Air Purifier With Variable Speed Fan and HEPA Filter





Introduction: Air Purifier With Variable Speed Fan and HEPA Filter

About: I am a professional embedded and electronic engineer with a degree in cybernetics and a masters in embedded as well as 10 years in the industry. I have a keen interest in electronics, gaming, green energy an...

Whenever bright sunlight comes in to our house through the window it always surprises me just how many particulates I can see floating in the air. Having several furry house pets makes the problem all the more noticeable. Combine that with a wife with allergies and you're in for trouble.

I could have gone out and spent a load of money on a shop bought air purifying system but where's the fun in that.

I did look at a few existing off the shelf systems and they seemed to have limited control in terms of fan speed usually only 2 or 3 settings. What I wanted was something that I could control from whisper quiet sleep mode through to a super charged lets get this house clean right now mode.

Using an off the shelf cleanable HEPA filter means I can clean the air very well (lots of quotes of 99.7%) and I can also easily clean out the filter whenever necessary. If the filter degrades over a few years then I can simply buy another and slot it straight in.

Step 1: List of Materials

Parts Required : Mechanics

• Sheet of 3mm Acrylic or Plywood
• 12 x 30mm Female to Female M3 Spacers
• 24 x 16mm M3 Button Head Bolts
• 8 x 120mm PC Fan Metal Finger Guard
• 32 x M5 Self Tapping PC Fan Screws
• 3 x Small cable ties

Parts Required : Electronic

• 8 x 120mm PC Fans - Good Quality & Low Cost - I used Arctic F12 fans
• 12V 3A DC PSU
• 7805 5V Regulator
• TIP122 NPN Transistor - A Logic Level N-Channel FET would actually be better
• Schottky Diode - Ideally with a high current capacity, low switch on voltage and fast response times
• PIC16F88 or Arduino
• 2 x 10K resistors
• 10K Potentiometer
• 5mm Green LED
• 2 x 100nF Ceramic Capacitors
• 1 x 10uF Electrolytic Capacitor
• 1 x 100uF Electrolytic Capacitor

Tools Required

• Laser Cutter or Table Saw and Drill
• PICkit 3 Programmer or Arduino Programming tool
• Soldering Iron

Step 2: Laser Cut Parts

Using SketchUp I created a CAD drawing of the mechanical bits I needed to bring everything together. This part took some thought to get everything in and in the right place.

I started by drawing the dimensions for a 120mm fan complete with 5mm mounting holes and a central hole for the air to pass through. This was then copied 3 times to make a 240 x 240mm square. The HEPA filter I'm using is 240 x 300mm so I extended one of the dimensions to allow for the entire filter to be housed inside the plastic. I added holes for the fan cables to come through the plastic without being in the way and slots for the cables to be cable tied in place. I then created more space for the electronics on the other dimension of the original 240 x 240 square and then added 3mm wide cut outs for the side panels to fit.

Finally I added a cutout to allow the HEPA filter to be easily removed or inserted, removed any unneeded lines from the drawing and made a second copy of the finished part.

The side panels were designed to fit into the slots and provide 30mm distance between the two Acrylic sheets. On one of the side panels I added holes for the D.C. power socket, the on/off switch, the 5mm LED, the speed control potentiometer and a couple of slots for cable ties.

The four pieces of the design were then laser cut from a single sheet of 3mm Acrylic.


Step 3: Assembly

Lets start with the two big Acrylic sheets. We need to fix 4 fans to each sheet. The first Acrylic sheet needs the fans pointing inwards and the other pointing outwards to push air onto the filter on one side and draw air out from the filter on the other. It doesn't really matter which is which for now just that all four fans are fitted the same direction on the sheet.

Each fan is fixed with two of the 5mm self tapping screws in two opposing corners ensuring the cable is in the correct position to go through the hole. The two unused corners are used to attach the finger protection grill to the exposed side of the fan.

Once all four fans are connected to the Acrylic sheet gather up the wires and cable tie them securely in position. The wires can then be stripped and commoned together. The wires we are interested in are the red and black. All the reds need to be together and all the blacks need to be together. Any other wires can simply be removed.

I also added the M3 bolts and M3 threaded spacers to one of the acrylic pieces.

Now both sheets have their fans fitted it's time to add the electronic components.

Step 4: The Electronics

The electronics consist of the following.

• An On/Off switch to control the power to the circuit.
• Voltage regulator to bring the 12V down to 5V to power the speed control electronics.
• A Microcontroller is used to perform the speed control.
• A Potentiometer is used to provide the microcontroller with an analogue input to control the fan speed.
• A Transistor is used to switch the 12V supply to the motors.

I used a small section of veroboard to connect the components together. I made a schematic of the circuit I wanted to create. Then with some thought to try and reduce the workload laid the circuit out on the veroboard.

The DC socket, Potentiometer, LED and On/Off Switch were added to the smaller Acrylic piece with the mounting holes and wires back to the veroboard circuit attached and cable tied in position.

