Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

http://hackedgadgets.com/2007/02/24/uv-led-pcb-exposure-scanner/

TANSTAAFL!

Myc

On Feb 15, 2008 2:36 PM, Stefan Trethan <stefan_trethan@...> wrote:

> if you use an old flatbed scanner you'd have all the moving parts for
> that (and the board could remain in place, which is always good).
>
> BUT the drawback is obvious, longer exposure time, ~100,000 times
> longer for the board you suggest to make ;-)
>
> ST
>
>
> On Fri, Feb 15, 2008 at 7:52 PM, agscal -AGSCalabrese <agscal@...<agscal%40gmail.com>>
> wrote:
> > Why not make a line of LEDs spaced 7-8 mm apart.
> > then push the photoresist board underneath the LEDs
> > at a constant speed ? If your line is 160 mm long you
> > can produce exposed boards 160 mm by 1,000,000 mms.
> >
> > Gus
> >
> >
> >
> > On Feb 15, 2008, at 5:39 AM, Myc Holmes wrote:
> >
> > All of the designs so fr have used a rectangular grid. Since the
> > output is
> > circular in shape, why not take a clue form the honey bees and set up
> > the
> > LEDS in a hexagonal grid? This should result in more even coverage.
> >
> > Myc
> >
> > On Fri, Feb 15, 2008 at 3:48 AM, agscal -AGSCalabrese <agscal@...<agscal%40gmail.com>
> >
> > wrote:
> >
> > > Even LEDs from the same batch cannot be trusted to
> > > have the same brightness for the same current. After
> > > they age, it is even less likely that they will match. If you
> > > want them to stay matched you would have to have some
> > > kind of feedback or periodic re-calibration of each LED.
> > > I think that is probably overkill.
> > >
> > > I recommend that you use a current limiting resistor and
> > > put 10 or 20 or more in a string. This guarantees that every LED
> > > in that string will get the same current.
> > >
> > > For example, if you had 20 LEDs and each one had 1.7V
> > > forward voltage drop, you would have a total forward voltage
> > > drop of 34 volts. If you use a 36 VDC supply you can tweak the
> > > resistance
> > > to get the current you want. To get 20ma you would divide 2 volts
> > > by .02 A to get a resistance of 100 ohms. The 2 volts comes from
> > > subtracting 34V from 36V.
> > >
> > > This approach can be modified to match whatever power supply
> > > you have laying around to the LEDs you have.
> > >
> > > Gus
> > >
> > > On Feb 14, 2008, at 10:19 PM, javaguy11111 wrote:
> > >
> > > > I went ahead and placed an order for some as well.
> > > >
> > > > I am wondering if just doing simple current limiting resistors is
> > > > sufficient or if a proper LED driver chip would be the better
> > way to
> > > > go. From what I have read, at least for visible LEDs, you want to
> > > > match the current in the LEDs to ensure that they all shine evenly.
> > >
> > > [Non-text portions of this message have been removed]
> > >
> > >
> > >
> >
> > [Non-text portions of this message have been removed]
> >
> >
> >
> >
> >
> >
> > [Non-text portions of this message have been removed]
> >
> >
> >
> > Be sure to visit the group home and check for new Links, Files, and
> Photos:
> > http://groups.yahoo.com/group/Homebrew_PCBs
> > Yahoo! Groups Links
> >
> >
> >
> >
> >
>
>
>


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