Introduction: Ozone Generator

Picture of Ozone Generator

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.



1st: 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.

2nd: 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?

Picture of 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 ΔHf0= -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

Picture of 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:

1st ) 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.

2nd) 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.

3rd) 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

Picture of 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:

1st) 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.

2nd) 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.

3rd) 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

Picture of 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

Picture of 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.


dknape (author)2017-11-11

Great looking and great deign. I am wondering if you could share with me the measurements and source of your project box.

I see your concern regarding providing a parts list but would like to still request a list if you would be willing to share it with me.

BennyOne (author)2017-07-16

Great job! I use a small ozone generator designed for hot tubs to sterilize and deodorize lacrosse equipment. It just has 2 small ports, where hoses are press-fit on. I put the long end of the ozone discharge side into a trash bag with 1 players' worth of equipment and tape the bag closed. Let the ozone pump run for 20 minutes or so, the bag puffs up, and then pull out the hose and shut down the pump. Once the bag collapses over the next hour or so, I year it open and hang up the gear.

Anyone who's ever been near lacrosse or hockey equipment knows how strong the smell can get, but ozone completely clears it up. I can imagine is works perfectly in a car's ventilation system!

Cassani (author)BennyOne2017-08-27

I was looking at this project to create a small Ozone Generator that would do a single pair of cleats/shoes and maybe a pair of shin guards. I figure that it would only take a few minutes to get the stink out.... If I built to look like a toaster oven (that sealed) it would not take up much space. Thoughts?

BennyOne (author)Cassani2017-08-28

I've thought about building some sort of cabinet too, or maybe using an old fridge. One of the problems with ozone is that its a rapid oxidizer, and any metal connectors or hardware - not to mention the box itself, if metal - will rust very quickly. Maybe make it out of plywood & glue and paint the whole inside with Flex Seal? It sounds like yours would be pretty small and therefore a great test case - build it and post it please!

JeffD51 (author)2017-08-03

Very well done! I've made 1 corona discharge plate myself MANY years ago and used the ballast from a neon sign. It was very dangerous but sure cranked out some O3.

My next try was via uv hid lights inside a piece of pvc backed by a pc muffin fan. Those work great, but can't compete w/ corona discharge plates for output. I've seen the exact idea sold as ozone cannons. If you only need to clear the air a little, these may work better for that app., but not for killing molds imo.

JohnC430 (author)2017-07-16

ok, i found the "plates" i did not notice the link.

In the winter we have the heating running and in the summer its the cooling so the house is more or less sealed from the outside "fresh" air with the internal air circulating throughout the house, and diffusing thru the closets and bedding and curtains. we cook and the stink permeates every nook and cranny of the house. I open the doors and windows in the evening and thru the night to "air out" the house. I think one of these running at low power for a few minutes after we have our meals will help clear the air after the "peak stink" hours.

BrittLiv (author)JohnC4302017-07-18

Yes, sorry. I decided to not publish a "stuff you need" list like I normally do, since I wanted people to read the whole Instructable before starting to build something that might harm them unless they take precautions.

JohnC430 (author)BrittLiv2017-07-21

sounds like a good idea. better be safe than sorry.
However, if u watch some YouTube videos, u will see many people making high voltage generators, much higher voltages than this corona discharge p/s and yet they don't seem to be perturbed by what they are doing.

ITPhoenix2 (author)2017-07-16

Great idea. Perhaps a typo, but odors come from the evaporator where the interior air circulates through it, not the condenser, which is mounted in front of the engine radiator.

BrittLiv (author)ITPhoenix22017-07-18

You are right, of course. Thanks a lot for your correction!

ggadget (author)2017-07-18

This is an awesome idea. I don't see why an arduino or any of the electronics are needed if all it really needs is a power source and the fan.

BrittLiv (author)ggadget2017-07-18

I wanted to add a boost function and a timer, but you can of course build it
however it works best for you.

oldfatnbroke (author)2017-07-16

Call me a "nit picker" but I can't help it. Its not the condenser that is causing your "stinky" A/C in your car because it is not inside the car. It is located under the hood right in front of the radiator. It is the evaporator coil that is located in the "heater" box under the dash where the offensive smell comes from. What causes the problem is usually leaves and other plant material that gets trapped in the heater box and gets saturated with water when the A/C cooling cycle is used. I'm sure you have noticed the water that drips under cars during warm weather. There is a drain hose with a "flat" end that lets the water escape from the box, the flat end is on it to keep insects or anything else from crawling into it. To clean the trash out of the box you will need to reduce the end of your garden hose as small as you can, 3/8 of an inch (9 mm) is about as big as you need. Drill a hole as high as you can in the box where the evaporator coil is located. WARNING! If you drill a hole in the heater box DO NOT jam the drill into the box or you will damage the evaporator coil of the heater coil! Get somebody to hold the drain tube open, just squeeze the sides so the hose opens and then squirt water in through the hole you drilled in the box and let it drain out through the drain hose. You can use the coil cleaning products at this time. After washing out the box you need to plug the hole you drilled or stick a piece of duct tape over it, if you do not plug the hole you will have air (hot, cold or ambient temp) blowing into the car while it is being driven.

