Building a Crude UV-C Mask/PPE/IPad/Phone Disinfection Chamber (1 hour, $30, minimal tools) PART 2: Door Interlock and Timer Automation (Arduino and analog)
This is Part 2, adding some features to the original build and part 3:
I decided to add an interlock and a timer to the sterilizer. This is another bare minimum build. Just demonstrating what is possible with almost no supplies.
In practice, it would be better to use standalone switches without any code. Since I think a simple circuit would be more reliable and safer, I included a video walkthrough and a simple diagram at the end for anyone that wants to try building that.
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If you are going to build a system used by multiple people (like setting this up in a hospital closet or something), you should really have a proper door interlock and timer switch. Someone without safety goggles could easily open the closet while it’s on and get hurt. Also, a UV-blocking window (normal window glass blocks UV-C, quartz doesn't) should be installed such that blue light can be seen through it when the bulbs are on.
You may also want to get a UV-C sensor to make sure you are getting the right dosage, but that might be going too far in a crisis where perfect is the enemy of the good.
In general, this build is meant to be the bare minimum, which should be avoided if possible.
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Nebraska Hospital UV Guideline:
General UV-C Safety Guideline:
New Features:
- Door interlock:
When the door is opened, for whatever reason, the UV-C lights should shut off to protect the user and those around them.
2. Timer:
Once the system is activated it should shut itself off in 20 or 30 minutes (even if the door remains shut) to ensure the lights don’t burn out too quickly and the items being sterilized don’t get excessively damaged.
Components
- Arduino knock-off
2. Relay (10A 125VAC rated, 5v input)
3. Power brick
The Build
- The control system Arduino wiring overview
2. A photo for reference:
3. Implementation (this could be much more properly set up using 3d printed brackets and additional fixtures/components)
The relay wiring:
The door sensor
Note that the underside of the relay needs to be isolated from the metal housing of the ballast (I later hot glued the cardboard and relay into place). The backside of the relay has pins that will short through a metal surface.
The system
Note that you want to separate your AC voltage and your low voltage DC lines as much as possible. That’s why I mounted the Arduino externally. Also do not mount the Arduino on a conductive surface (Aluminum foil). It will short the pins which are soldered on the backside.
Arduino Code
If you are using an Arduino for the first time, there are a ton of resources. This code worked for me, but YMMV. The knock-off board I used kept getting false HIGH and LOW signals at the door sensor pin (possibly due to noise from AC wiring), so I had to add some code to create redundancy/resilience to this jitter.
I am not very good at coding and didn’t clean this up but it’s pretty straightforward.
Not everyone knows how to use github, so I put this on Drive.
Testing
Interesting note… There is a fairly strong smell of ozone after a 20 minutes disinfection cycle, which is probably a good thing — the ozone can probably get into folds/seams better than UV.
The fact that enough light leaks through the door for the camera to capture confirming that the bulbs are on inside is, of course, not a good thing. I leave the room when I turn this thing on. Will probably at some baffles for a more permanent fix.
“Dumb” Circuit
The relay would be installed and wired as explained above. The switches should be installed into the sides or top of the AC wiring box. The DC power source would still be necessary to run the relay and sense the door closing.
A third switch could be used to sense the door state but it should not be switching AC. You still need it to control a relay because it would be better not to run AC into the interior of the box.
