A solution for monitoring multiple air conditioning systems

This is a quick overview of a system that I developed to monitor the status of multiple air conditioning system (A/C) for a friend's company.

Background 

The friend's company has a 2-story building with over 20 rooms. Each room has at least one A/C installed. The A/Cs are controlled by the controller installed in the individual room and they cannot be centrally controlled / monitored. From time to time, the A/C will be left turned on for long period of time (over the weekend, etc.) until someone finds out about it and turn it off.

Due to safety concern, my friend doesn't want me to mess with the wiring of the A/C. He also doesn't want me to install additional powerline and power outlet around the A/C to provide power to the sensor used to monitor the A/C.

Monitoring Interface

Below are the screenshots of the monitoring interface of the solution.

A login screen is provided for user to login to the monitoring system.

An overview screen is provided for user to quickly find out about the status of each A/C (on/off status, battery level, etc.).

The on/off history of individual A/C could be viewed by clicking on the corresponding "Temp. Diff." field.

It's possible to zoom in / zoom out on certain time period for a closer look of the data.

System Architecture

Temperature sensor data are sent to the backend database via WiFi.

The backend system (Apache Server, PHP, and MySQL Database) are running on Raspberry Pi or cloud server such as DigitalOcean.

Temperature Sensor Board

The temperature sensor board is based on ESP8266. The board could support 2 DS18B20 temperature sensors for measuring the temperature at the A/C air outlet and the surrounding environment.

The sensor board could be powered by 2 AA batteries or by an AC to DC power supply. The board is programmed to measure temperature and send the measurement to the backend database at 10 minutes interval. After the data is sent, the board will go into sleep for another 10 minutes until it's time for it to wake up to take another measurement.

When the board is powered by 2 AA batteries and taking measurement at 10 minutes interval, the battery could last for about 1.5 months.

A Chinese introductory post about this system could be found at the link below:
https://wei48221.blogspot.com/2019/06/blog-post.html