Looking for some advice/feedback on a planned circuit, it's a combo 12vdc / 110ac thing -DIY UVC sterilization chamber safety concerns

Thread Starter

fatair4me

Joined Apr 1, 2022
23
Howdy, its total amatuer hour here.

I am trying to design some circuits to basically control a 80w fluorescent lamp. Wow, sounds easy, but not for me. The kicker is that the lamp is a very strong UVC lamp, so I have to ensure the door is closed on the enclosing box in order for the lamp to turn on (an important safety thing). The whole thing is supposed to be an UVGI chamber more or less used to sterilize household items like tv remotes, cutting boards... Since I am going to make 2 of them, one for my sister and one for me, I'd like the circuit to be designed safely and not have anything overloaded. I have plastic boxes with UVC aquarium lights that I have been using to sterilize stuff occasionally, but they make ozone and the whole design is relatively dangerous, not exactly a good gift idea unless you want to blind or mame someone. I've used them for a couple of years, they work. This project is meant to make a real sterilization chamber, one that is safe for the operator.

So here is the general idea that I am trying to accomplish (see attached pictures of the circuit and the box that will house the light/electronics):

There is an overall on/off switch (DC). If that switch is turned on, then a 1) couple of DC fans start running, and 2) a normally open reed switch is energized (rated at 1A switching current). Ok, if the door is closed then the reed switch is closed and 12Vdc passes on to a momentary switch and a countdown timer relay. So, the power switch is on (it lights up), the fans start, the momentary switch is illuminated, and the countdown timer is powered on but not counting down (it's waiting for a trigger signal). So if they hit the momentary switch, the countdown timer starts counting down on its display which then 1) energizes a furnace relay's coil which powers on the fluorescent lamp, 2) lights up a cute led that looks like an automotive highbeams facing down.

I'm a software guy, not a EE, and it's been about 35 years since undergrad courses touched on electronics. Any and all feedback would be much appreciated! I just don't want to burn down my sister's house (if you see smoke coming from Denver, then my circuits probably did not work so good).
Thanks, I can't believe how hard it is to study, learn, and order parts for such a relatively simple electronics project (search for a diode on Mouser, they have like 5,000 of them to choose from. Electronics seems to be something that takes a lifetime of knowledge to be able to wip together circuits and procure the parts (without starting a fire, I can do that).
 

Attachments

dl324

Joined Mar 30, 2015
14,458
You need a safety switch to interrupt power to the tube if the door is opened while it's on. Is that what the reed switch is supposed to do? I can't understand what you're trying to indicate with 4 wires going to the switches. How many poles do the switches have? What are the inputs to the timer? Why do you think you need diodes?

Using connection dots and humps went out about 50 years ago. Did they draw wiring diagrams with colored crayons when you studied electronics?
 

Thread Starter

fatair4me

Joined Apr 1, 2022
23
Thanks so much for looking at my schematic and providing feedback. My dad was a self taught EE (only high school educated, I have a masters, still can't understand this circuit, go figure), but he died from covid last summer, which of course really sucks. He would have helped me (probably would have laughed at the design too), but that's not an option. This project has made me think about him alot, which is good. I certainly have gained an appreciation and respect for those that choose electronics as a profession, there is nothing easy about it even for such a lame thing I am trying to build.

To answer your questions, well, the switches are illuminated, they each have their own socket with wires sticking out. There are many ways to wire those things. I choose to have the 1/0 switch only light up when the switch is activated, the momentary switch lights up once the 1/0 switch is on AND the reed switch sees its magnet (no magnet, the countdown timer and the momentary switch are meant to be killed). Here are links to the 2 switches and the timer thing:

* https://www.amazon.com/dp/B07932DMF4?psc=1&ref=ppx_yo2_dt_b_product_details [latching 1/0 switch]
* https://www.amazon.com/dp/B07DFMWH6V?psc=1&ref=ppx_yo2_dt_b_product_details [momentary switch]
* https://www.amazon.com/dp/B081N5NG8Q?psc=1&ref=ppx_yo2_dt_b_product_details [count down timer]

The reed switch is meant to shut down power to anything that matters (i.e. die die die). If the NO reed switch is not seeing the magnet (i.e. the door is open), then my hopes were that it will cut power to the momentary switch and the countdown timer, that is my thinking anyways. If the countdown timer has no power, then I would think that the omron relay could not possibly be activated. I probably didn't need 2 ac/dc power supplies, but they are only $20 and I was trying to ensure that they are not overloaded.

