Air Purifier With Variable Speed Fan and HEPA Filter

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.

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

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.

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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.

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.

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.

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.