LED UV irradiation box design and production process (illustration)

In order to build it yourself you must first decide what size you need. With my setup, boards up to 10 x 15 cm may be exposed. For this you will need the following materials:

- 168 UV LEDs, OSSV53E1A;

- 56 91 ohm resistor 0.25W 1% (5% is also OK);

- 66 red diffused LEDs;

- 18 220 ohm resistors for the red LED;

- 6 330 ohm resistors for the red LEDs;

- 2 prototyping boards, 13 x 25 is the smallest size I found to fit;

- 1 cover for 1.5A minimum switch;

- 2 hinges;

- 1 box;

- Others: screws, nuts, glue, etc.

The box may be made of the material of your choice. I have built it from two identical halves, with internal dimensions of 18 x 11 x 5.5 cm each. The material is 8mm thick OSB so you will need the following parts:

- 4 11 x 5.5 cm pieces;

- 4 pieces 19.6 x 5.5 cm;

- 2 19.6 x 12.6 cm pieces.

Timer

Continuing with the development of my UV exposure box I have designed and built the Countdown.

There are several out there for the same purpose, but I wanted to add a few more features to get things done.

As I mentioned in my previous post, safety is a big issue for me, so I added a switch that should prevent the UV LEDs from rotating when the cover is opened. Of course, this is not enough, so the switch works with the timer to make sure everything goes well.

Key Features:

- Accurate time display

- Adjustable time, 10 seconds resolution

- Start/Pause function

- Automatically pauses and UV LED turns off when lid is opened

- Some predefined values ​​available for convenience.

Implementing the safety measure is easy because the switch on the box can make the connection to either the red or UV LED. The timer needs to control the UV LED, so I used a MOS transistor. It also needs to know the state of the box lid. I built a circuit that can achieve all this. The conceptual schematic can be seen below:

The diagram shows, symbolically, that the red and UV LEDs are repeated in arrays across the panel via the base unit: three series LEDs and a resistor. As you can see, there is no way to turn the UV lamp on when the lid is open, even if such a command would come from the microcontroller. The microcontroller can turn the UV lamp on via a MOS transistor only when the lid is closed. This is controlled via software too. To detect if the lid is open or not, the microcontroller monitors the state of the cathodes of the red LED array. When the lid is open, this is shorted to ground and the micro sees a low logic level. When the lid is open the red LED will conduct current, forcing the microcontroller to see a high. This current, although small, is limited by the 10K resistor and therefore the LED does not come on. I could have chosen to leave the red LED on all the time, as this does not affect the functionality of the exposure box, but it adds extra consumption and more heat needs to be dissipated by the lower panel.

Although it doesn't totally make sense, I decided to go through the effort and make a display for this timer. There are probably many ways to do this starting from a simple 555 circuit. I wanted the circuit to be a challenge in its own right so that as small SMD parts as possible were used.

The display is a KW4-361ASB from Luckylight. It is small, 9mm character height and has 4 multiplexed digits. There were two basic criteria for choosing this particular part, firstly I wanted it to be small and efficient so that a small current is enough to brighten the display and secondly I wanted it to be multiplexed as this simplifies the board layout.

I chose the ATTINY2313 microcontroller because it has all the features needed to perform all the tasks: enough pin drivers, timers, enough pins and memory. It has an internal oscillator too, but I missed that the tolerance was within 10%, which I thought was too high for this project. I later added the crystal on board, but adjusted the value of the timer at the start for my particular circuit to compensate for the internal oscillator deviation. This was easy because I knew that the refresh frequency for the display is 50Hz, so the frequency meter is all I need. I believe that this should have no fluctuations and no future compensation will be needed to maintain a smaller value error, maybe less than 2-3%. I recommend using a quartz crystal, but it is not mandatory, you can use my method or simply accept a maximum 10% tolerance.

To turn on the UV LEDs I used a TSM2302 MOS transistor which has a 3.6A capability, more than enough for both boards. Other transistors can be used, such as the IRLML0060TRPBF.

Besides that, there are 4 buttons on the board, a 7805 voltage regulator and connectors. The 4 buttons have the following functions from left to right: Start/Stop timer, UP? adds 10 more seconds, DOWN? subtracts 10 seconds, MEM? recalls some preset value from memory.

Since the software can be flexible in the connections displayed, I chose to make the PCB layout for the connections as convenient as possible, that's why the schematic is not very simple.

The connections for the switch and LED array are as follows:

Because of the function of the box, I decided to make its PCB a challenge: small display, small SMD components. The finished PCB is as follows, the board is very compact and measures less than 6 x 3.5 cm:

You might be wondering why there is no programming interface. There are two reasons: I wanted to make it as compact as possible and the fact that it is the type of circuit that requires a single programmer (for the end user). Since all the required programming pins are connected to the display it is easy to wire up a few programmers, as you can see in the picture below what I did: