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UV Light box design

Discussion in 'LEDs and Optoelectronics' started by Brenton Pettitt, May 27, 2014.

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  1. Brenton Pettitt

    Brenton Pettitt

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    May 27, 2014
    Hi,

    I'm just about to complete my second strip board of uv leds for a two sided uv light box, based on an online instructable. The specifications for the recommended led (168 in total) have been provided, although the instructable, other than in a general way, is very light on in regard to detail for the housing of the led boards. Other online help similarly lacks detail for my needs. I'm not talking of general dimensions of the overall construction but rather internal aspects that are important to the design. Basically, I want to make a simple, light weight housing of modest dimensions to accommodate the boards.

    In particular, I would like to know a couple of things, although there may be more issues that you also consider important and relevant.

    Firstly, is there an optimum design distance between the circuit artwork & the the upper & lower led boards for the construction? (I'm thinking that for commercial boxes there is probably an engineering formula for that distance, but I don't want to get too anal about it). Also, some commercial units use a vacuum seal for the artwork. I don't know if it is possible to simulate that in a simple way, without going overboard.

    Secondly, is perspex a satisfactory option, as opposed to glass, to sandwich the artwork.

    Any help that you can provide would be helpful.

    Thanks.
     
    Last edited: May 27, 2014
  2. BobK

    BobK

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  3. shrtrnd

    shrtrnd

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    Jan 15, 2010
    If they didn't get specific in your instructions, it may be hit and miss when you fire it up. Be prepared to reset the distance as needed for your artwork.
    The composition of the surface of the artwork may also impact the UV illumination, as some composites might absorb different spectra.
     
  4. Brenton Pettitt

    Brenton Pettitt

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    May 27, 2014
    BobK,

    Maybe "overthinking it" by your standard of appreciation @ 3229 (obviously experienced-I bow to your seniority), but for inexperienced me there are unknown unknowns here ( quoting Rumsfeld) about this project.

    A good teacher tailors his teaching to the needs of his students, not criticize them for asking questions, given that his students are in a learning phase. Surely that is what this forum should be about.

    Actually the question wasn't answered with any exactitude, so I figure you don't know the answer.

    Anyhow thanks for your contribution.
     
    Last edited: May 28, 2014
  5. BobK

    BobK

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    My theory is you start simple and only get more complex as you need it. It sounds like you are trying to build the ultimate device without ever having tried simpler and less costly methods.

    Bob
     
  6. Brenton Pettitt

    Brenton Pettitt

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    May 27, 2014
    Bob,
    Yes I accept what you say, but that's a generalization many experienced people fall back on and it can often mean they can't be bothered with a meaningful response because by implication they feel they are too experienced for what the questioner is asking.

    Actually, your saying has some truth, but on the other hand I view it often to be a cop out. In other words you can't necessarily assume that everyone is dead raw & challenged in their ability to grasp medium level technical matters and that they need to start at the very sub-basement. That can be a waste of time, particularly for those of us with more than just a modicum of technical ability.

    No mate, I only want a simple, straightforward & cheap setup ( I have budgeted $60 for it), but I don't want to have to rebuild the project because I have made basic errors. I could house it in a small, medium or large box and in any event there is plenty of room for error, particularly regarding the led distance from the artwork. There surely must be some relationship, taking all relevant factors into account, but in fairness to you, without that relationship being spelled out (most of aren't engineers here eh!) I just need to 'suck it & see' by experimentation, and go the long way round.

    The online projects I am using as a guide, if you are interested are at these links;
    http://www.instructables.com/id/UV-LED-Exposure-Box/?ALLSTEPS
    http://www.doctormonk.com/search?q=UV

    You may see my question as' over thinking it' and I appreciate that there can be outrageous limits but I like to have at least a plebs understanding of whatever I am doing. That's how I'm wired.

