How to build an Ultra Violet Exposure box using LED's.

Your last Veroboard project!

A UV exposure box is an extremely useful piece of kit. It can be used to make proper PCB's. It can also be used to make other things such as intricate photo etched parts (a subject for another instructable). The trouble is they can be a little pricey for the hobby enthusiast especially if you want the double side type.
This instructable outlines the construction of a double sided UV exposure box using the recent generation of high brightness UV LEDs.

Why use LEDs?
LEDs are far more energy efficient than either incandescent or fluorescent lamps offering between 5-10 times more efficiency making them cheaper to run and kinder to the environment. They also (unlike fluorescent tubes) do not contain mercury. LEDs have a far greater life span than the other types of lamp measured in decades rather than months. The frequencies being emitted are also in a tighter band making UV LEDs safer than the traditional UV tubes. There's also just something cool about LEDs, I can't put my finger on it, but ever since I was a kid I've found them to be one of the more fascinating electronic components.

Is there a disadvantage to using LEDs?
Not really, however the UV exposure box I have detailed here is a little less powerful than the commercially available ones. This means that your exposure times will be around 2 ~ 3 minutes as opposed to 30 ~ 40 seconds, but come on, do you really need your PCB's to be produced that quickly? Anyway sometimes having a slightly slower exposure time can be an advantage allowing you a little more control.

This UV Exposure box will consist of 2 UV panels; each having 84 LEDs a total of 168 LEDs. Each panel will draw about 700mA at 12v. This makes each panel 8.4watts a total of 16.8 watts for the whole thing.

Step 1: Materials

The most critical parts of this project are the UV LEDs, you are looking for 5mm Ultra Violet LED 2000mcd 395nm, 3.4V 20~25mA.
I bought two 100psc packs from eBay.
If you find something better then ensure that they are;
- At least 2000mcd in brightness
- Have a peak wavelength of less than 400nm.
- A viewing angle of at least 20 degrees.

You will also need 2x 160mm x 100mm pieces of Veroboard and 56x 75R resistors.
Another important choice is the PSU. I used a plug in, 12 volt 24 watt switch mode power supply. Switch mode power supply's are far more energy efficient than most other types and are also very stable.

All the other parts and materials are easy to find. Some I bought, some I salvaged. This is where you own judgement and taste comes in. In the end it's up to you how closely you follow my design. I've included all the CAD drawings and schematics as metafiles so they're easier to read when you print them out.

Step 2: Box Parts

First cut out all of the parts as shown in the drawings. I used some salvaged 6mm MDF. Then cut out the holes in the glass lid, the apron and the recesses on the side inserts. And route the recess for the glass using a router cut the surface recess on the underside of the glass lid.

Step 3: Box Sides

Now glue together the 4 outer sides of the main box using the base as a guide (make sure you don't glue the base though). Then glue the inner sides in place so that when fitted, the apron is flush with the edge and the base is slightly recessed.

Step 4: Lid Assembly

Glue together the lid just as you did the main box but the lid can be assembled all in one go.

Step 5: Fit, Fill, Sand and Drill

Fit the lid, hinges, catches and glue the apron and support in place. This step will require a lot of test and adjustment to get things just right. Pay particular attention to the glass lid. I've slotted the holes for the glass lid hinges to make for easy adjustment. I've also chosen some hinges that only open to 95 degrees and some toggle catches.

Step 6: Holes

Drill holes for the PSU connector, and for a cable to go from the box to the lid. Make one final check that everything fits, drill pilot holes for the base screws. Then remove all the hinges etc. give everything one last going over with filler and sand paper and then paint all the wooden parts. I recommend using white for the inside to help reflect and diffuse the UV light but the outside can be whatever colour you like.

Step 7: UV LED Panels

I've mounted everything except the LEDs on the copper side of the board to keep the LED side uncluttered.

Step 8: Prepare the Veroboard

First cut the tracks with a spot face cutter as per the track diagram and drill the 6 holes (3.2mm). Buzz the tracks with a multimeter at each stage to check for short circuits and bad connections.

