July 1998
ATM's sometimes find the need to make their own
PCBs (printed circuit boards) in order to complete a portion of their current
project. Shown here is an Ultraviolet Light Exposure Box for exposing
sensitized circuit boards. Parts for this project may be obtained
from your local hardware or home improvement outlet. I used four
18 inch GE black light fluorescent bulbs in my design. You will also
need four pairs of 15 watt fluorescent bulb sockets, four 15 watt ballast
transformers (inductors), and four 22.5 watt fluorescent bulb starters
(capacitor and thermal switch combination.) A full parts list appears
at the end of this article.
I have also found this device doubles as a handy light table for other uses. By replacing the UV bulbs with normal fluorescent bulbs, it can be used as a small general purpose light table for many of the other tasks faced by ATM's.
The method of making circuit boards at home involves
using a copper-clad fiberglass or phenolic board which has been coated
with a photo-resist sensitive to UV light. The circuit pattern you
wish to imprint on the board is then placed between the sensitized board
and a source of UV radiation. After this exposure, the image of the
circuit pattern on the board is then "developed" which removes the extra
resist from areas you want to remove the copper. The board is then
etched in an etchant solution to remove this unwanted copper leaving the
pattern of circuit traces represented in the design.
The bulb sockets, ballast, and starter are wired as shown in the wiring
diagram. Each bulb's circuit is wired in parallel with the others.
You may choose to use more or fewer bulbs in your design. I used
four because it seemed to give very even illumination of the exposure surface
and fit the maximum dimensions of any circuit card I am ever likely to
make myself.
The base of the box with UV fluorescent tubes is assembled and wired first.
Each bulb requires a socket set, ballast, and starter. The bulb sockets
and ballasts are screwed to the masonite base board using two short sheet
metal screws each. In order to make the design more compact, the
starter components were removed from their metal cans and directly soldered
into the wiring for the bulb sockets. The starters and wiring were
then secured to the masonite base board using a few dabs of "hot glue"
to keep everything from rattling around.
The base is next assembled into the plywood enclosure. The switch
and power cord are installed through openings in the end of the box.
The dimensions of the plywood box may be modified to suit your own requirements.
Be sure to leave enough room between the UV bulbs and the diffusing screen
to ensure adequately even illumination. I allowed about three and
one half inches between the bulbs and the diffuser and spaced the bulbs
about three inches apart. This proved to be a good compromise in
my unit. The exposure times with this unit run from about 10 to 16
minutes. The actual time depends not only on the distance from the
bulbs to the exposure surface, but also on the thickness and make-up of
the glass or Plexiglas exposure surface. The different types of materials
pass UV light in differing amounts.
This photo shows the front end of the box and illustrates the mounting
of the fluorescent tube sockets, ballast transformers, and power switch
with wiring. It also shows how the wires along the bottom of the
box are held in place with small areas of "hot glue." Note the narrow
shelf inside the top edge of box used to hold diffusing screen and glass
exposure surface in place. This shelf is made up of one quarter by
three quarters inch cabinet molding glued around the inside of the enclosure.
The diffuser frame rests on this shelf and the glass exposure frame rests
on top the diffuser. I also installed short sections of three quarter
inch cove molding into the corners of the box to add strength to the assembly.
All joints in the plywood enclosure were glued and I found no need for
brads or screws to hold everything together. The bottom is screwed
to the enclosure in case I ever need to disassemble that part of the unit.
The
masonite base board of the box holds all the electrical parts. The
sockets and ballasts are screwed to the base and the wiring and starter
components are held lightly in place with spots of hot glue at strategic
locations. All these components are hidden by the poster board liner
when the box is finally assembled for use.
This photo shows the inside of box after installation of the reflective
liner. The liner is made of poster board with aluminum foil glued
to the top side. The foil is cut back in areas near the electrical
connections for the fluorescent tubes to prevent shorts and electrical
hazards. The shape of the liner was laid out by carefully measuring
the inside dimensions of the box and part locations, then transferred to
the poster. The sides were laid out at the same time and a small
amount allowance was made for overlap in the corners. The aluminum
foil was then glued to the poster board using "white school glue."
The board was next bent into the proper shape and fit into the box.
There was no need for anything to hold it in place as the upper edges of
the liner tend to spring outward and are captured under the shelf which
holds the diffuser and exposure glass.
I should note that I originally intended to just
use the white poster board to help diffuse the light throughout the box.
I overlooked the fact that most white paper products actually absorb the
UV light and re-radiate in the longer portions of the spectrum - they fluoresce
quite well. This meant that the amount of UV available to expose
the circuit board was reduced. The aluminum foil works quite well
to help spread the light around the inside of the box and doesn't significantly
change the wavelength makeup of the spectrum.
This photo shows the diffusing screen. It is made from a piece of
matte drafting velum taped to a foam core poster board frame. The
foam core poster board is about 0.25" thick. The side with the velum
is placed on the shelf in the exposure box and the glass top is placed
on top of it. This provides for a 0.25" gap between the diffuser
and the glass giving good, even illumination.
Here the unit is assembled (minus the lid) and turned on. The color
is actually a very deep violet-blue but is not recorded well by the camera.|
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| 15 watt black light fluorescent bulb. |
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GE F15T8/BLB or equivalent. |
| Flat-bottomed "medium" fluorescent bulb socket. |
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| 15 watt fluorescent ballast. |
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Valmont Electric 120PH-120 or equivalent. |
| 15 to 22.5 watt fluorescent starter. |
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Surelite Products FS-25 or equivalent. |
| 18 to 20 gauge 125 volt stranded hook-up wire. |
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Cut to length as required. |
| 120 volt wall mount light switch with cover plate. |
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Optional - spring driven wall mount 30 minute timer switch (Intermatic FD30MWC or equivalent.) |
| 120 volt power cord. |
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| Masonite or Plywood Base |
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| Plywood Lid (3/8" or 1/4") |
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| Plywood Sides (3/8" or 1/4") |
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| Plywood Ends (3/8" or 1/4") |
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| 3/4 " cabinet cove molding. |
4 - 1 " |
Used to strengthen the enclosure. |
| 3/4 " x 1/4 " cabinet molding |
2 - 14 " |
This forms the shelf for the diffuser and the exposure glass. |
| 15 mil thick poster board |
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| Household aluminum foil |
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| Drafting velum |
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Matte on one or both sides. |
| Foam-Core Poster Board (1/4") |
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Only one 14 " x 20 " is required if the diffuser frame is made from 3/4 " wide glued strips. |
| Glass or Acrylic Plastic |
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| Cabinet Hinges (Light-Duty) |
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| 1/8 " long sheet metal or wood screws. |
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| Solder and electrical tape or wire nuts for wiring connections. |
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| Glue or tape for securing loose parts and wiring.
Woodworker's glue for box assembly. |
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| Cork or rubber "feet". |
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Self adhesive electrical project case feet. |