The diode in the motor driver circuit is used to prevent back EMF (high voltage spikes generated by the spinning motors and or high frequency PWM control) from traveling back to the Micrcontroller and causing damage. Make sure you fit the diode to help ensure the system remains working long term.

Once the Microcontroller was programmed with it's firmware we can test everything works before finishing the assembly.

Step 5: The Firmware

The firmware was written in Flowcode and is very simple. We take an analogue voltage reading from the analogue pin connected to the potentiometer and convert this into a digital value. This value then sets the mark/space ratio for a Pulse Width Modulated output pin which drives the base of the control Transistor. Add a small delay and a loop and we're done.

The configuration settings for the 16F88 device are as shown, internal osc, internal MCLR, watchdog on.

If you're using an Arduino and don't want to use Flowcode then just find one of the many Arduino PWM tutorials and you will be up and running in no time. Make sure you connect the two signals to Arduino pins with the required functionality: 1 - Analogue Input and 2 - PWM output.

Step 6: The Finished Unit

Now when it's sunny I can simply enjoy the light and warmth without gazing in horror at the swamp of airborne particulates surrounding everything. My wife is even sneezing less.

Unfortunately the filter unit I purchased was not the dimensions stated on the website. It was a bit smaller and wider which gave me problems installing it. I made it fit by fitting it slightly diagonal and removing two of the supporting threaded spacers. It works but is not easy to take out and clean the filter as it should be. So I recommend buying the filter first and then basing your design around your own measurements to be sure it will fit.

Here are some photos of the finished unit. Why not have a go yourself?

Thanks for viewing.



    • Water Contest

      Water Contest
    • Clocks Contest

      Clocks Contest
    • Oil Contest

      Oil Contest

    23 Discussions


    1 year ago

    Where have you placed the filter for good/best results?

    3 replies

    I currently have the filter sat on the stairs landing where warm air can rise up from the ground floor and pass through the filter. As the cleaned air cools it should hopefully return to the ground floor and the cycle repeats. It's pretty portable so if I have a messy or dusty job e.g. vacuuming, sawing or sanding then I take it to the source of the dust and place in the same room.

    Near to a radiator is also good as you can tap into the air stream created by the radiator and simultaneously clean the air and increase the flow of air over the radiator.

    In a forced air system, it would seem to be best to clean the air as returns but some of the newer systems have a low speed fan so you can run air through the system year around and, with good filters, keep that winter dust storm on startup.

    can you post some pictures of the hepa filter or a link from where to buy

    great project by the way

    Very good! I will refer to the production, Thank you!

    Very nice work, using what some would have around their workshop, some of which we'll have to fabricate some things. I wish I had all the equipment, so I may work with the "Wally World" (WalMart) fan and work my way around to the more sophisticated design you offer here. My laser cutter is a smallish 500 mW "starter" machine I built. Good work, well presented. KUDOS!


    1 year ago


    I am assuming these are 12v DC fans and something like this will work ?

    2 replies

    They look like they will work but they seem a little expensive. I used Arctic 120mm fans and they were about half that price.

    Cool thanks; those are Delta fans, compared to the Artic these provide more than double air flow and have a more robust construction will last a lot longer, so I guess they are worth the extra price.

    I can see a issue with PC fans by design are not made to "push" air through a medium. Your better off with a $10 box fan from wally world, a bungee cord, and a HEPA filter. +1 if you make use of a NEW fish tank filter and use the activated charcoal to deodorize while your at it.

    If you wanted to add some real CFMs check out your local residential HVAC company's scrap yard (Ask first!) for a fan/squirrel cage out of a natural gas furnace. If they refuse say no try offering $10/15. The best part is it runs off 120v with 4-5 speed steps or variable speed if you lower the AC frequency from 60 Hertz downwards. Those suckers no pun intended move 350-400 cubic feet a minute and are designed to pressurize ducts or in our case a filter. Just make sure your terminate any wires your not using and tuck away the capacitor to avoid some unpleasant pain.

    2 replies

    wally world? I'm in the UK so not sure where you mean? A single large fan would be more efficient at shifting air then several smaller fans. Also less turbulence. Activated charcoal is good but from doing research I'm told it only lasts around 24 hours max in this type of application.


    1 year ago

    Am i correct that there are 4 fans pulling and 4 Fans pushing air from the other side?

    3 replies

    Yes 4 pulling and 4 pushing air through the filter. I thought about adding a second switch to be able to set if push, pull and both were active but decided against it in the end and have both all the time.

    I made a small prototype with a single PC fan pulling air and was a bit disappointed with the performance. Doing just one is going to be a lot more efficient then doing both in terms of power usage per cubic meter of air cleaned. However I like the symmetry and being small DC fans they hardly use any Wattage anyway.

    That Potentio without knob is really giving me OCD

    1 reply

    Just be aware that there are different grades of Hepa filters depending on how much and what you want to filter.