Thanks again!
 

MisterBill2

Joined Jan 23, 2018
11,876
Welcome to this site. This is not the first UV sterilizing system project, but the most ambitious one so far.
What is not clear to ne is why the count down timer and display. Are those for the entertainment value? Or What?

One thing about the ozone generation is that the fans will only circulate it, they will not stop it from being produced. And consider that ozone is a disinfectant to some extent, anyway. Also, it does not last, it breaks down fairly rapidly.

THe reed switch and relay, along with the start button are an effective way to interlock, but not tamper proof. But in this case tamper proof is not required.
 

dl324

Joined Mar 30, 2015
14,458
  1. You can get rid of the Omron relay (saving it's 112mA current draw) and use the relay in the timer. That will allow you to use one 12V 500mA adapter.
  2. Why do you think you need 2 fans? Do you really want fans? You're just going to circulate the ozone the UVC lamp generates.
  3. Instead of using a magnet and a reed switch, which can't carry much current, you could use a push button (I'm thinking Cherry snap action) switch that the door closes, rated at 1A and line voltage, to interrupt power to the UVC fixture. The timer would still be functional.
  4. Both diodes are unnecessary.
  5. If you want the 12V LED to indicate when the UVC fixture is on, you could use a neon bulb with a resistor across the line input for the fixture. That will reduce the load on the power supply by 15mA. I think UVC includes visible light, so you could also put a light pipe in the box so you can see when the tubes are on. That's what most of the EPROM erasers I have do.
  6. If you use a PTC fuse, you won't have to replace anything if there's an unlikely overcurrent condition.
  7. What is the purpose of the momentary switch?
 
Last edited:

Thread Starter

fatair4me

Joined Apr 1, 2022
23
Thanks for your feedback! Yeah, I am very familiar with ozone and health hazards it can cause (I have a daughter with asthma), which is why I chose Steril-Aire brand components. They are the only company that I have seen that actually narrows the UVC range down to 254nm wavelength, which does not generate ozone. Most other bulbs are sloppy and produce ozone even if they market that they don't (even Philips). Here is a link to a dual element fixture that I installed in my furnace over the evaporator coils, it makes no ozone:

* https://www.airwater.com/proddetail.php?prod=Steril-Aire RSE-210-HO X-mount UVC Emitter fixture

The radiation is supposed to be enough to sterilize the air as it passes by.

The fixture that I am using for this project is a Steril-Aire 30" tube fixture. There stuff is real nice, but not cheap. They have electronic ballasts and their products are made in the USA. Here's a link to what I am using:

* https://www.airwater.com/proddetail.php?prod=Steril-Aire+DE+301VO+High+Output+UVC+Emitter

I paid about $300 for a fixture and bulb (their bulbs are just under $100 each). But, they are made in the USA so I have no problem paying that.

I figure I would try to make this thing right if I am going to give one to my sister.

The fans are just intended to draw in air from outside just for cooling purposes. The box will be fairly sealed up. An 80w bulb will make some heat, not sure how much really. I think the bulb will only have to run for 60 seconds to generate enough radiation to kill all the cooties. I have a meter for measuring UVC output, but I also use these cards to prove out time/distance requirements for UVC radiation dosages:

* https://store.auvhealthcare.com/

The cards are real nice and effective, the yellow center dot changes color to indicate the dosage received. They make yellow sticker dots too. Hospitals use these sorts of things when they check UVC dosage during sterilization. I have even seen a cool robot that roams the halls and they put the sticker dots on the walls just to verify that enough radiation is produced to sterilize. The cards are over priced at about a dollar each, but I don't have to use that many of them.

Covid requires only a dosage of 50mj/cm2 to kill it from what I have read, but I generally apply 100mj/cm2 just to be on the safe side, which is the darker pink color on the cards. I will test out dosage times once the thing is built. The nice thing about this setup is that the timer is programmable, which is cool.
 

LowQCab

Joined Nov 6, 2012
2,037
It looks like You did a respectable job of designing this project.
However, I would do a few things differently.