This is not hard to do just be careful drilling the hole!

MimiE091209 (author)oldfatnbroke2017-07-17

Glad you posted this because my head was about to explode with the condenser/evap mixup!
Thank you.

GTO3x2 (author)oldfatnbroke2017-07-16

Was also going to comment that they probably meant the evaporator coil.

mattyw21 (author)2017-07-17

Years ago I heard about them on a hotel makeover reality show and immediately bought one online. It has worked beautifully for both odors and mold in a number of different situations. However, it has a timer, and the instructions forcefully state that no breathing creature (humans, animals, etc.) should be in the room while it's working. This one covers 4000 square feet (the largest), and when I use it I close off the area -- I even stuff towels underneath the door. I have been very pleased with the results, and recently loaned it to a person who had a small fire in their home to get rid of the smoke odor. After four hours, the odor was gone -- permanently. You can get a good one for under $280, and I highly recommend them.

Antzy Carmasaic (author)2017-07-15

Nice project. I had no clue that ozone could be used to kill mold and fungus.

I have a tiny garage which I tried converting into a workshop. But during rainy season, it got realllly damp in there. Everything metallic got rusted(includes all my tools) and anything else developed fungus on it(even plastic). On the surface of wooden compartments, the fungus grew a few mm thick within a month, which I had to remove with bleach and scrubber. Could this be used to prevent and kill such fungal growth?

BrittLiv (author)Antzy Carmasaic2017-07-16

In principle yes. However, I guess the amount of mold formed on the condenser of a climate control unit is much smaller than the one you described. Taking into account that you also have a lot of other surfaces the ozone can react with (tools, etc.) you would have to use a lot of ozone.

I had exactly the same problem with my workshop and used a sodium hypochlorite based cleaner/bleach. The mold practically vanished into thin air after the first spray.

Antzy Carmasaic (author)BrittLiv2017-07-16

Thanks BrittLiv for the advice. I looked into ozone generators in detail and it turns out in presence of humidity, the ozone generator will create nitric acid, which might react with the tools and cause further rusting.
I'll try getting a dehumidifier instead. If only they weren't so expensive...

JohnC430 (author)Antzy Carmasaic2017-07-16

make one. (easier said than done. LOL)

Antzy Carmasaic (author)JohnC4302017-07-16

If you're pointing to making a DIY dehumidifier, I believe that can be done with peltiers. There is even an instructables for that. But the main problem is that peltiers are very inefficient for this. I could install a mid sized solar panel and run it off of that but that too won't run any decent amount of time. Especially since it is most required on rainy days and that's when solar panels will give least output. And also we've veered off the topic of ozone generators here.

snoop911 (author)2017-07-14

Do you have any thoughts on those continuous flow Oxygen Concentrators? Supposedly, they can help problem sleepers by pumping oxygen into a room...

Not sure what's inside, or how they work, but they're super pricey. I imagine they must have some kind of consumable material and/or generate some kind of by-product, no?

And if you're room starts at 80% nitrogen and 20% oxygen what is reasonable to expect after running for a few hours?

BrittLiv (author)snoop9112017-07-16

Hi! To my knowledge such oxygen generators use a membrane technique or an absorbtion/regeneration principle and are able to produce significantly more oxygen than my 20 g/h of ozone.

I would, however, not expect too much of those "home" oxygen generators. At elevated levels of oxygen (~30%) humans develop signs of oxygen poisoning including vertigo and nausea... Personally, I would not like to sleep next to an oxygen generator.

JohnC430 (author)BrittLiv2017-07-16

do u have a lot of other 'ibles? i am fascinated by the wide breadth of your knowledge.

JohnC430 (author)snoop9112017-07-16

Yaeh, they are extremely overpriced!

DieterP2 (author)2017-07-16

I just found out that California has banned Ozone generators.

DieterP2 (author)2017-07-16

Tesla_Shock (author)2017-07-13

Nice work! Really innovative; now just a way to transport it to the arctic circle...

Cyberchipz (author)Tesla_Shock2017-07-16

lol, I thought about that too... I wonder how many Amps it uses.... and I also wonder if it would work that way.

