMD leds and normal 5mm IR Leds, and not only that I also noticed some AC units pull out something I called "heavy" air or something that somehow doesn't let the IR data transmission to get to the receiver of the minisplit unit, some appliances do require even more powerful Leds and so I also tested 850nm IR leds and 940nm. This particular point was a pain and tooke me quite sometime to came up with a conclusion but up to this day, I recommend if you use one of my boards to point the led in the direction of the minisplit, most of the interactions have better results this way.

• SMD chips, I opted for as tiny as possible chips to use, the idea is to have a reduced board, so, in the beggining for instance I used an IR receiver that needed soldering leg by leg, but now I made the transition for SMD as much as possible, at some point I used IR smd leds, but noticed that sometimes it is better to decide thinking about performance.

After my initial experiments I finally had a prototype which then I proceed to test, my surprise was so big and almost went into tears :') when I saw it working, with some little software glitches, but working, I took some time to test it and let it run.

BTW the renders, animation, design and all related to the simulation has been with the help of Fusion360, I do need it as a must to avoid some errors that I know for a fact can be seen before going into the real production of the board.

I started to add some more interesting features, I will enlist them here:

• You can select from a predefined list of Minisplit brands
• Or you can use your remote so that the adapter tells you what brand is suitable for selection
• You can also configure it manually, in case no brand is compatible, you can make capture the cool/heat commands directly from your remote.
• It supports homekit, you can control your minisplit with homekit
• You can record a TV command, so not only can you control your minisplit, you can also control a TV (only for ON/OFF)
• And finally you could record another minisplit, you 1 nest can control 2 minisplits beware they need to be in line of sight

This is the process I wanted to achieve, be able to unbox a nest thermostat and wire it as in the original instructions. For the people not familiar with thermostat terminology there are several letters representing the temperature.

Most of the thermostats, the ones with all the available temperature stages available will have these letters, if you have a thermostat that doesn't contain all, don't worry, this guide still works for you.

• C this letter is one of the 24vac output legs
• R/RC/RH the other output leg of the 24vac transformer goes here
• Y1/Y this represents "cool air please"
• Y2 this represents "even cooler air please", beware not all the systems have this, if you do, it means your thermostat is able to switch even lower the temperature.
• W1/W this means "warm up please"
• W2 even warmer air please

Most of the people that are used to using the Minisplit system select the "temperature" directly on the remote and the temperature changes in the screen however the thermostats work in a more efficient way as seen in the table above. By just letting you know it will start to cool down the room and when it reaches the temperature it will stop the minisplit from working, thermostats do this as a way to save energy.

Once the thermostat is wired and ready to connect to the wall, a 24v transformer is needed, remember that as mentioned before the Nest thermostat needs 24vac in order to boot up, that's where the transformer comes in, the reason why the adapter also receives this high voltage is to take energy from the same transformer and feed itself then it manages the energy back to the nest thermostat which finally will enable the full nest thermostat capabilities as shown in the video.

So, I previously mentioned that this adapter has homekit features. Homekit is basically the name of the protocol to interact with smart homes based on Apple's specification. Homekit is only available on Apple's products through the app "Home" very similar to the app "Home" from google but it is much more integrated into the device, the laptop and the TV.

So, even though you can now control the thermostat with the "Nest" app you can also control it with the Apple's home app, giving you more freedom in terms of temperature selection, when the adapter is used with the nest, it can only control 4 points of temperature as explained int he previous points, but opening homekit you can go simulate a remote with more options. Here in the image you can see several "nest" titles which represent the homekit enabled thermostat and how it looks when opening the cockpit for temperature interaction.

First of all, you might be wondering if you could be able to buy this adapter and yes I put it on for sale on tindie, I realized that other people were also looking for this solution so, you can buy it from here:

https://www.tindie.com/products/beckmx/minisplit-adapter-for-nest-thermostat/

That would let me keep researching and improving other devices as well.

I started this project some time ago but I took some time to make a real testing and correct as many bugs as possible, all these weeks I have loved how well the nest installations have worked so far.

Some great advantages I have got after using this technology is:

Nest has learned from my room/house and is able to manage the temperature very well

At nights I don't wake up to turn on/off, lowering down, or increasing the temperature, all that is gone.

If i forget to turn off one of the minisplit units I know the nest system will take care of that activating the "away" and "eco" temperatures, which basically mean no energy waste.

I can have several Minisplit units runing and working in parallel, at some point they go off and energy is heavily reduced because of this.

Thanx for reading this long story! and thanx for your support :) heavily appreciated