Instructables
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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.
 
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Step 1: Materials

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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

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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

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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

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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

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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

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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

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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

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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)

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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

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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

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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

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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

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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

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

Step 16: Solder Links

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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

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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

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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

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

Step 20: Control Panel

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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

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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

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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

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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

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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

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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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dalpets2 months ago

Decided to go thro' hole with the resistors instead of a facsimile of smd. I had no problem with solder track shorts with this method, mainly because I used a 2mm BEVEL tip with .015" (.381mm) 62/36/2 silver solder

dalpets3 months ago

I've come to the conclusion, despite the foregoing, that the only way to make sure that each led is flat is to put something flat directly below the led being soldered. Even so you need to check there is no slack.

Sometimes it might not quite work out that way so it's a good idea to solder only one leg & check for slack. If there is, just apply more heat to that leg. I don't like doing this as there is the risk of lifting the copper track, which has happened to me, so I'm very wary (leary) now on that score. A lifted copper track can cause big problems & if you are near completion of the board it is a real bummer to have to correct with bridges.

Hope this helps.

dalpets3 months ago

Just remembered-I found the the recommended spot tool not nearly sharp enough & very very laborious work when you consider the number of spots you have to do. Save your money on this excuse for a cutter.

On the other hand rolling a new drill bit back & forth between the thumb & forefinger was surprisingly quick & effective. No comparison really.

I originally used a Proxxon hobby drill press with a Dremel drill using an adaptor & a burring drill bit (the Dremel drill press is well known not to be an accurate design). This method is quick but requires a degree of control, and its OK if your doing them all at once, but the real downside is that the resulting dust from strip/Vero board, albeit not huge, is apparently very toxic to the respitory system. If doing it this way make sure it is done in the open air.

Hope this helps.

dalpets3 months ago

A few more tips.

Before doing any soldering I found it obligatory planning to do a dummy placement of leds on the top horizontal row & the left vertical column to make sure that the design is workable for the board size, also making sure that there is room at the bottom of the board for the two power rails. I then marked component placements with texta around the side of the board. When you start soldering use your continuity meter for every solder joint to ensure there are no short circuits between adjoining rails. Its easy to slip up otherwise.

Don't be tempted to follow the instructable blindly, particularly the pics. Make sure you understand how the first of all the blocks works. If necessary do a practice circuit of 2 vertical blocks (of 3 leds each) on scrap board to make sure you have continuity working between blocks.

The author says it tricky to get the leds to sit flat with only two hands. Yes it is, but that can be overcome by using the following simple method. Place temporary unsoldered leds in the 3 corners, ie., other than the corner where you are soldering the first led. Bend their leads over flat to the board. That's a reasonable start. Then place the board on an even flat surface with the temporary leds facing down, whereupon they act as stabilizing legs. There will still be some small slack between the board & leds but if you place a reasonably heavy weight on the top of the board ( I used a cheap steel sticky tape dispenser) the slack will be taken up completely & voila the leds then sit perfectly flat.

I hope this helps for those of us that are challenged with circuit design.

dalpets3 months ago

Not immediately obvious what the diagonal spot cuts are for? They are necessary to electrically isolate each vertical bank of 3 leds.

Saw elsewhere a tip recommending cutting the leads before soldering to avoid mechanical strain on the solder joint and the potential for later breakdown of the joint. Seems a worthwhile precaution, although too long with soldering iron heat may destroy leds with such shorter leads.

Probably wise to test each LED before soldering because if you have to wrestle with removing a dud LED, depending on technique, you could destroy the copper strip, which too easily lifts with excessive heat on a standard board, & if you don't have room to bridge the broken continuity you've wasted a hell of a lot of time & energy building the array.

I found it preferable to install a vertical column of 3 led sets, rather than horizontal banks in order to make sure from the outset that the there are no errors in the design. I found it easy thereafter to duplicate succeeding columns use the first as a guiding template.

dalpets3 months ago

Got it to work at last. This is very, very detailed work. Easy to make mistakes along the way. Needs some serious time, effort, continuity testing & unlapsing concentration to get it to work first time. Don't do it when you're tired. Screw up with the spot cutter too many times & you'll need to toss the board.

Saves heap on buying a commercial unit, but hey you need to be a masochist to do it.

jscoppetta3 months ago

Can i use the plexiglass instead of normal glass ? Or the result will be worse ?

wheelsfalloff5 months ago
Great project mate, followed as best I could but am having problems getting all of the LEDs to light up despite checking the circuit and replacing components. I am using .5w 75r with 3.2-3.8 V, 75ma peak, 20ma forward LEDs. And a 12v 7.5a power supply to test. Could it be a problem with this combination? Any advice appreciated!
Bertz5 months ago

I just completed my own UV exposure box. I used 96 UV LEDs in a 12 x 8 matrix (good for 4" x 6" boards). LEDs were approx. 3" from exposure window. The LEDs were 1200 mcd with 160 degree viewing angle. Exposure time was 2 minutes.