MY FRIEND, TAKE CARE DOING AND USING OZONE O2 or O3 .... CLASS OF BASIC CHEMISTRY: Oxygen is the common element to react in the RED-OX chemical reactions making this in this way a very corrosive agent. I had long time ago one of those and everything that was metal got rust and rusted all staining and completely damaged.... It is too dangerous too for our lungs in high And lots of exposure to this Chemical Compound.... Try to use a little doses, no more please... The project is cool, but watch it my friend!!! : D

Also, while I'm at it, I was just thinking that a whole home unit that could be installed in the A/C & Heat air intake for the whole house. I wish there'd been an instructable for something scaled to general usage.

How would someone modify this to make a simple ozone generator for the home, like the ones that can be bought in the store. I guess since it can be ramped down, I'd have to research the specs for safe ozone levels, it wouldn't need but one generator, and instead of 220, just upconvert from 110 to 220. And I suppose a little math would give us the ratio of output of the O3 IC and desirable levels... it that enough?

JohnH508 (author)2017-07-16

Very nice, but i hardly think there is a need for an arduino in this project. Another word of caution. When running this and running your air conditioner, make sure the car is not in a closed garage. Doing so would be deadly, of course then you wouldn't need to worry about mold smell anymore because you are dead.

JohnC430 (author)JohnH5082017-07-16

he is using the Arduino as a timer and so what? there are many ways to skin a cat and that is the easiest way for him.

how do you make an automatic one hour timer?

JohnH508 (author)JohnC4302017-07-16

My only reason for mentioning it was the fact that they left their car running to run the air conditioner. This made the need for a timer in the ozone generator pointless. You are going to babysit this thing anyway and set a timer on your phone or something. I would have just plugged it in straight and turned it off in an hour, but perhaps they have other projects planned for the generator.

jtechian (author)JohnC4302017-07-16

If you just wanted a timer, the manual dial timers for ac can switch on and off power each hour. The kind that you set with sliders for on and off time.

So if a person would want to go quick and low tech, use switches for power settings and let this type of timer handle on/off part.

JohnC430 (author)jtechian2017-07-16

you are right. I guess i only thing in terms of electronics devices (e.g. LM555 timers, etc.) so i did not think of big mechanical devices.

mrandle (author)2017-07-16

I worked at a swimming pool where we used ozone to clean the water. We still used chlorine but we were able to use less due to the ozone. I might have to try this!

JohnC430 (author)mrandle2017-07-16

how did u get the ozone gas to mix with the water?

ve6cmm (author)2017-07-16

One thing to remember is that Ozone is very corrosive. I built a large ozone making device (3KW). It had to be in a very air tight room with a lot of ventilation to the outside. It still corroded anything that was in the room.

Ozone also has health implications. In small doses, it can be helpful. Larger doses can be deadly.

autotech1 (author)2017-07-16

I am so glad that you put the warning about ozone being dangerous. As a
retired automotive technician we used ozone generators in vehicles that
had water intrusion into the vehicle interior and that wasn't repaired
quickly enough to prevent the odors occurring. We ran the ozone
generators for the recommended length of time, sometimes longer if the
odor was especially bad, and when it was finished we opened all of the
doors for a while to let it air out before we could start reinstalling
the new jute, carpet, (new if needed), seats, etc back into the vehicle.

JohnC430 (author)2017-07-16

by the way, why the following:

"After letting the air circulate for 120 minutes while the generator was running"

Is that a typo?

two hours seems such a long time for such a small space. i think even 10 minutes should be good enough. especially if you're "cleaning" the car daily which you should be doing under the climatic conditions you described.

JohnC430 (author)2017-07-16

thanks for sharing. i liked the "quick" chemistry lesson. i did not see a 'parts list' so i am wondering from where you got the "corona discharge" plates.

by the way, i clear the stink in my car by opening the windows and turning on the heater for a few minutes. very simple and quick and the stink is gone. however it looks like you're probably somewhere in Europe so the humidity and rain there is huge and not the same as here in Southern California which is mostly desert. So the heater may not work where you are.

Again, thanks for sharing.

Jerry Gioia Sr (author)2017-07-16

Very informative article. However the part of an A/C system that can gather mold is the evaporator not the condenser.

HazimA3 (author)2017-07-16

I like it.

Dan AnthonyV (author)2017-07-12

Does this really work?

BrittLiv (author)Dan AnthonyV2017-07-12

Yes, it does. Better than I expected

About This Instructable




Bio: Hi, my name is Britt Michelsen, I'm a Chemical Engineer especially interested in Computational Fluid Dynamics. To balance all the theoretical work I like ... More »
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