Building (And Testing) A DIY Air Purifier

Whether it’s the usual pollution of the city, or the fact that your corner of the globe happens to be on fire currently, poor air quality is a part of daily life for many people. One way of combating this issue is with a high quality HEPA filter in your home, but unfortunately that’s not something that everyone can afford to even has access to.

Which is why [Adam Kelly] decided to design this DIY HEPA air purifier that can be built for less than $100. That might still sound like a lot of money, but compared to the $500 sticker price he was seeing for the models recommended by health officials, it’s certainly a step in the right direction. Of course, it’s only a deal if it actually works, so a big part of the project has also been verifying the design’s ability to filter particles out of the air in a timely manner.

To build his purifier, [Adam] found a HEPA H13 rated replacement filter that was cheap and readily available, and started designing a low-cost way to pulling air through it. He eventually went with a 120 mm computer case fan coupled with a step-up converter that can produce 12 V from a standard USB port. Then he just needed to design a 3D printed “lid” which would position the fan so it draws air through the center of the filter.

In terms of testing, [Adam] wasn’t worried about the purifier’s ability to actually filter out smoke particles; unless the manufacturer lied about the capabilities of the filter itself, that part is a given. But he was curious about how effective the fan would be in terms of circulating air through a room.

By installing a pitot tube from one of his drones into the lid of the purifier, he determined the airflow in the center of the filter to be approximately 160 CFM. By his calculations, that means it should be able to circulate all the air in his 25 cubic meter office around 10 times per hour. That’s a promising start, but [Adam] says he’d still be interested in a more detailed analysis of the design’s performance by anyone who might have the equipment to do so.

As he lives in Australia, this project is more than just a passing fancy for [Adam]. He only has to look out the window to see that the air he’s breathing is filled with smoke from the raging bushfires. They say that necessity is the mother of invention, and breathable air is pretty high up on the list of human necessities. Our hat’s off to anyone who sees their fellow citizens suffering and tries to use their skills to come up with a solution.

Adding Drone Instrumentation With No Additional Parts

Soon the skies will be filled with drones, or so the conventional wisdom goes, and these flying droids will deliver pizza, mail, packages, and medical supplies right to one of the taller trees in our backyards. To date, advanced fixed-wing UAVs and toy quadcopters have proven themselves to be exceptionally dumb; they have no idea what their airspeed is, and no, ground speed measured by GPS will not keep you in the air.

The sensors to measure airspeed and angle of attack can be adapted to small drones, but [gallinazo] has a better idea: why not estimate these figures using sensors a drone already has? He’s measuring synthetic airspeed, something that would have already saved a few hundred lives if it were implemented passenger airliners.

Small drones are able to take a few measurements of their surroundings using standard accelerometers, magnetometers, and of course recording the position of the throttle and control surfaces. All of these variables are related to airspeed – at a constant throttle setting, with no movement of the control surfaces, an aircraft will eventually settle at a stable airspeed.

The trick, though, is to tie all of these variables together to produce a number related to the airspeed of the drone. This is done with a Python script implementing a radial basis function and eating all the memory on [gallinazo]’s desktop. This Python script is effectively a black box that turns the throttle position, bank angle, elevator position, and pitch rate into an airspeed.

Does this black box work? Judging by the graphs comparing synthetic airspeed to measured airspeed, this is amazing work. [gallinazo]’s airspeed estimator accurately and reliably matches the measured airspeed. It does this with zero extra parts on the airframe.

All of the code required to implement this synthetic airspeed indicator is available on GitHub, and could conceivably be implemented in a small RC plane after all the variables are pre-computed. Awesome work that pushes the state of the art forward quite a bit.