DIY LED light box for PCB manfacture... I give up.

Discussion in 'The Projects Forum' started by Evil Lurker, Apr 29, 2014.

  1. Evil Lurker

    Thread Starter Member

    Aug 25, 2011
    Ugh, I give up. Been trying to build an LED based UV light source and it has been nothing but problems from the get-go. The biggest problem has actually been with component sourcing and cost. Since I can't etch my own boards I have to rely on perfboard for mounting my LEDs. Perfboard is a royal pain in the rear to work with, especially the cheap single sided bakelite variety as it is very very easy to lift a pad. For the time and aggravation involved in working with it I find that it is much much better to just go ahead and get some of the double sided fiberglass with plated through holes even if it costs a bit more. Problem with that is there are no sellers of double sided fiberglass perfboard larger than 10x16cm in the USA other than major electronics distributors. If you have to go through one of them get ready to shell out some serious cash. For example a 6x9" board is $20 at digikey.

    Then there is the cost of the LEDs. If I understand correctly 365nm is the best output wavelength. 5mm emitters using this spectrum output exist, but are god awful expensive at anywhere between $1-2 each. Supposedly 395nm LEDs will work and fortunately these are much more affordable, but again there are not too many sellers in the USA that offer these at reasonable cost.

    In an attempt at sourcing some suitable LEDs I ended up purchasing a lot of 150 off ebay recently, and unfortunately for me they turned out to be junk. The purported spectrum was 385nm and while they do actually put out UV light they simply do not put out very much of it. I highly doubt if measured it would total 200mcd per emitter. On top of it all they have an extremely narrow viewing angle, which I'm going to guess is going to be 15 degrees or so so any PCB will have to be held away several inches to get an even exposure.

    While these might be unsuitable for actual use I went ahead and ended up building an array of 128 of these LEDs with 16 parallel strings of 8 emitters in series on breadboard as a "proof of concept" using some creative wiring techniques and a $2 HP printer power supply from Goodwill. Current limiting was accomplished via a single 150 ohm 1/4w resistor per string which allowed on average 18ma of current.




    It may be sloppy, but overall it works. What I did was take two breadboards and remove a power rail off each since they are more or less held together with dovetail joints and double sided sticky tape. Then I mounted those two together so as to allow for more or less even spacing of the LEDs without having a gap in the LEDs two power rails wide. Construction was simple. Put LED in length gauge and trim the leads to proper length, insert into breadboard, take lead that was just trimmed and form it into jumper wire using the case of a TO-220 transistor. Rinse lather and repeat. 128 times.

    Should I ever decide to attempt to build something with a bit more "oomph" using a different bunch of 5mm LEDs with a higher output this is exactly how I would do it all over again, except this time using 3 new "virgin" breadboards put together instead of 2. Thing is I can get three of these exact same 830 tie point boards off ebay for less than $10 shipped, there is no soldering involved, mounting is simple thanks to the sticky tape, and it costs less than using an actual PCB. And if you ever have anything go wrong with your array repair is easy.

    But for now, I have decided to go with one of the 36w nail polish curing units. These have 4 x 9w 365nm standard G23 base fluorescent tubes in them. Total cost for one of the units, 4x replacement tubes, and some fake nails and decorations is ~$25. Truth be told if I had known about these I would have never even bothered with the LEDs to begin with. These units have a slide out panel on the bottom so in theory all that is needed is to put your transparency on top of your blank PCB then sit the entire curing unit on top of it. Not for sure exactly how large the opening is on the bottom, but it looks like it might actually do an entire 4.5x6" rat shack sized PCB with zero modifications.



    Hopefully it will be in a few days and I can get some pictures. I'm anxious to see how well this photoresist paint works but that is going to be a whole other thread.
    SgtWookie and Bernard like this.
  2. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    Those fluorescent tubes should work nicely..

    FYI.. (great company..professional boards)
    HARD to beat that pricing (example 10 boards 5cm x 5cm = $0.99/ea)..
    I stopped doing "DIY" boards a while ago.. I'd rather wait the 2 weeks for delivery and have a "professional" board complete with soldermask/silkscreen,etc...
    djsfantasi likes this.
  3. wayneh


    Sep 9, 2010
    And there's a hard learned lesson.

    Regulars around here know I tend to lean very hard towards the "buy" end of the make-or-buy decision. Buy when you can, build when you have to. I'm an avid DIYer in most things in life (food, car repair, woodworking, electronics, etc.), so I'm not afraid to launch a project, sometimes just for fun.

    But a little wisdom goes a long way, and I've found that it rarely makes sense to build something when you can go out and buy something close. Learning new skills and the satisfaction of completing a project are great, but it takes the wind out of your sails when you learn you could have saved time and money - sometimes a LOT of both - by doing more research upfront.
  4. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    The fluoro tube won't give an even coverage like the LEDs will. You will need more distance between the tube and PCB to try to even out the coverage.

    Regarding the perfboard and lifting pads, if you use stripboard (veroboard);

    The tracks are ideal for wiring long lines of LEDs into arrays (much less work) and never lift off from soldering.
  5. Wendy


    Mar 24, 2008
    A schematic is always good, even if it is something simple like LEDs.