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LEDs for sterilization with UV-C light

2020-10-29

LED UV

As the coronavirus pandemic spreads through the world, we often wonder how to keep ourselves and others safe at home, at work or at other meeting places. Disinfection of all the rooms in which we stay and the objects we use with alcohol-based liquids is possible, but not practically feasible. It is difficult to reach every place, moreover, not all surfaces and materials should be wetted. Could state-of-the-art devices help us to increase the level of security in our environment?

According to the information published by the CDC and WHO, one of the possible ways to contract the coronavirus is when a person touches contaminated objects and then they touch their face (nose, eyes or mouth) with the contaminated hand. The best we can do to prevent the virus from entering our body is to practice social distancing, wash our hands frequently, and disinfect everyday items. This applies especially to the objects that we often use outside or bring them close to our faces, e.g. smartphones.

Fortunately, the tedious and inconvenient wiping with a liquid disinfectant is not the only way to maintain hygiene. To decontaminate everyday objects, UV radiation sources may be helpful, for example, ultraviolet sterilization lamps, used for many years in hospitals, clinics and other medical facilities.

The use of ultraviolet for disinfection and sterilization

UV light is a very effective way of eliminating various pathogens. As mentioned, medical facilities and laboratories have been using ultraviolet lamps for years to sterilise the tools and to disinfect the hospital rooms. Smaller models of such lamps are becoming more and more popular in small businesses and even in our households, providing a tool to facilitate the protection of everyday items against viruses and bacteria.

There are three basic types of UV light: UV-A (wavelength: 315…380 nm), UV-B (280…315 nm) and UV-C (100…280 nm). UV-C light has the shortest wavelength and carries the highest energy. Thanks to those features, it can be successfully used as a bactericidal and virucidal agent.

Research shows that ultraviolet light is able to eliminate 99.9% of all pathogens. Disinfection and sterilization with UV lamps is an effective way to sterilize tools and everyday objects, without the necessity to use any toxic chemicals.

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Will the ultraviolet lamp destroy the SARS-CoV-2 virus?

The coronavirus is believed to be spread primarily from person to person via airborne droplets, when an infected person coughs or sneezes. The disease can also be caught by touching the object contaminated with the virus, and then touching one’s mouth, nose or eyes, although scientists believe that this is not the main way the virus spreads.

The light of UV lamps has been tested in laboratory and clinical conditions and proven to be effective in killing various pathogens, but the SARS-CoV-2 virus is a new disease. Although experts claim that there is no unambiguous research that would indicate that UV light is capable of inactivating the coronavirus, it should be able to do it, just as it is in the case of e.g. influenza virus and other viruses. It is therefore very probable that UV light may prove to be a very effective measure to help contain the coronavirus pandemic and maintain hygiene in our environment.

Use of ultraviolet – what items can be decontaminated with UV light?

The light of ultraviolet lamps can kill 99.99% of viruses on surfaces made of materials like glass, metal, wood and plastics with added UV inhibitor. Most high-quality plastics have the addition of the inhibitors to protect the items used outdoors from sunlight. However, some cheap plastics do not have any UV inhibitors added. Elements made of those low-quality materials may become discoloured after continuous exposure to UV light and/or may become brittle.

Disinfection and sterilization with UV light

UV light is an effective biocidal agent in the fight against various types of microorganisms. It can not only inactivate viruses and kill bacteria, but also destroys fungal spores. However, it is important to understand how to use the radiation for disinfection to be effective.

There are many commercially available UV-based sterilizers. They are widely used in medical facilities, treatment rooms, beauty parlours and other similar places. The most popular UV lamps are designed for disinfecting rooms; they are mounted on mobile stands and can be moved to the required place. UV-based sterilizers can be also built as relatively large boxes or chambers resembling microwave ovens. They can be used to sterilize objects such as scissors, tweezers and other items that can be placed inside the chamber.

The amount of radiation reaching the surface is expressed in joules per square meter (J/m2). As we can remember from the physics lessons, one joule is the amount of work done by the power of 1 W in 1 second, so 1 J = 1 W × 1 s. It can be seen from the formula that if the radiation power is constant, the energy reaching the surface of the object depends on the time of exposure. The longer the time, the larger fraction of viruses is inactivated.

Ultraviolet light with a wavelength of 270 to 290 nm is used for disinfection and sterilization. However, the whole UV spectrum covers wavelengths from 100 to 400 nm. The resistance of microorganisms to UV radiation varies. Bacteria die most quickly under UV light, while their spores, viruses and fungi spores are destroyed more slowly. Most of the known bacteria, viruses and spores are effectively destroyed after exposure to 8 mJ/cm2 of UV. Fungi spores have the greatest resistance to UV radiation, but the good news is that pathogenic fungi are less resistant to UV radiation than bacterial spores.