1)
The Reed-Switch is not particularly "mechanically robust",
I would use a very heavy-duty "Micro-Switch" ( Limit-Switch ),
to switch the entire AC Power-Supply,
( the first component that the Power-Cord attaches to ).

2)
Take the Fuse out of the Low-Voltage Circuitry,
The Switching-Power-Supplies should limit the Current just fine.
If You feel that You really want or "need" a Fuse,
position it right after the Door-Safety-Switch.

3)
Use an AC Solid-State-Relay, (SSR), to switch the Bulb, instead of the clunky mechanical Relay.
If You want to keep the Relay, it needs a spike-suppression Diode across the Coil.
They usually have a fairly wide, DC-Voltage Input-Range,
and require only a few Milliamps to operate, and are silent.
( They do get warm, and need some Air circulation, or, need to be mounted to a metal-panel of some sort ).
.
.
.
Suggested Safety-Switch .......
https://www.digikey.com/en/products/detail/c-k/ASKHC2J04AY/3752320
.
Safety Switch .PNG
Suggested SSR .........
https://www.digikey.com/en/products/detail/autonics/SR1-1210-N/13148186
.
SSR .PNG.
.
.
 

Attachments

Thread Starter

fatair4me

Joined Apr 1, 2022
23
Thanks for the advice dl324, the momentary switch basically triggers the count down timer, its needed. Once trigger, it counts down and if triggered while it is counting, the subsequent trigger is ignored.
 

Thread Starter

fatair4me

Joined Apr 1, 2022
23
For the timer, the triggering is sort of left to the imagination of the person that buys it. The hi/low trigger is inside (which is goofy), you have to remove the bottom cover to see it. It has a little white 3 prong socket where the pins 1) DC high voltage (I think 3v-24v, something like that), 2) a ground pin (which I assume is -dc), and 3) a low pin that takes pretty low DC voltage. So giving the high pin 12 volts DC starts the timer (I tested it, it works).
 

Thread Starter

fatair4me

Joined Apr 1, 2022
23
You guys are giving me some real good ideas on how to simplify the design while also making it much safer. Just this morning while testing one of the reed switches I picked up, I noticed that sometimes if I slowly removed the magnet, the switch would stay closed, which won't work for me and would be really dangerous in this application.

I have a question about the SSR, I have seen some of them that say they need a heat sink, would the one suggested previously need that? I think the SSRs that I was previously looking at were strictly DC and were switched at a pretty high frequency (like for suspension system reactionary sort of controls.
 

Audioguru again

Joined Oct 21, 2019
4,671
Your extreme sterilizing kills germs and viruses. Asthma is not caused by germs or viruses, usually it is caused by an allergy to pollen or animal dander. It can also be caused by poor nutrition and lack of breast feeding is mentioned by medical experts.
 

Thread Starter

fatair4me

Joined Apr 1, 2022
23
My kid with asthma is all grown up now, so she is unrelated to the purpose of the chamber I am building.

In my understanding, ozone is a problem for people with asthma because it is an eye, nose, and lung irritant. Living in Denver, we have a lot of high ozone days due to pollution and the elevation. I think there is something about high elevation that makes it easier for ozone to stay around longer too, not sure if it is due to the low humidity here or the elevation.

I have an ozone generator that I only really use to kill the smell of smoke in our vehicles. It works great, but the smell of ozone and some of the formaldehydes generated from oxidizing the carpet sticks around for a whole week inside a vehicle. I kind of plan on not using the vehicle for a week if it is in the winter, or keep the windows open all the time in the summer.

The intent of this sterilization chamber is for things like cutting boards, TV remotes, car keys, sandals, cell phones, tablets, chrome books, laptops... I have some very large cutting boards, which is why I chose such a large box. I still wipe down things before treating them with UVC because one thing that I have noticed is that UVC does not penetrate a surface. Steril-Aire makes plastic coated bulbs for hand held sterilization wands. The idea behind that is if the bulb breaks, you don't get glass everywhere since it is enveloped in a plastic bag essentially (good for a hospital setting). Out of curiosity, I asked John at Airwater what the loss of energy was for their super thin coating, and he said it was about 9%. So, I get gunk off of stuff before blasting it with UVC.
 

dl324

Joined Mar 30, 2015
14,458
For the timer, the triggering is sort of left to the imagination of the person that buys it. The hi/low trigger is inside (which is goofy), you have to remove the bottom cover to see it. It has a little white 3 prong socket where the pins 1) DC high voltage (I think 3v-24v, something like that), 2) a ground pin (which I assume is -dc), and 3) a low pin that takes pretty low DC voltage. So giving the high pin 12 volts DC starts the timer (I tested it, it works).
Don't you just set the timer and start it with its front panel switches?
 

