How to sterilize corona virus masks

Introduction

First of all, I want to reiterate that I am not responsible for what I will expose in this article, it is just an idea that came to me reading that ultraviolet rays alter the DNA of viruses and make them harmless. To have a scientific validity it is necessary to know other information such as the intensity of UV radiation and the exposure time necessary to eliminate them. In these times of forced isolation, I thought of reusing the lamp of an old eprom (Erasable Programmable Read-Only Memory) eraser that I had at home. These non-volatile memory chips had a fused quartz window to erase them by exposing them to a special ultraviolet lamp for a few tens of minutes.

The lamp

The recommended exposure time for eproms was 20-30 minutes with a UV lamp with a wavelength of 253.7 nm with an intensity of at least 15W / cm² and at a distance of approximately 2.5 cm.

The lamp of my eraser, shown in figure 1, may have similar characteristics.

Figure 1

It is a transparent fluorescent tube, having no internal phosphor coating, with a power of 6W. The lamp housing is made of fused quartz or Vycor as the common glass filters UVC rays. The efficiency is around 30-40%, which means that the effective radiated power is at most 2.4W so it is necessary to use an aluminum reflector to put under the lamp, as seen in the prototype photos. Even a thin sheet of aluminum, like the one used in the kitchen, acts very well as a reflector.

As you can see from the photo, the lamp has the writing "GERMICIDAL" G6T5 GL-6. It is not difficult to find them on the market because they are widely used for disinfection of air and water. Philips also produces several lamps with a wavelength of 253.7 nm (UVC) from the TUV TL series. The cost is less than 10 €.

Warning, UVC lamps are very harmful to the eyes and skin, therefore all precautions must be taken, as I did in my project.

The lamp, like all fluorescent tubes, requires a ballast suitable for its power and a starter.

A tanning lamp (UVA) is not good or, in any case, does not have the same effectiveness.

Even the black fluorescent tubes, coated with special phosphors that filter the visible light (> 400 nm), emit UVA radiation.

Germicidal lamps made with UVC led arrays are also on the market, but these are of higher wavelength (265-290 nm). From what I have seen in the datasheets, these LEDs have not high powers and low efficiency (10%), therefore different ones are needed and they are also quite expensive.

My realization

I made my system with what I had at home. I disassembled the lamp, ballast, starter and timer from the old eraser and mounted them in a suitably sized aluminum cabinet, as seen in the images in figure 2.

Figure 2

On the bottom right side I mounted a micro-switch to turn off the lamp when the cover is open, as seen in figure 3. The timer used is mechanical.

Figure 3

Of course you could make this system in better ways, perhaps by inserting a support for the mask, but I made it with what I had at home.

The green light that can be seen at the top acts as a simple slide, lights up with the light of the lamp and indicates that it is working.

The scheme is shown in figure 4.

Figure 4

Parts list

• 6W UVC lamp (254 nm) Philips TUV TL or Osram HNS 6 W G5 or compatible.

•        2 lamp holders with G5 socket.
•        Starter complete with 6-9W ballast or electronic ballast.
•        Micro-switch.
•        0.5 A fuse and fuse holder.
•        Double switch, power cord with earth.
•        1-5 minutes electronic or mechanical timer.
•        25x15x10 cm aluminum case.
•        21x12 cm aluminum reflector, 4 + 4 + 4 cm folded (trapezoidal section).

References

1.       “Ultraviolet”, https://en.wikipedia.org/wiki/Ultraviolet

2.       “Philips TUV TL Mini”,

3.       “HNS 6 W G5, PURITEC HNS UV-C”, OSRAM GmbH, April 17, 2020

A DIP switch for Arduino

How to read a quad dip switch with only one analog pin with Arduino

The DIP switch is a set of small switches in a Dual In-line Package. It finds numerous applications, such as address configurator, select a type of operation, enter a remote control code.

Normally the dip switches require as many digital input pins as there are dip switches used and also with pull-up resistors enabled. In the proposed circuit I use only one analog input for four dip switches.

This article is also particularly useful for applications with ESP8266 which has few I/O pins but has an analog input with a full-scale of 1V.

After developing a project to read a 4x3 keypad with three analog inputs [1], I was thinking of applying this methodology to dip switches too but things are not at all similar. The buttons close only one at a time and are monostable while the dip are bistable and also can close all together.

After several attempts, simulated at computer, I arrived at the synthesis of the circuit shown in figure 1.

Figure 1

In theory, the values of the R1-R4 resistors should be 1, 2, 4, 8 kΩ, but even with the standard values I used, good results are obtained. If we measure the resistance between Vo and Gnd, we have a value that is proportional to the binary number set, but the Vo voltage will be equal to:

Vo = Vcc*R14/(R5+R14)

Where with R14 I indicated the sum of the resistors R1 to R4 not short-circuited by the dip switches. As can be seen, now the law is not linear, but it suffices to put R5 large enough to minimize the current variation. This will reduce the output voltage, but by setting the Arduino reference voltage as "INTERNAL" we get a full-scale Arduino UNO ADC converter equal to about 1.1V.

Indicating with "ON" the closed switches and with "OFF" the open ones and setting Vcc = 5V the following table is obtained.