jmatharu8 months ago
hey! very useful post!
I was wondering instead making a new circuit board can we use the LED light strip?
I am a graphic student and want to use it to expose my screen printing screens will it work on them?
Many thanks.
Dillon123 8 months ago
Parts express has UV led kits of 100 for $9.99
jbromley1 year ago
This is a very nice instructible. It is the most complete UV box I've seen. I do have one question. Is there a reason you put the jumpers and resistors on the copper side of the board and not on the component side? Was it just an aesthetic decision? Or will the UV cause component degradation of some type?
agodinhost3 years ago
Hi, please, were did you bought your leds? I did looked in ebay but I can't say for sure that those leds there really have 20º view angle. All leds that I found there are kinda suspicious, chinese cheap fake stuff, even tough they say they are 20º I can't trust. I'm in Brazil and at here ebay is the more easy way to get electronics by a decent price but not in this case.
I found these in Germany. They are more expensive, but because of the angle of 120° they are more suitable for illuminating flat surfaces. This is exactly what we need. http://www.ebay.nl/itm/50-UV-Leds-4-8mm-120-200mcd-Led-STRAWHEAD-schwarzlicht-ultraviolett-/370543226494?pt=LH_DefaultDomain_77&hash=item564618ee7e#ht_1926wt_1139
Isnt 200mcd WAAAY too little?
jeddar2 years ago
I have a question, would the output from this UV box be enough for it to be used as a sterilizer box? I do my own beer/wine brewing and I'd love to be able to fit a rig I could use to sterilize equipment in between batches..
Sterilization also takes place at 365nm, however, to get the type of power needed would require a much more robust, i.e. $$$, set of LEDs that can output some real power. You'd also have to do some extensive testing to make sure sterilization is taking place, the research articles I've seen were using 15 to 30 minutes exposures to achieve complete sterilization of bacteria.

I think using a non-toxic spray method would be a lot quicker and cheaper in the end.
CODIY jeddar2 years ago
No, the wavelength for sterilization is much shorter than this type of LED can emit (<300 nm where these are likely closer to 400 nm). UV LEDs for sterilization are expensive and difficult to find in small batches.
MFXPYRO1 year ago
I built a double sided UV exposure box with vacuum pulldown around 10 years ago using tubes and it is still going strong without replacing the tubes. Also my box will do boards up to A3 size, How many LED's and what would they cost for an A3 sized double sided unit? Yes they're more energy efficient but no one is going to have a unit running for more than a few minutes at a time so cost is negligible. You've made a nice looking unit but I just can't see any real benefit of LED's over tubes both in cost and construction time. TBH. Hopefully I'll post an indestructible of my unit soon but as I built it so long ago I'll need to dismantle it for photos.
Aquafuz1 year ago
I'm a bit lazy to research this so I guess I'll just ask,
Which will give better and faster results; total watts, total mcd or both?
lsilva141 year ago
I managed to finish this project, but with some mods of my own :p

Instead of painting the wood white, I covered it with metallic duck tape which gives me extra brightness.

I also tested some pcb that usually take 120s in a retail exposure box with UV lamps and they took the same 120s in my box with the LED.

Thanks for the idea and the design, it is excellent!
Peterflick1 year ago
So i found some LEDs but they peak at 400-405nm, everything else checks out. Will they still work?
Jake3131 year ago
Awesome project! Are those Metal Film Resisters?
Jakob28031 year ago
Looks very nice, I might copy your project and make a scaled-down version! ;)
rlorenzo2 years ago
can i just use a normal LED.....?
no because a normal coloured LED does not operate at the right frequency to be used for this purpose due to the wavelengths being different this diagram i found on google images shows the uses of the wavelength. :)
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rlorenzo2 years ago
can i just use normal LED's ??? this parts are hard to find here back in Pilippines.

UV LED is available at e-gizmo (taft ave, along DLSU). 3.50 PHP a piece.
dwah2 years ago
this glass too thick for LED UV going pass thur glass..
Vogavt4 years ago
I built the box, but I seem to keep blowing LED's. I'm using the supplier that you recommeded and everything appears to be okay. However, I noticed that when I turn off the wall-wart type 12v power supply, I have some latent voltage residing on the board. I can see some residual glowing of some of the LED's immediately after turning the unit off. It does eventually fade out, but if I turn the unit back on too quickly, I have a bright flash of the LED's. It's as if there is a capacitor somewhere that's holding current in the 12v transformer. Any ideas or suggestions to bleed off the current? I'm a newbie at these sort of things.
Vogavt Vogavt4 years ago
Okay. After surfing around the web I've found out what a "Bleed Resistor" is, but I haven't been able to find out definitively how to calculate the amount of resistance needed or exactly where to put it in the circuitry. Any suggestions would be welcomed.
Why guess resistors? Calculate:

Most efficient way is to use make strings of few LEDs in series with resistor (and you can connect many such strings in parallel)

