Air Conditioner Measurement - Part 1 - First Cut

I was getting ready to update the hardware on my air-conditioner measurement system when I realized that I had never written about the initial set-up. So, several months after the fact, here's what I've been doing.

In my previous temperature measurement system, I was able to fairly easily measure the air-conditioner state by tapping into the relay that activated the central circulation fan. In this new house, that was not going to be so easy so I decided to try something more direct: measuring the condenser fan in the outdoor unit. I thought about tapping into the relay on that fan unit but that relay switches 240V and I didn't want to have to worry about making sure the wiring was properly protected for that higher voltage. Instead, I decided to measure the air pressure change caused by the fan turning on.

To do that I employed a BMP180 which measures both pressure and temperature.  I was able to get the sensor mounted on a little PCB and strap down an old milk jug to catch some of the air when the fan turns on, making a little pocket of slightly higher-pressure air. I wasn't sure if this was work but testing revealed the pressure increase was high enough for the sensor to clearly detect.

Air pressure capture for the fan. I've pulled the system for maintenance and it isn't shown.

I wanted to try to make correlations between outdoor temperature and air-conditioner run-time under the assumption that the air-conditioner would run longer when the outdoor temperature was higher. To do that, I employed one of the temperature sensors I previously used for indoor temperature measurement. Not being weather proof at all, I stuck it under the out-cropping on our house under, behind the air-conditioner. This is not ideal as it is very close to the ground (less than 6 inches of clearance) and natural air circulation is likely to not be very high. Its not great but it is out of the rain and is better than nothing. (I'm not able to use the BMP180's temperature sensor because the air blown by the air-conditioner fan will be hotter than ambient. The whole point of the outdoor half of an air-conditioner is to cool the hot coolant as a part of the thermal cycle.)

Since I don't have a real-time measurement system up and running to record the data collected, I decided to use an SD card logger commonly available for the Arduino platform. It is fairly easy to write specific values to the card and it includes a real-time clock to time-stamp each log entry. With this I measured the BMP180 pressure and temperature (the later just for fun), and the TMP36 outdoor temperature. I did this every 15 seconds and, through the use of an extension cord, was able to leave the unit plugged in, running indefinitely.

Here's the final schematic:

The two LEDs are diagnostic lights. The red one flashes every time the SD card is being written to and the green one is lit up when the system is powered.

Here's the hardware, pulled out for maintenance:

From left to right: Interface PCB for LEDs and TMP36 module, BMP180 module, SD card logger on top of an Arduino Uno, and TMP36 module.

And here's the code. Not all of it is working at present, such as the time measurement, but the core measurement functionality has been working great.