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

#### 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).

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#### 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?

#### 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]

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#### 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.

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#### 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.

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#### 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