R=(V-n*Vf)/If

P=R*(If)^2

where

R= resistance (Ohms)
V= power supply voltage (Volts)
Vf= LED forward voltage drop (usually about 2.2-3.8V for most LED, read specs)
If= LED forward current (usually 20mA, read datasheet)
n=number of LEDs in series (in one string)
P = resistor power (Watts, minimum rating)

note: you should aim to get the voltage difference to be about 2V:
(V-n*Vd) ~ 2V

reason:
if the difference is greater, the more energy is wasted into heat by resistor (you need more powerful resistor and power supply).
if the difference is too small you loose current regulation because LEDs are non-linear device.

calculation example, assuming you are using

V=12V
Vf=3.2V
If=0.020A (that is 20mA)
n=3

R=(12-3*3.2)/0.020=120 Ohm
P=120*0.02^2=0.048W (you can use 1/8W or 1/4W without problem)

note how P is small. that is because we have (12-3*3.2)=2.4V

using large resistors makes no sense because to get current to match spec, you would need high voltage (this is not just dangerous but also means that most of the energy is wasted in heat so those resistors would have to be for high power).

for example using same scenario
Vf=3.2V
If=0.020A (that is 20mA)
n=3
R=47k

we would need resistor to have power rating of at least
P=47000*0.020^2=18.8W

(and that was just power three LEDs)

Also DC power supply would need to provide much higher voltage which is
V=n*Vf + R*If
V=3*3.2 + 47000*0.020
V=9.6 + 940
V=949.6V

That is way too much just for poweering few LEDs.

So that was introduction for current limiting resistors.
For bleeding resistors we calculate it from time constant.

t=RC

where
t = time (seconds)
C=capacitor in power supply
R= resistor (note this is combined resistance of load and bleed resistor)

R=t/C=Rb*Rc/(Rb+Rc)

Rc=V/(m*If) this is load resistance (for m LED strings drawing current If)
Rb=bleed resistor

Say you have simple 12V PSU with bridge rectifier and 10000uF capacitor and 6 LED strings drawing 20mA

C=0.010 F
m=6
Rc=12/(6*0.020)=100 Ohm

Without bleeding resistor t=RC=0.010 * 100= 1 seconds
which is shourt enough

adding bleed resstor in parallel, we can bleed the capacitor faster (total resistance is smaller and t=RC is smaller).

Say we use Rb=18 Ohm

R=18*100(18+100)=15.25 Ohm

t=RC=15.25 * 0.010 = 0.1525 sec

so turn off time was reduced by about 6x.

note that value of bleed resistor was quite small. usinglarge value such as 10k or 47k would not make any difference.

also keep in mind power disipated by bleed resistor P=12^2/18=8W
(which is more than used power for LED circuit in this example).

good luck
http://youtu.be/qPt0X5s-1JU
djhamer (author)  Vogavt4 years ago
You’ll need to put your bleed resistor across the whole circuit, i.e. between the + and – supply. Make sure its in the 10s of K ohms e.g. 47Kohms.
dedson13 years ago
By varying the types of LEDs to get the proper output you could also use this as a light box for seasonal affective disorder. Just a thought.
djhamer (author)  dedson12 years ago
I really wouldn't, UV is very bad for your eyes. The amount of UV energy coming out of this lamp is more than enough to do some damage. Even a few second is enough to cause discomfort. Since the consecration of light intensity is in a band invisible to the human eye, your eyes cannot adjust properly and you natural reaction to look away as you would if you looked at the also does not occur.
agodinhost3 years ago
There is a site that provides one wizard to help you with any different setup in mind: http://led.linear1.org/led.wiz

Just enter your number of leds and the wizard will help you to decide the resistors, the arrange of leds and the PSU that you will need.

My 5 cents, I hope it helps.

Cool instructable by the way, really cool stuff.
Love the link.
rykonen3 years ago
I love this instructable. How do I figure out how much power my LED's will need? I want to do four times the amount you have here... Could I just repeat the LED corn rows a few more times before I run it to the 12v ? (That idea doesn't seem right some how...)
Nazdro rykonen3 years ago
Hi,

I'll assume you're using the same kind of LEDs and resistors that are specified then for every veroboard you've got 84 LEDs in bundles of three connected in parallel. Thus for every bundle you've got a forward current of:

I_f = (12V - 3x3.4V)/75ohm = 1.8/75 =24mA

So one veroboard, with 84 LEDs, will draw a current of

I_vero = (84/3)x24mA = 672mA

So if I've understood you correctly you wish to have 8 veroboards of UV LEDs then after you've connected them in parallel the total power drawn from the source would become:

P_total = 12Vx(8x672mA) = 12Vx5.375mA = 64.5W

So you'd need a 12V 65+W power supply, though to be on the safe side I'd personally use a 75W. Though as I said, you might be using LEDs with a different forward voltage so here's a formula where V_f is the forward voltage of your diodes:

P_total = 2304(4-V_f)/25 W
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