At some point last year my car’s air conditioning unit started to develop a bad smell, which is a sign that mold has formed on the evaporator. Unfortunately, this part is hard to reach and cleaning the evaporator coils is hence a big problem. I went through a set of commercial climate control cleaning products and found most of them pretty useless:

1) Two different disinfectants in spray cans that are sold as climate control unit cleaners: The spray cans are easy to use as you just put them into the car and let them spray their content while the ventilation is running. Unfortunately, the smell was back two days after their application. I guess the sprays are a combination of a perfume (they leave a strong lemon-like smell in the car) and am mild disinfectant like isopropanol. The latter one is not good for killing mold if it reaches the evaporator at all.

2) A spray can with a long tube to spray disinfectant and cleaner directly onto the evaporator. This is actually a pretty good system and preferred by most workshops due the simplicity and easy application. It actually removes organic matter from the evaporator and typically protects it with a good fungicide. However, you have to be able to reach the evaporator with the tube which in my case sadly wasn't possible. It would have meant a lot of extra work to make it accessible. Also, the ventilation shafts are not cleaned by this method.

Talking to a mechanic I found out that some workshops use ozone generators to deodorize such cars. They use it also to remove cigarette smells or other unpleasant odors. Due to its gaseous state, short lifetime and exceptional reactivity ozone can reach the radiator, reacts with the mold (killing it) and vanishes without a trace after some hours. As professional ozone generators and cleaning services are pretty expensive, I decide to build a simple ozone generator from cheap available parts.

So should you have a smelly room, car or van simply build your own ozone generator.

Step 1: DISCLAIMER

1^st: In this instructable you are going to work with dangerous voltage (110/220 V up to several kilovolts) to create Ozone which is a hazardous substance. Make sure to always use a ground fault circuit interrupter during building, testing and using the device. Also, make sure that all metal casings are properly grounded. If you are not sure, I strongly advice to use a portable GFCI plug like this one to keep yourself from harm.

2^nd: Ozone is a very aggressive substance which irritates and damages the eyes, throat and respiratory system at already very low concentrations of 1 ppm (0.0001 Vol-% or 2 mg/m³). Luckily, the substance has a very characteristic odor and the odor threshold for humans is below 0.1 ppm. This means that you will smell the gas well before it reaches dangerous concentrations. NEVER run the generator in an enclosed space together with living animals (including yourself, your pets,…) and ventilate the room for several hours before entering it again.

Step 2: What Is Ozone and Where Does It Come From?

Most of you probably mainly know ozone from the so called ozone layer, a naturally occurring layer in our atmosphere that protects life on earth from UV radiation.

Ozone is a molecule build purely from oxygen atoms. But in contrast to our “breathable air” oxygen (that consists of two oxygen atoms) ozone is built from three. Chemists call this difference in atomic constitution an allotropy. Although this difference sounds insignificant at first, the ozone molecule is much more active than our “standard” air oxygen. Also, Ozone is metastable, which means that it is energetically favored to dissociate according to the following equation:

2 O₃ → 3 O₂ ΔH_f⁰= -286 kJ

To give you an idea of this value: The decomposition of 2 mol O₃ (96 g of ozone) alone (not including any heat from reaction of ozone with other substances) is enough energy to heat 1 kg of water from 10°C to 80°C.

Step 3: The Science Behind It

As described in step 2 ozone is not a very stable substance that can decompose without any other partner into oxygen. Consequently, ozone cannot survive very long under ambient conditions and cannot be stored without major obstacles. To use it, it has to be generated on spot.

There are three major sources for ozone:

1^st ) In our upper atmosphere ozone is build up by irradiating oxygen with energetic ultraviolet radiation. The highly energetic radiation cleaves the O-O bond and forms two oxygen radicals that react with other oxygen molecules to ozone. You can find a lot of detailed information about that on Wikipedia. You can buy UV based ozone generators mainly to disinfect water (e.g. here), but for our intended purpose of deodorizing a room the amount of ozone is typically too small.

