This is the ambient dust collector I made to gather dust missed by my main dust collectors. Dust is most effectively captured near the source and I work hard to make that happen, but fine dust always seems to escape and cloud the workshop air. This pictured collector captures most of these fine particles and provides other benefits as described below. 

I use a Panasonic bath fan rated at 258 cfm.(Panasonic FV-20NLF1 WhisperLine™ In-Line Fan 1.4 Sones 258 CFM Energy Star) Its quiet, rated for continuous duty and uses very little energy. I have it wired to my light switch. I chose a common filter size (24 x 24) so its easy to find at retail, though I shake it out carefully and expect it to last for years.

You can see the 6 inch exhaust feeds into a T fitting with a valve on the leg that blows into the shop. The other leg blows outdoors. I seldom blow fully outdoors but the T will always direct a small portion of the filtered exhaust outside or a large portion when I close the valve at the bottom of the T which blows toward the floor. This way the workshop is always under a slight negative pressure which helps keep dust from filtering into the other parts of my basement. In the dead of Winter I often close the outside leg completely. In the Summer the shop can get pretty warm on active machine days. Then I close the valve and the fan takes the warm air near the ceiling and moves it outside. The resulting negative pressure pulls more cool air from my central air conditioning supply register at the opposite end of the shop. There are no HVAC returns in the workshop so this supply usually doesn’t blow much. It all works surprising well. 

More photos at

http://johnthetoymaker.wordpress.com/2013/10/14/shop-built-ambient-dust-collector