See Figure 3 for a driver solution we suggest here. An inexpensive ATtiny2313 AVR microcontroller is used. High precision timing is not needed here, so timing is 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 as soldering points on the board. Connectors facilitate the assembly, disassembly and launch of the chamber, but they are not mandatory.
The UV diodes are powered by a current source based on the popular LM317 circuit which is used as the voltage regulator. This is a much better solution than the use of a resistor, because it not only protects the diodes under changing operating temperature conditions, but also makes it possible to power the chamber with a wide range of voltage levels.
Here, we calculate the LM317 output current using the following equation:
UV diodes work in the series-parallel connection, with a rated voltage of ca. 6.2V. If we take into account the voltage required by the LM317 and the transistor switch, we need to connect a 15V source with a load capacity of at least 1A to the G1 power connector. A higher voltage may be applied, but with caution, as the excess voltage is converted to heat, which has to be dissipated.
The microcontroller is powered by 5V voltage supplied by the 78L05 regulator (U2). The user interface includes S1…S3 buttons, a 7-segment SEG1 display and a safety switch connected to P5. The microcontroller can be programmed in the system using the P6 connector. The display is operated straight 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 the use of any language for AVR microcontrollers, such as AVR Studio and the GCC AVR, Bascom AVR or another compiler. 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. With only one display available, the exposure time should be set with a certain interval, for example every 15 minutes. For instance, "1" could be 15 minutes, "2" – 30 minutes, etc. You can also use letters if you run out of numbers (unlikely in this specific application, but theoretically possible), so e.g. "a" could be 420 minutes, etc.
Press "start" to begin disinfection. During disinfection, the number shown on the display changes every 15 minutes. Once disinfection is completed, "0" is displayed and – obviously – the UV LED is switched off.
The software checks if the lid is closed before switching the power on and during operation. If the lid is open, the UV LED power gets switched off immediately. PD3 earth connection means that the lid is closed. If the lid of the device is opened during disinfection, the irradiation will be continued after the lid is closed.
Figure 3. Proposed solution for the disinfection chamber controlle
Final remarks
Above, the idea of creating a disinfection chamber for smartphones has been presented. It should not be viewed as a ready-made design of a DIY system, but as a general concept to be pursued individually. You can adapt and modify the design, depending on your needs. Instead of 3D printing, you can buy a ready-made chamber casing or you can make it yourself from the material of your choice, e.g. wood, but remember to use the material that is resistant to UV rays. The same applies to the microcontroller-based switch. A ready-made timer can be used instead of a DIY one, but you will surely enjoy developing the program yourself according to the recommendations above. The LCD module can be used instead of the LED display, if you want more information to be presented. Instead of buttons, you can use an encoder with a button, which has the full functionality of user interface manipulators.
To power the UV LED, you need a power supply. Whatever the threshold voltage on the diode connectors is (it varies as the semiconductor structure heats up), a constant supply current will be maintained, which will greatly prolong the life of the diodes (and these are far more costly than regular LEDs).
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