Quiet, low cost, low noise, no-solder DIY air filter that keeps PM2.5s below 10 in a medium-sized bedroom

I’m in the process of building a manually controllable air filter using an Arduino and an SDS-11 particulate sensor. I wasn’t sure whether to be delighted or disappointed when my first prototype turned out to be pretty effective on its own. This is a quick howto.

Background

I live in Jakarta, where PM2.5 levels are consistently above 30ug/m3 (micrograms per m3) – an AQI of about 90 and often above 50 ug/m3 (more information on Jakarta levels here from the US Embassy or IQAIR). In the long run, this is bad news.

We live in an open-windows-with-fan setup rather than a hermetically-sealed-with-AC one, so getting indoor air quality consistently good isn’t an option, but closing up bedrooms (which do have AC) at night is.

We’ve tried the Xiaomi Mi Air 2, which is relatively affordable, well designed and very effective in small rooms, but has three problems:

  1. The default automatic setting doesn’t recognise PM2.5 at 30ug/m3 as “bad” – so doesn’t increase fan speed
  2. In theory it’s adjustable through the MiHome app, but I couldn’t get it to work and don’t like the idea of giving Xiaomi access to my network
  3. The “high” setting cleans the air well but is loud. Our kids got used to it, but I’m not sure we would.

That said, the filter cartridge is really good – so I decided to use it as the basis for my homemade air filter…

“Design”

The hard part is adapting the fan to fit the filter. This was supposed to be a test to see how well the filter worked with a computer fan alone, so the design is more or less “find a way to seal the fan on top of the filter.”

We’re making this:

The fan is pulling air through the filter, so that dirt is caught / collects on the outside and inside the filter. It can be cleaned a few times before the filter needs to be replaced.

Click here to fast-forward to results.

Parts

  1. Xiao Mi Mi Air 2 filter cartridge (Indonesia / UK): approx IDR 400,000.
  2. Six inch drain cover cap to seal around the hole at the top of the filter (we’ll cut a hole for the fan). (ID / UK): IDR 30,000.
  3. A high-volume, high static-pressure, ultra-silent PC fan. Speed controllable with PWM if you want to add speed control later. I used the Gelid SIlent Pro 14 from QuietPC.com, (about £9 / $12) – if you want to spend more the Noctua NF-A14 looks good.
  4. An old 12v laptop power supply.
  5. Some fine wires like these for connecting things together.
  6. Some screws the right size for the holes in your fan.
  7. Electrical tape.
  8. A bit of glue or sealant if you’re feeling keen.

Tools

  1. Drill and hacksaw blades (or similar) to cut the shape in the drain cap and make screw holes.
  2. Screwdriver.
  3. Wire cutters / snips.
  4. OPTIONAL soldering iron.

How to

1. 12v Power Source for fan

  1. Cut the barrel-jack (the bit that goes into the laptop) off your adaptor.
  2. Strip the wires. Hopefully they’re colour coded (red positive and blue negative 12v DC).
  3. Find a way to join those wires to the positive and negative terminals on the fan. I did a bit of soldering here (it’s very therapeutic) but you could probably twist the various wires together and stick with electrical tape.
  4. Plug the whole thing in to see if it works. My fan takes a few seconds to start (5? It always feels like 10 and I start wondering if it’s broken).

2. Fan Assembly

This is the hardest bit:

  1. Draw around the outline of the fan on the drain cap (it’s probably symmetrical but make sure that it’s the right way round – you want it to extract air from the hole you’re about to make it). Leave a bit of a margin to make sure the hole doesn’t allow leakage around the sides.
  2. Drill / cut around the outline you’ve made. I used some circle-cutter bits and a hacksaw blade. It took about 20 mins all in.
  3. Mark and drill holes for screws. Make sure you screw the fan on the right way up – you want it to be pulling air through the hole, not pushing air in. My temporary wood screws seem to have done the job on a semi-permanent basis.

You should now have something like this:

3. Put it all together

Simply sit the fan assembly on top of the filter. You could add a foam seal or use silicone, but mine seems to work well enough without.

Finished DIY Air Filter

And that’s it! Close the doors and turn it on.

Results

My bedroom is about 3.8 x 3 x 3.1 meters, a volume of about 35m3. The fan claims 80 cubic feet per minute (140m3/hour) max airflow, at about 28 decibels.

I’ll share a graph another day, but the summary is that a single unit got PM2.5s down from 50ug/m3 to below 10ug/m3 in about 45 minutes, and kept them hovering between 5 and 10 for the rest of the night, with the door closed but a fair amount of going in and out. The fan was tolerably quiet – say, like an AC with low or medium fan.

45mins to get levels low isn’t very fast (I’m planning to experiment with a dual-fan model to speed this up), but you just need to plug it in half an hour before you go to bed and hey presto, you’re sleeping in clean air for less than IDR 800,000 / $55 / £40.

Please use the comments below for any error-correction, questions or feedback – I’d love to hear from you.

I'd love to hear your thoughts and recommended resources...