MisterBill2

Joined Jan 23, 2018
11,876
I am still wondering about the purpose of the short time timer. UV sterilizing takes a bit longer than a few seconds, if the UV is not super intense. At least that is my understanding. And in ten seconds it does not seem like you should have much ozone produced.
 

Thread Starter

fatair4me

Joined Apr 1, 2022
23
Hi MisterBill2, I won't know how much time to set the timer for until the thing is built and I do some testing basically, but my guess is about a minute. Due to the inverse square law of light, the distance from the light source is a huge factor, light falls off very quickly just like in photography, there is no difference really. So, the old box had the lights much closer to the floor compared to the new one I am building, but the new one has a much more intense bulb, so I'll see later just how much time will be required.

In the past for my old cheesy UV box (see pictures), it took 2 minutes and 30 seconds to achieve the UV dosage I was targeting. That old box was built with some aquarium 15watt fixtures off eBay, HVAC foil tape, and a curling iron timer that has a 5 minutes as its lowest setting. The whole thing cost about $50 to make a couple of years ago. I'm easily distracted in general so anything like these sorts of builds need to have a time based shutoff in case I forget about it, the last thing I want is a fire. The original thing that got me curious about UVC is the Lumin box I bought about 5 years ago which I use for sterilizing CPAP stuff daily, it has a 15w tiny Philips bulb. It makes ozone, but not much. My old box I built makes a ton ozone, I have to hold my breath just to take stuff out of it.
 

Attachments

Thread Starter

fatair4me

Joined Apr 1, 2022
23
One other thing, my decision to add fans had something to do with the design of the box I bought, it came with 2 large holes in the back panel (see attached photo, which was a surprise to me when I got it), so I thought I might as well fill the holes with fans because for one thing, I don't want the box to leak light or it hurt someone or kill an unsuspecting nearby plant.
 

Attachments

Thread Starter

fatair4me

Joined Apr 1, 2022
23
Hi LowQCab, I decided to take your advice and use the SSR and mechanical switch, thanks a bunch for posting links to the parts, I spend countless hours trying to order parts since I know next to nothing about them. I am going to review the rest of the feedback and post a new schematic pretty soon. This weekend I spent time working on some preliminary build items. I lined the inside of the box with HVAC foil tape. It should improve efficiency of bouncing light around for surfaces that are not in direct line of sight to the light source, although that is probably insignificant. The other reason I used the foil tape is because the wood looks like it has a laquer fininish which will brown up pretty fast when exposed to UVC.

I also worked on making the "control panel", which was kind of fun. Basically it is made out of 1/4" aluminum plate, so I had to cut it, bevel it, regrain the finish with 220 grit sandpaper, sprayed it with 3 coats of SprayMax 2K clear, sanded the finish with 2000 grit and 4000 grit, and then hit it with headlight polish. The spray urethane is nothing short of amazing, never tried it before. It is a 2 part spray can and the urethane was completely cured over night. The urethane finish ended up looking like glass, and it only took about 30 minutes to sand and polish the 2 plates this morning. I'll never use any other urethane again. The stuff is kind of pricey at $15-30 a can, but it is truly amazing.
 

Attachments

MisterBill2

Joined Jan 23, 2018
11,876
Now that I look back, I see that the digital timer is already allowed for.
I had an idea for a more classic and simpler scheme:
One option that could simplify the wiring a whole lot would be to use a mechanical, rotary dial, timer. A company called "Mark Tome has made them for many years, and they are (were) available with many different ranges. Probably one with a five or ten minute, or even just a one minute maximum setting would work for your application. All of them that I have seen had two sets of contacts rated for at least ten amps.
But I see that concept is a bit late. Oh Well

But
 
Top