I have decided that i wanted a better air quality in my house.
In case of cats, dogs, smoke, unsufficient air circulation or whatever, an air purifier is always appreciable.
Well, this is my filter!
3D printed case: here.
The fan and the power supply should/could be easily reclaimed.
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Step 1: Actual How To
I've purchased a bunch of active carbon sheet filters, their supposed use was for my solder fume extractor and 3D printer enclosure air filter. The active carbon filters are 13x13cm, and therefore they should be fed by a 120mmm fan. Unuckily 120mm fan are not so common and aren't easy to scavenge, therefore i've decided to design something for 80mm fan, which are ubiquitous. I didn't want to print huge and unestetic adapter cone, preferring idea to keep the same filtering surface (130x130mm) but splitted on four 75x75mm surfaces, creating a "cubic" filter. This filter is compact and nice, you can simply place it on one side, or "vertical" using its feet. To power the fan i've used a reclaimed wall charger, 7.8V 1A, which is perfecly fine for the job. The fan is a standard 12V pc fan, and its (as usual) rated for 12V, but it can reasonably work down to 5V. Less voltage means less air flux but also less noise, i think that 8-9V are the sweet spot to reduce the fan noise by a lot without over-sacrificing the air flux. It depends on the specific fan you are using, but 3-400mA are usually more than enough. Since i've planned to have the filter working 16h/day i've choose a power supply that can provide twice as much current, so it doesn't even get warm. I've also added a simple switch, to stop the filter without removing the wall charger. The standard fan has 3 wire: positive (red), negative (black) and speed control (yellow). Just ignore the yellow, connect the negative from the wall charger to the black cable, and put the switch between the positive wire from the wall charger and the red cable. The screws are four M3x40mm (at least 40mm, the hole is actually more than 80mm deep), i've decided to put a small fan grid, wich i didn't designed because there are literally hundreds fan grids on thingiverse (and because of that reason i can't find the source). That's all, now you have your really cheap air purifier, enjoy!