    Thanks you again very much for your attempt to help me with my project.:)

    Best regards
    dalpets
     
    Last edited: May 28, 2014
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hello

    dal pets, lets get one thing straight here Bob is a very experienced member of this forum and he was only trying to help you by saying that there maybe an alternative method to what you are trying to do. But I guess from your response to him that you feel passionate about getting the LED light box to work.

    I don't think from the extremely small amount of information you have given us, how could us brilliant engineers help you. You haven't even given us the details of the LEDs and the PCB material you plan to use. But with that aside you can help your self by doing the following.

    1) Get a Stouffer 21 Step Sensitivity Guide

    2) Initially calculate exposure time by working out the output power of your LEDs in watts and dividing by the total areas of the PCB in cm^2. Then divide this result into the total energy (mJ/cm^2) requirements of your dry film PCB materials then multiply by 2

    3) Expose the PCB and the Stouffer guide.

    3) Develop you material as recommended by the manufacture.

    4) Examine the guide and do the following. If say #5 has been partly etched away the #4 is called the held step. The ratio of exposure time between two numbers on the guide is 1.414. So say your PCB material needs an exposure number of 10 then you would increase your time 5 fold so it didn't etch any number lower than 10.

    As far as distance is concerned you have to think about the divergence and uniformity of luminance so the half angle of the LED plays apart and the distance plays a part in this also. Not forgetting to mention the further away you are the intensity of the light will vary by the inverse square law. So the exposure time will be increased.
    Adam
     
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Other than the dangers of working inside microwave ovens... I recall that someone fitted a UV source inside a gutted microwave, keeping only the timer and the turntable.

    The timer (obviously) controls the exposure, and the turntable is used to help ensure even illumination of the board by rotating it under the light source.

    The only issue is that you should block off the door window to prevent the UV from going places it shouldn't (i.e. into your eyes)

    The microwave interlocks also help keep you safe.

    Just be VERY careful because there are things inside a microwave oven that will kill you if you look at them sideways.
     
  9. Brenton Pettitt

    Brenton Pettitt

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    May 27, 2014
    Hi Arouse 1973,

    Thank you very much for your detailed reply. That's is the of the sort of information I was after. I'm not really passionate or a perfectionist in this area, just let's say measured in what I do.

    It's hard, isn't it, in the context of a forum to let people know where you are coming from & your level of understanding in order to develop a meaningful dialogue, ie., without writing a book that bores people to death. Indeed, one can be wasting ones time, with perhaps an expectation of a bland reply, depending on ones past experiences.

    Actually, I can't but smile at your remark that I supplied scant information. That's another forum saying that is regularly trotted out that can be seen to be a put off and can put distance between the so called experienced & less experienced user. If it's a genuine statement I have no qualms but if it's just one of those forum late night 'throw away lines' it doesn't contribute. If on the other hand, if it's 'this Is what we specifically need to know' type of questions, that's a great start, rather than glibly putting the OP 'on the spot'.

    Maybe I was somewhat harsh about Bob's reply but I thought I made it clear that I knew he was an experienced contributor.That was really the problem. It's not my way to antagonize people. Maybe I've been around too long & have developed a cynical attitude in some areas.

    Anyhow have a great day & thanks for your thoughtful inputs.

    Best regards
    dalpets
     
    Last edited: May 29, 2014
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Hi dalpets.
    It is really important to supply enough information so we can help. It was a genuine statement and not a late night throw away line as you put it. Which is why I wrote the rest of it. The thing is if we advised you incorrectly because of lack of information and you ended up damaging an expensive component or even worse hurting yourself. We are going to feel pretty bad. You will find this a lot on forums and when extra information is given the replies come flooding in.
    Adam
     
    Last edited: May 29, 2014
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    You've got to have a bit of sympathy. There's a thin line between too little information and tl;dr. Of course the worst is when you get no information in a long rambling post.

    In most cases, if there's no easy balance to be found, erring on the side of too little information is generally better.

    Let me offer you 2 answers to your questions.

