This project was put together because I wanted to check the performance of my home ventilation system because I was having some condensation issues. This primarily had to measure the temperature and humidity inside and outside of my home. I was not concerned with air flow and air changes per hour, but that should be relatively easy to monitor and may become an add on later where I could have an indication of when the filters were getting clogged and needed to be changed along with keeping tabs on the performance of the heat recovery core that periodically has to be cleaned.
My plan was for a few sensors scattered around in various rooms so I could instantly check the temperature and humidity levels, plus store the data in a database for later retrieval and create some historical graphs. The project I had planned was to use a combined temperature and humidity sensor, connect this to an arduino pro mini and then have the data transmitted via wireless to a receiver unit attached to a PC to store and process, where the data would be finally accessed and displayed within a web browser.
I initially decided on a 433MHz transmitter receiver pair for the link but along the way I discovered that there was something cheaper and better that had hit the market quite recently by the way of a company called Espressif with their ESP8266 WIFI chip. The 433MHz link is old school and is very popular in consumer gear, it is currently used in a lot of the off the shelf wireless devices you can buy at the local hardware store. That technology has its advantages, one being low cost and low power but some of the disadvantages are that there is very little if any error checking, they all share the same frequency so data collisions and other interference can be problematic, the modules do not have very high frequency tolerances and some additional range issues, so I was getting a bad feeling about using them.
This project seems to be evolving to the point where I am keeping the hardware as flexible as possible but keeping costs low. My printed circuit board will support various sensors such as the DHT-11/22 temperature humidity sensors and the DS1820 family of one wire temperature sensors. A simple relay board can be added for control applications if needed and the whole circuit was designed to be powered via USB power or a USB power adapter. A higher voltage power supply or battery combination can be used due to the onboard regulator, however a downside to the ESP8266 is the current consumption when WIFI is being used. This can be offset by configuring a longer reporting interval if needed.
I believe that my circuit can eventually be used as a low cost building block for others to use, simply as I had initially intended, or integrated into a home automation project through the efforts that the ESP8266, internet of things and home automation community are working on.