I recently moved from the North of Tokyo to a new apartment situated 40km south but just 25min by train from my workplace. Bathroom and toilet separated, big kitchen, a proper living/dining room and an overall size of twice what I used to have.
There is 2 air conditioners to cool/warm up all that space:
- one in the bedroom, quite recent and powerfull
- one in the living/dining room, fairly old and just powerful enough
In my previous apartment, I had only one air conditioner that I controled with the dedicated remote control but also with my phone thanks to a WIFI enable IR remote control and Android app I made.

It was my first project since I moved to Japan 9 years ago and took me 7 months to complete.
Arduino, ESP8266, FUSION360, MIT App Inventor, ThingSpeak... I learnt a lot and was able to brush up my skill in C/C++ a little bit.
Only downside of this solution was that I couldn't know if the air conditioner properly received the command sent and once I used the normal remote control my system would be completely desynchronized.
For this reason, this time I want to implement a kinda of feedback so I dont' have to worry about my system being synchronized or not. I don't plan to make another IR remote or repurpose the one I built before but rather investigate to see if there is a way to talk directly with the mainboard of the air conditioner.
Moreover, I'm also digging in home automation and I plan to install a rather simple system based on Home Assistant and Xiaomi temperature sensor (Zigbee type).
The ultimate goal here is to be able to control both air conditioners directly via Home Assistant so I don't have to fear a really cold room when coming home in the winter neither to deal with an overwhelming hot apartment in the middle of summer.
Wow! cool! Can you attach a wiring diagram?
I have a modern air conditioner Hitachi (RAK18RPD), but the connection is similar to yours, I wanted to try to repeat your project for integration into HA
Thanks!