    The first relates to dimensions. With 168 LEDs, you either want to cover a large area at low intensity, or a smaller area at a higher intensity. Whatever you choose, you'll end up with a certain distance between them. Also your LEDs will have a half angle that describes where the intensity of light falls off to 1/2.. The height of the LEDs from the board being exposed should be at least sufficient for this 50% point to reach to the center of the furthest neifhbour's beam. If they're on a square grid, that height will be the grid spacing times sqrt(2) divided by the tan of the half angle. A hexagonal "grid" will give you more even lighting and allow you to cover an area more efficiently. Oh, and the higher you place your light source the more even the lighting will be for a given half angle. The advantage of a narrow half angle is that the light tends to be more perpendicular

    As for the material to hold the artwork down, i cant recommend anything other than glass due to its rigidity. A collegue of mine recommends nothing thinner than 6mm due to the flexibility of thinner glass.
     
  12. Brenton Pettitt

    Brenton Pettitt

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    May 27, 2014
    Thank you all for your mature insights & your understanding of my quibbles.
    Thankfully no overt flaming on this issue.

    I will go away and digest all your contributions.

    For the record the leds I am using have these specs;
    * Emitted Colour : uv/ purple * Size (mm) : 5mm
    * Lens Colour : Water Clear
    * Peak Wave Length (nm) : 390~395
    * Forward Voltage (V) : 3.2 ~ 3.8
    * Reverse Current (uA) : <=30
    * Luminous Intensity Typ Iv (mcd) : Average in 2000
    * Life Rating : 100,000 Hours
    * Viewing Angle : 20 ~ 25 Degree
    * Absolute Maximum Ratings (Ta=25°C)
    * Max Power Dissipation : 80mw
    * Max Continuous Forward Current : 30mA
    * Max Peak Forward Current : 75mA
    * Reverse Voltage : 5~6V
    * Lead Soldering Temperature : 240°C (<5Sec)
    * Operating Temperature Range : -25°C ~ +85°C
    * Preservative Temperature Range : -30°C ~ +100°C


    The plastic hinged light box I will be using is the same as that covered in the doctor monk link in an earlier thread, except it will measure 8" x 5" . The strip boards measure 100 x 600mm.


    best regards
    dalpets
     
    Last edited: May 29, 2014
  13. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Let's assume that they have a 20 degree half angle for 50% power.

    Let's also assume you have them arranged in a hexagonal pattern.

    The for each unit of width they are apart, the must be 1/tan(20) units above the board. That's about 2.75 units.

    So, if you made a box 5cm tall (inside dimensions), you'd probably want the LEDs to be about 1.5cm apart to ensure a reasonable coverage. Personally I'd make the 1cm.

    As a rough guess, 168 LEDs would let you cover an area of around 12 x 12 cm (it's less than you think because the rows are spaced at 0.87 times the spacing between the LEDs in a row)

    How does that sound?

    I'd probably make a small MDF box with the LEDs in the lid so that you place the sensitized board, the artwork, and the glass in the bottom, then put the lid on and turn on the power for the required time.
     
  14. Brenton Pettitt

    Brenton Pettitt

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    May 27, 2014
    postcript: I just realized, when you boil it all down there's not much information at all that needs to be mulled over, in the quest for a more detailed OP question, as suggested should be the case. To suggest otherwise would be making a mountain out of a mole hill. For me, the separation distance & the led specs are the only significant issues, really. The rest of the build is pretty much common sense.

    dalpets
     
  15. Brenton Pettitt

    Brenton Pettitt

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    May 27, 2014
    Thanks for that *Steve*. I guess I'm committed to the boards I've already made up now (16 x 10 cm), based on the earlier 'instructable' link I posted. By your reckoning I'm a bit short on with led coverage, however, the author of that link says it shouldn't matter-just needs a little more exposure time. I presume that his comment will also need to be subject to the separation distance, which unfortunately he doesn't specify and which brought me to ask the question here.

    dalpets
     
    Last edited: May 29, 2014
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    16 x 10 is probably fine. Assuming it's on a square grid, you're still probably fine as long as it's 5cm or more above the board, although I'd go a little higher
     
  17. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
     
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