Project Log – UV LED Exposure Box – Building the UV Panels

UV LED Exposure Box – Building the UV Panels

Warning - this is a work in progress!
You won't be able to just follow these steps and get something that works.
Don't follow these instructions.

This article is the third in my series on building a box for making PCBs from UV-sensitised copper-clad boards. Eventually  I’ll cover everything from construction through to developing, etching and preparing for assembly.

  1. Intro and Design
  2. The Box
  3. The LED Panels
  4. The Controller
  5. Lists, References and Acknowledgements

All the files are available on GitHub.

Building the LED Panels

The schematic for the two LED panels is pretty straight forward, there’s just lots of the same stuff. Using the online LED calculator, I determined that a single bank of 84 LEDs is nicely broken down into 28 sets of 3 LEDs and a resistor. According to the place I got the LEDs from, they have a forward voltage of 3.2V, a forward current of 20mA, and I’ll have a source voltage of 12V. The resistors will need to be 120 Ohms for each set of three.

UV Box - Panel Schematic

The calculator also tells us some important facts: each 120 ohm resistor dissipates 48 mW of power, so standard little ¼W resistors are fine. Combined, all resistors dissipate 1.3W, and all the diodes dissipate 5.4W.  That gives a total power dissipation of 6.7W, and the array draws current of 560mA from the power supply.

Remember that there will be two of these panels, so the total power drawn by the LEDs will be over 1.1A – make sure the PSU can handle that.

UV Box - BootstrapNote that at this point I thought to skip the Veroboard and just make a breadboard panel. That didn’t work, as the LEDs were too bunched together, so even though small tests came out okay, trying to expose the full panel board showed that only a small central stripe was actually developed – the rest wasn’t touched at all. If you do go the breadboard route, make sure you space the LEDs out about 12mm (½”).

Putting all these LEDs on stripboard is a pain, but it’s an easy way to get a quick panel made.

This is how I built the tracks up. By alternating the direction of the LEDs, it was possible to reduce the number of cuts and links required.

  • The little white dots correspond to the holes in the stripboard.
  • The purple circles indicate where to cut a gap in the track – I used a 5mm drill bit for this.
  • The red lines are links joining tracks together.
  • Green lines are the tracks already on the stripboard, to show the connected parts.

The top view is the layout as looking down on the component side. Note the polarity of the LEDs when you install them!

LED Panel Stripboard Top
(
click to enlarge)

The bottom view showing just where to cut holes and where the links will end up.

LED Panel Stripboard Bottom
(click to enlarge)

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