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UV Box

UV boxes are used to expose photo sensitive PCB material to transfer the layout to the board so it can be etched. There are commercial boxes but they are quite expensive and usually way over sized for stompbox needs. Keep reading to learn how I built mine.

First I bought some candy in a metal box and ate the candy (eating the candy is optional). Next I bought a 1W ultra violet led on ebay, that set me back all of $7.25 (I think you can find them cheaper now). I also bought some acrylic plastic (3mm thick) and an LM317. I drilled a hole close to the bottom of the box to get the cables out and used a small piece of strip board to configure the LM317 as a current source (check the datasheet and/or online LM317 calculators). I use my (diy) lab supply to power the UV box.

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This is the box (old protective film stored on the lid). The height isn't a bad thing as the led has a somewhat narrow beam of light, the intensity gets higher the closer to the center you get. With this height beams of light will bounce off the inside walls and help distribute the light more equally. As you can see below the inside walls are metallic so they reflect a fair amount of the incoming light.

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This shows the acrylic glass cut and shaped to fit the opening and the glued on pieces of wood holding it up. The board design that's on that transparency is 33 by 63 mm (1.3 by 2.5 inches), maximum size is about 10 by 10 cm (4 by 4 inches) with this box. When exposing I place a photo sensitive PCB (cut roughly to size), copper side down, on top of the transparency and put the lid back on before powering up the led.

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Here you can see the led glued to the bottom with a glue gun. You can also see the heat sink for the LM317 (the box acts as part of the heat sink too).

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Here's the LM317. The resistors need to handle the current needed for the led, not wanting to buy a 1W resistor of appropriate value (mostly didn't want to wait for it) I paralleled a few 1/4 watt resistors. The led handles a maximum of about 400 mA and the voltage drop over them is 1.2V, that's 0.48W of power dissipation we need to handle. You could theoretically do that with just two resistors, but instead of running close to the limit I opted to be on the safe side. Not that this circuit runs for extended periods, but pushing close to maximum power through resistors will make them fail much faster than if you stay well below the rated limit. Using four resistors instead of two also made it easier to hit the desired resistance. Voltage divided by desired current in amps, 1.2 / 0.4 = 3 Ohm, that would be easily hit with four 12 ohm resistors, I used three 12 ohms and one 15 ohm for a slightly under 400mA current.

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