D4	D3	D2	D1	N	R14	I [mA]	Vo [mV]	DV [mV]	NADC	DN	NADC_L	NADC_H
ON	ON	ON	ON	0	0	0.073	0.00	0	0	0	0	30
ON	ON	ON	OFF	1	1000	0.072	72.36	72.36	67	67	47	87
ON	ON	OFF	ON	2	2000	0.071	142.65	70.29	133	66	113	153
ON	ON	OFF	OFF	3	3000	0.070	210.97	68.32	196	63	176	216
ON	OFF	ON	ON	4	3920	0.069	272.15	61.18	253	57	233	273
ON	OFF	ON	OFF	5	4920	0.068	336.89	64.75	313	60	293	333
ON	OFF	OFF	ON	6	5920	0.068	399.89	63.00	372	59	352	392
ON	OFF	OFF	OFF	7	6920	0.067	461.21	61.32	429	57	409	449
OFF	ON	ON	ON	8	8200	0.066	537.35	76.14	500	71	480	520
OFF	ON	ON	OFF	9	9200	0.065	595.08	57.73	553	53	533	573
OFF	ON	OFF	ON	10	10200	0.064	651.34	56.26	606	53	586	626
OFF	ON	OFF	OFF	11	11200	0.063	706.18	54.84	657	51	637	677
OFF	OFF	ON	ON	12	12120	0.062	755.42	49.24	703	46	683	723
OFF	OFF	ON	OFF	13	13120	0.062	807.68	52.26	751	48	731	771
OFF	OFF	OFF	ON	14	14120	0.061	858.67	50.99	799	48	779	819
OFF	OFF	OFF	OFF	15	15120	0.060	908.44	49.76	845	46	825	865

Where NADC is the output number of the 10-bit ADC converter. With four dip we represent 2^4 = 16 different states that must be identified precisely. The program must discriminate each state with two thresholds NADC_L (lower) and NADC_H (upper). The number of 10 bits, at the converter output, must be between these two limits which I determined with ± 20 units with respect to the NADC value. The diagram in figure 2 clearly shows the intervals and the output voltage Vo as a function of the number set on the four dip.

Figure 2

As can be seen, the trend is fairly linear and the intervals DN range from a minimum of 46 to a maximum of 71, so the thresholds of ± 20 do not overlap.

As shown in the table, by powering the circuit with Vcc = 5V, the maximum output voltage is 908.44 mV, lower than the maximum voltage (1100 mV) accepted by the converter.

If you need an 8-pin dip switch, just duplicate the circuit and use a second analog input. Figure 3 shows a connection diagram of the individual components.

Figure 3

The circuit is realized with few soldered joints and it is also easy to draw the printed circuit. Figure 4 shows the appearance of my prototype seen from the component side and from the soldered side. I used a small perforated circuit board of about 40x30 mm.

I have optimized the circuit for Arduino / Genuino Uno and the program is fine for MCU type ATmega328 / 168. But it can also be used, with minor modifications, with other MCUs, such as SAMD21G18 mounted on Arduino Zero and Arduino MKR1000 boards and also ESP8266 and 32.

Figure 4

I used a 3-pin strip connector for wiring with Arduino. The resistors must have a low tolerance, I used those with 1/4W metal film with a tolerance of ± 1%, very common and easy to find. Figure 5 shows the wiring between the DIP card and Arduino Uno.

Figure 5

List of components

component	description	component	description
R1	1 kW ± 1% metal film	R5	68.1 kW ± 1% metal film
R2	2 kW ± 1% s metal film	-	0.1” perforated circuit board
R3	3.92 kW ± 1% metal film	-	3 pin strip connector
R4	8.2 kW ± 1% metal film	Dip4	DIP switch with 4 positions

The program

The sketch is an example of use of the circuit and provides both the number and the individual dip arranged in the DipSw char array. In this example I use Arduino Uno's pin A0 as an analogue input and in the setup I insert the analogReference (INTERNAL) instruction to set the internal Vref at 1.1V, more stable and less noisy than the default at 5V.

In the loop () function I set the discrimination to thresholds and in the readDIP function (byte mybyte) I create the array of characters ordered as the dip switches. This last function is useful to use every single switch.

/* program DIP4to1.ino

 *  only one analog input for 4 dip switches

 *  Giovanni Carrera - 20/08/2019

 */

int DipPin = A0;// DIP4 analog input

// limits of DIP4 output values:

const int NADC_L[16] = {0,47,113,176,233,293,352,409,480,533,586,637,683,731,779,825};

const int NADC_H[16] = {30,87,153,216,273,333,392,449,520,573,626,677,723,771,819,865};

char DipSw[5];

void setup(){

 analogReference(INTERNAL); // internal ADC reference input = 1100V

 Serial.begin(115200); // used with serial monitor

}

void loop() {

  int val = analogRead(DipPin);// read analog keyboard

  for (int i=0; i < 16; i++){

     if (val >= NADC_L[i] && val <= NADC_H[i]){// has found the right value

        Serial.print(val);

        Serial.print(" , N = ");

        Serial.print(i);

        Serial.print(" , Dip = ");

        readDIP(i);

        Serial.println(DipSw);

        break;

     }

  }

  delay(500); 

}

void readDIP(byte mybyte){

 byte mask = 0x01;

 for(int i=3; i >= 0; --i){

   if(mask & mybyte)

       DipSw[i]= '1';

   else

       DipSw[i]= '0';

   mask <<= 1;

 }

}

References

1.      “Very few wires for a numeric keypad for Arduino”, Giovanni Carrera, 10/11/18, http://ardupiclab.blogspot.it

2.       “Only Three Pins for a 4x3 Keypad”, Giovanni Carrera,  November 19, 2018, https://www.hackster.io