The radiation dose is calculated as the product of the intensity of the radiation and the exposure time. The required irradiation time can be calculated from the intensity of the ultraviolet light source. For example, if a UV disinfecting device with the radiation intensity of 70 µW/cm2 is used to irradiate the object’s surface from a short distance, the radiation dose corresponding to 100 µJ/cm2 can be calculated from the following formula:

Please note that UV sterilizers may be harmful to health and adversely affect eyes and skin. Therefore, the UV light source should be covered and switched off when the disinfection chamber’s lid is open. If the UV light source irradiates only one side of the object, it should be turned after the specified time so that the other side is also disinfected. If a UV light source is used in a room, it is best to switch it on when the room is empty. The UV light may be switched on when someone is in the room, but not longer than for 30 minutes, and personal protective equipment must be used at the same time (skin cover, UV-blocking glasses). Disinfection of water or other liquids may be performed by an internal source of radiation (immersed in the liquid) or an external one. If an internal source is used, the UV light source should be equipped with a quartz glass cover. Regardless of the method, the thickness of the water layer should be less than 2 cm.

No official studies have been published confirming the amount of energy needed to inactivate the virus that causes COVID-19. However, its structure may be compared to the hepatitis A virus (HAV), which is more resistant to UV-C rays, and which is inactivated by an irradiation dose of less than 7 mJ/cm 2. Based on the data above, it is estimated that the dose needed to inactivate the coronavirus causing the COVID-19 disease is approximately 4 mJ/cm2. This means that an ultraviolet lamp emitting 1 W/cm2 of UV-C radiation, placed 1 m away from the surface, needs 400 seconds or about 7 minutes, to disinfect the surface in 90%.

Smartphone disinfection chamber

Light emitting diodes producing UV radiation were developed some time ago, but initially they were mainly useful for exciting the white-light-emitting luminophores. Nowadays, many manufacturers produce LEDs that emit UV light of various intensities, suitable for a variety of applications. In recent years, due to the development of semiconductor technology, the luminous power of such sources has significantly increased, and the spectrum of their applications has expanded.

UV diodes can be used in new applications, where typical UV light sources (e.g. a mercury lamp) would not be suitable. For example, a small UVC LED lamp can be mounted inside the tank of a coffee machine. As the time of exposure to the radiation is practically unlimited, this solution enables inhibition of microorganisms’ growth in water. In this application, the LED has a very convenient, low-voltage power supply, which eliminates the risk of electric shock. This is especially important as the light source works in conditions of increased humidity. Additionally, the long lifetime of LED lamps eliminates the need for expensive service.

Those properties of UV LEDs are very useful in compact air purifiers or deodorizers. In these devices, UV-A light irradiates the titanium-dioxide-coated catalyst to generate free radicals that break down large organic molecules. Such purifiers can be mounted in refrigerators and air conditioning systems to neutralise unpleasant odours. Combined with a germicidal UV-C light, the purifier keeps your air conditioning system fresh and free from airborne pathogens. Also, it reduces the frequency of filter cleaning and replacement.

Liteon is one of the leaders among LED manufacturers. Products of this brand are a perfect choice when it comes to constructing a disinfection chamber. To build the chamber, LTPL-G35UVC275GZ diodes have been selected. It is a diode with a maximum power of 3W, in a ceramic case. The viewing angle that can be used in the constructed chamber is 120°. High LED power ensures fast and effective disinfection. The LED’s supply current is relatively high. i.e. 0.35A, and the rated voltage is typically 6.2V. Under conditions of our application, the power of the diode’s supply current will be 2.17W.

The compromise between the viewing angle and the external dimensions of the designed chamber determines the distance between the UV LED and the disinfected object. Let us assume that the maximum dimensions of the smartphone placed in the chamber are (160×80×12)mm. Figure 1. shows a sketch of the designed chamber. Plans for the chamber construction can be made using graphic objects – it is enough to sketch the UV LED, the irradiated object and to arrange these components accordingly. The chamber and the object are not large, so it’s best to use the 1:1 scale. Then, the drawing should be dimensioned with a programme or a ruler, and that’s it.

Figure 1. Sketch of the smartphone disinfection chamber enabling the determination of the location of the LEDs in relation to the disinfected object (e.g. smartphone)

What can we learn from the sketch? The UV diodes should be installed as close to one another as possible. However, the distance between LEDs and the object must be a compromise, because we have to remember that LEDs have relatively high power and are going to heat up during operation. Therefore, four UV LEDs have been used, two at the top and two at the bottom. If the distance between the LEDs is about 80 mm, the shortest distance between them and the object (so that the UV beam covers the entire smartphone) will be 25 mm. The shortest distance is important here, because it directly affects the dimensions of the chamber, and our intention is to make the disinfection chamber ergonomic and as compact as possible. At the same time, we must remember that the LEDs have relatively high power, so the cooling airflow should be provided. If necessary, it can be forced with a fan.