2^nd) In the presence of nitrogen oxides and volatile hydrocarbons ozone can be build up in the lower atmosphere, which is (in contrast to upper sphere ozone) harmful for humans, animals and plants. This phenomenon is typically known as “summer smog”. NO₂ (which can be formed e.g. in combustion engines, fired heating systems,… ) is decomposed by UV radiation into NO and ozone.

NO₂ → O + NO

O₂ + O → O₃

This process would essentially be completely reversible, however, in the presence of hydrocarbons (e.g. only partially burned fuel) NO is converted again into NO₂.

R-CH₃+ 2 O₂ + 2 NO → R-CHO + 2 NO₂ + H₂O

With better combustion engines and exhaust catalysts the emission of hydrocarbons and NO₂ (and hence also summer smog) has been drastically reduced.

3^rd) Electricity: Do you know the smell of an old laser printer when operated? You will get the same near an electric arc or near a place where lightning struck during a storm. All have in common that high voltage is used to ionize the air. During this process large amounts of ozone can be generated. Most of the electrical ozone generators do in fact not use electrical arks (as those are bad to handle and lead to rapid material degradation) but corona discharges. Again, you can read a lot about those e.g. on Wikipedia.

There are some pretty impressive electrical driven ozone generators out there. I decide to go with these as they are quite cheap (around $30 and even less if you get them from China), easy to handle and generate an astonishing amount ~ 10 g/hour of ozone. This does not sound like much in the beginning, but keep in mind that for deodorizing cars rates of 0.5 g/h are enough and professional room cleaning generators are aiming for 5 to 10 g/h.

Step 4: Concept and Layout

As you can see in the picture, the layout is pretty straightforward: The ozone generator panels are put into a metal box together with some powerful fans (located in the backplane) that cool the whole system and transport the generated ozone to the outside of the box. A relay board together with an Arduino Uno board are used to control the whole system.

Two 5-position rotary switches are used to set the desired power level and runtime. To save some DIO pins on the Arduino we’ll use a resistor network and the analog inputs to read the switch position. The “start” button is used to start the generator (duh) and some LEDs indicate which panel is currently running and if the fans are on. A “boost” button is used to optionally have a strong start (both generators running continuously for 10 minutes) and a lower ozone formation afterwards. This can be useful e.g. for cars where you want to fill the room with ozone in the beginning and keep it at a lower level for a few hours afterwards.

The generator uses 220V AC for the ozone generator panels and powers a 12 DC converter for the Arduino Uno controller and for the fans. You will find a more detailed electrical layout later in the instructable.

You have to consider three important points in the layout:

1^st) The fans need to be powerful enough to cool the O₃ generators and transport new oxygen to the generator panels. I used two 80 mm 12V fans for servers. Also the 12V power supply has to be good enough to supply the fans, the Arduino and the relays.

2^nd) High voltage (the O₃ generator uses a few kV) means that voltage flashovers can occur. Keep cables at least 5 cm away from the panels and if you are using a metal enclosure (I used one for fire safety reasons) be sure to ground every part properly.

3^rd) The high voltage generators produce some electrical noise that can be harmful to small electrical components. Be sure to keep the Arduino and the relay board away from the generators. I also found it helpful to put the AC-DC transformer in between those systems as the (grounded) metal case of the transformer acts as a barrier.

Step 5: Drilling the Front and Back Plane

I used metal holesaws to drill the openings for the buttons, the power cable, the air inlets, the LEDs, and the fan outlets into the metal front- and backplane. I found it very helpful to put the metal backplanes between two pieces of wood to keep them from bending and used a printout of my intended design to position the drill.

Step 6: Electrical Layout and Code

I have attached an image of the final layout as a guideline. You can find the electrical layout in the second picture. The Arduino code to control the system is given below.

I have tested the generator two months ago, by putting it into the passenger cell of my car, guiding the power cable through an open window (seal the slit with tape) and turning the air conditioning on. If you have an active carbon based filter in your ventilation system you will have to temporarily remove it since otherwise it will capture most of the generated ozone. After letting the air circulate for 120 minutes while the generator was running on the lowest setting I unpluged the generator (you can also set the timer) and opened all doors to air out the passenger cell. After 4 hours the ozone smell was gone and the whole car smelled as good as new. Until now the moldy smell has not returned.

So if you are having the same problem, give this method a shot.