If you have allergies than you've likely considered picking up an air quality sensor for you house at some point. However, a good air quality monitor will run you upwards of $US290. DIYer Chris decided to make one for himself and test it against against the $US290 model.
In total, Chris' parts come up to about $US150, which isn't cheap, but it's a good chunk less than the store-bought model. He uses a cheap, $US12 sensor with an Arduino and a Radiation shield and monitors the data on his computer. Surprisingly, he found the DIY sensor was nearly as effective as the commercial sensor in his tests, but struggled a little with low particle counts. Still, it's a handy project if you need to monitor and track the air quality in your home. Hit up the link below for the Arduino source code, a build guide and all the details of the tests.