UVC decontamination chamber for community use

An easy and practical way for collective use, to avoid covid-19 contamination by supermarket purchases.

IntermediateFull instructions provided8 hours918

UVC decontamination chamber for community use

Things used in this project

Hardware components

STMicroelectronics Blue Pill (STM32F103C8T6) board

SparkFun RGB LED Breakout - WS2812B

Seeed Studio Grove - Relay

Switch Actuator, APEM A01 series Illuminated Push-Button Switches

Reed Switch, Flange Mount

UVC Lamp

ballast for UVC lamp

Plastic Enclosure

Software apps and online services

Arm Mbed Studio

Hand tools and fabrication machines

STMicroelectronics STLINK

Story

The ideia

My wife has always complained that one of the unpleasant activities that the new normality has brought about is the concern of cleaning supermarket purchases. So I thought about using a tool widely used in Biotechnology called decontamination by applying UV light and after some studies and calculations I decided to develop a sanitization chamber, not only for our personal use, but for all the residents of my building.

The knowledge

Ultraviolet germicidal irradiation has been a mainstay for killing and inactivating microorganisms for well over a century. This process utilizes short-wavelength ultra-violet C light to kill microbes. UV-C light encompasses a range of 100 to 280 nm, but the most effective wavelength for decontamination is between 250-260 nm. Exposure to the light inactivates microbial genomic DNA. UV-C light creates lesions called thymine dimers, which cannot be resolved by cellular-DNA repair mechanisms. This injury to cellular DNA impairs vital cellular functions and ultimately leads to the death of the microorganism. Thus, ultraviolet germicidal irradiation can be an effective means of sterilizing surfaces, instrumentation and facilities. (source: https://www.laboratory-equipment.com/blog/two-leading-decontamination-methods-uv-c-vs-hpv/).

Figure 1

The solution

I decided to develop a UV box using the sterilization method described above, using components that are easily accessible to most people and that would provide an easy and safe use, as exposure to UV light is harmful to health.

First of all it was necessary to choose the correct UV lamp and based in the correct UV dose necessary to inactivates the virus.

UV Dose = UV Intensity (μW/cm² ) x Exposure Time (seconds)

According to Arguelles (2020), the dose necessary to inactivation os virus must be 150000µW.s/cm2 (Table 1), so according to Keitz equation described bellow, I found the correct UV intensity for the UVC lamp that I used in this project (Puritec HNS S 13W). Then I needed to adjust the exposition's time for about 18 minutes.

Keitz equation

Table 1

The Circuit

For this first version I used a BluePill board (the final version will use an Arduino MKR WiFi 1010 to give IOT characteristics to the project). The controller circuit needs these followings characteristics:

One indication by LED when the UV-Box is ready to operates. Used green color on RGB LED;
One indication by LED to show if the cover of UV-Box isn't present. It's very harmful the UV-C light radiation in the yes, so the lamp never be turned on when in this condition. Used red color on RGB LED;
One microswitch to detects the cover of UV-Box;
One indication by LED when the UV lamp is ON and the timer is working. Used the blue color LED in flashing mode;
Ballast for the UV-C lamp;
One button to start the process.

The firmware

As I decided to work at that first moment using an ARM microcontroller, I chose to use MBED Studio as the programming IDE.

With an RTOS core based on the widely used open-source CMSIS-RTOS RTX, Mbed OS supports deterministic, multithreaded real time software execution. The RTOS primatives are always available, allowing drivers and applications to rely on features such as threads, semaphores and mutexes. So, as I needed a security guarantee in the case of opening the UV-Box cover, the use of an RTOS came to a great ideia.

Code

Code UV Box

#include "mbed.h"
#include "platform/mbed_thread.h"
#include <InterruptIn.h>


#define ESPERA_DESINF_MS                                                    100
// main() runs in its own thread in the OS
InterruptIn E_switch(PB_12, PullUp);
DigitalIn botao1(PB_11, PullDown);
DigitalOut test_led(PC_13);
DigitalOut UV_lamp(PB_9);
DigitalOut redled(PA_5);
DigitalOut greenled(PA_6);
DigitalOut blueled(PA_7);

void outputs(int state){
        switch(state){
            case 0:
            { // pronto para ligar
                test_led = true; // test led  OFF
                UV_lamp = true; //  UV  lamp OFF
                redled = false; // red led OFF
                blueled = false; //  blue led OFF
                greenled = true; // gren led ON
                break;
                
            }
            case 1: // turn ON UV
            {
                test_led = false; // test led  ON
                UV_lamp = false; //  UV  lamp ON
                greenled = false; // gren led OFF
                blueled = true; //  blue led ON
                redled = false; // red led OFF
                ThisThread::sleep_for(703s);
                break;
            }
            case 2: // emergency stop
            {
                test_led = true; // test led  OFF
                UV_lamp = true; //  UV  lamp OFF
                greenled = false; // gren led OFF
                blueled = false; //  blue led OFF
                redled = true; // red led ON
                break;
            }
            break;
        }
    
}

void emergency(){
    outputs(2);
}
int main()
{
    test_led = true; // false liga o led
    UV_lamp = true; // false liga a saida
    E_switch.rise(&emergency);
 
    while (true) {
        
        if (E_switch) {(outputs(2));}
        if (!E_switch) { outputs(0);
            if (botao1) { outputs(1);
            }
        }
    }

}