Let us attempt to put the dimensions given in Figure 1 together into design plans. In order for the object to fit into the chamber and for its edges to be irradiated, certain space must be provided between the chamber casing and the object. Suppose it is 4 mm. If the smartphone dimensions are (160×80×12)mm, and the distance from the object to the UV diode is 25 mm, the internal dimensions of the chamber are (168×88×62)mm.

An example of the chamber design is shown in Figure 2. We also need to include a space for the safety switch (which turns off the UV light as the lid gets opened), the mounting holes for UV LED plates, the holes for wires and connectors, the mounting for the electronic board with buttons, the window for the display or other indicator of the set switching time, the lid hinges etc. This could be the design of the smartphone disinfection chamber, e.g. to be made on a 3D printer. As you can see, the lower part of the chamber has 4 brackets designed to support the smartphone. Of course, they can also be made in a different way and redesigned for the most frequently disinfected objects.

Figure 2. Case design in progress

Figure 3 shows a proposed driver solution. The heart of the driver is an inexpensive ATtiny2313 AVR microcontroller. High precision timing is not required here, so it operates with timing based on a built-in RC generator. A simple transistor switch that activates the UV diodes is connected to the PD4 pin configured as an output. The diodes in the diagram have their P1…P4 connectors, which are not obligatory and can be made in the form of soldering points on the board. The use of the connectors facilitates the assembly, disassembly and launch of the chamber, but it is not obligatory.

The UV diodes are powered by a current source based on the popular, well-known LM317 circuit, operating as the voltage regulator. This solution is in many ways much better than the use of a resistor. Firstly, it protects the diodes under changing operating temperature conditions, and secondly, it enables the chamber to be powered with a wide range of voltage.

In this configuration, the LM317 output current is known from the following equation:

UV diodes work in the series-parallel connection. Their rated voltage is about 6.2V. Counting the voltage needed for the correct work of LM317 and the transistor switch, a 15V source with a load capacity of at least 1A should be connected to the G1 power connector. It is possible to apply a higher voltage, but with care, as the excess voltage is transformed to heat, which has to be somehow dissipated.

The microcontroller is powered by 5V voltage supplied by the U2 stabilizer type 78L05. The user interface consists of: S1…S3 buttons, a 7-segment SEG1 display and a safety switch connected to P5 socket. The microcontroller can be programmed in the system using the P6 connector. The display is controlled directly from the PB port of the microcontroller. The buttons are connected to the PD0…PD2 pins, the safety switch is connected to PD3, and the transistor switch for the UV diodes is connected to PD4.

The microcontroller software can be developed with any language for AVR microcontrollers, for example AVR Studio and the GCC AVR compiler. However, it might as well be the once popular Bascom AVR or another. The programme’s operating algorithm could be as follows:

  • After the power is switched on, the UV LEDs are off and the display shows "0" or a horizontal dash.
  • The user sets the exposure time using the "plus"/"minus" buttons. There is only one display, so the exposure time should be set with a certain interval, for example every 15 minutes. Thus, "1" could be 15 minutes, "2" - 30 minutes, etc. The letters also may be used, although the "9" time of 405 minutes seems to be very long. However, "a" could be 420 minutes, etc.
  • Press "start" to begin disinfection. During disinfection, the number shown on the display changes every 15 minutes. The end of disinfection is signalled by "0" on the display and – obviously – by switching the UV LED off.
  • The software checks if the lid is closed before switching the power on and during operation. If not, it immediately switches off the UV LED power. Closing the lid is signalised as PD3 earth connection. If the lid of the device is open during disinfection, the irradiation is continued after its closing.

Figure 3. Proposed solution for the disinfection chamber controller

Final remarks

The article presents the concept of building a smartphone disinfection chamber. The above solutions should be treated as an idea, and not as a ready-made design of a DIY device. Whoever makes such a chamber, they can freely modify the project to suit their own needs. The casing of the chamber does not necessarily have to be made with a 3D printer – the user can buy a ready-made casing, or they can make it e.g. from wood. It should be emphasised, however, that the material must be resistant to UV rays. It is also relevant for the microcontroller-based switch. A ready-made timer can be also used instead of a DIY one, although developing the program yourself according to the guidelines given in this article can be very gratifying. Instead of the LED display, the LCD display module can be used, as it can present much more information, and instead of buttons, you can use an encoder with a button, which includes the entire functionality of the user interface manipulators.

A power supply should be used to power the UV LED. Regardless of the threshold voltage on the diode connectors (it undergoes change as a result of the semiconductor structure heating up), it will maintain a constant supply current. This significantly prolongs the lifetime of the diodes, which are much more expensive than standard LEDs.

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