Step 9: Negative and Positive Rail Links

Next solder on the links putting some insulating tube between the solder joints. Put kinks in the wire where it contacts the board.

Step 10: Soldering the Resistors (surface mount style)

Put dogleg bends in the resistors wires. Then solder in position testing each one with an Ohmmeter to check for shorts.
Take care not to melt the paint on the resistors and cause a short!

Step 11: Soldering the LEDs

Solder all the LEDs in place, note their polarity. The diagram shows which side the flats should be. This step can be tricky, as all the LED bases need to be flat against the board to ensure an even spread of light. Resist the temptation to insert them all and then solder. The best method I found follows in the next few steps.

Step 12: Insert LEDs

Do one row at a time. Insert all the LEDs in the row checking you've got them the right way round.

Step 13: Solder 1st Leg

Then place a block of foam rubber (or something similar) on top and flip over. Then solder just one of the legs of each LED.

Step 14: Position LEDs

Now hold the board in your hand supporting an LED with your finger. Reheat the solder, as the solder melts the LED will become free and you can wiggle it with your finger till you feel it is flat against the board. Hold for a few seconds as the solder cools. Repeat this step for each LED in the row.

Step 15: Finish the Row

Now solder the other leg of each of the LEDs in this row and clip all the legs to length.

Step 16: Solder Links

You need to create a bridge at the end of each series of three LEDs to ground. Use a small piece of wire or an offcut of the resistor wires.

Step 17: Test that Block

After you've completed every 3rd row you can test that block by applying up to 12volts to the board. I recommend using a bench PSU and turning the voltage up slowly. Be careful not to go over 12 volts and watch your eyes, don't stare directly into the LEDs!

Step 18: Test that Panel

Finally add the red and black flying leads to the positive and negative rails.
Do a final test with your bench PSU. If any of the LEDs are duds then replace them (you should have 32 spares). And remember, check the polarity!

Step 19: Make the Second Panel

Now repeat the last 10 steps for the second panel, and fit standoffs to the six holes on each board.

Step 20: Control Panel

Make a control panel out of 1 ~ 1.5mm sheet steel and cut a hole to fit your power switch.

Step 21: Fitting the Glass

First cut the glass to size. Then stick the upper glass into the recess on the glass lid using silicone sealant.

Step 22: Glass and Foam

Cut some thick foam rubber (about 1 inch thick) to the same profile as the shelf. Make the cuts in the foam by compressing it with two rulers side by side and then run a craft knife between them. Then Place the foam on top of the shelf and the lower glass on top of the foam and then run a fabric strap around the ends of the glass, adjust the length of the straps so the glass sits flush with the top of the box and fix the straps to the shelf.

Step 23: Assembly and Wiring

Fit the lid, its hinges, the toggle catches and the LED panels. Run a wire between the lid and box and either fit connectors or solder it directly to the LED Panel. You may also want to cover the wires in PVC tubing. Attach the control panel with short screws, and fit the power switch. Then fit the power connector and the power switch and wire it all up as per the schematic.

Step 24: Final Assembly

Fit the shelf, foam and glass assembly and the glass lid and its hinges and check that everything still opens and closes smoothly.

Step 25: Testing

You may want to check everything is OK by hooking up a bench PSU and turning the voltage up slowly. You may also want to check the voltage of you PSU. When you're confident everything is OK plug it in and switch it on.
Safety note! Do not stare directly into the UV LEDs. UV light is harmful to your eyes. It's also a good idea to get hold of some laser goggles, these should block all the light below 532nm. To get an idea of the amount of time you should expose your PCBs for you can do an exposure test. Coat a piece of scrap metal with Photo resist on one-side and mark minutes on the other. Then with a piece of card mask of the metal expose for 1 minute then move the card to the next, mark expose for another minute and keep going until you reach the end. Remember start at the 10-minute mark and work down.

Step 26: Go Make Some PCB

And you're done. Go and experiment with the photo resist and get a feel for how it responds to the UV light and the chemicals you're going to use with it.
A great first project might be a timing device for your UV Exposure Box. I've deliberately left plenty of room on the control panel for this and in fact it will be the subject of my next Instructable.

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