Though electronics can seem pretty complex to the
uninitiated, learning to use simple circuits (or to copy
more complicated ones) really isn’t difficult. Moreover,
that knowledge can help you save a tremendous amount of
time and money while allowing you to accomplish tasks that
might otherwise be impossible. That’s why you’ll find, in
nearly every issue of MOTHER EARTH NEWS, one or more useful projects
that require at least a rudimentary low voltage circuit . .
. and those devices are often mounted on what is called a
printed circuit board.
Furthermore, for over a year now, my electronics articles
have each been accompanied by access information, which has
given a source for reasonably priced, commercially produced
printed circuit boards . . . and I plan to continue that
practice in the future. Nevertheless, some readers may want
to learn how to make such boards, so that they can
complete projects that are shown in this magazine, as well
as start laying out their own electronic devices.
Fortunately, Radio Shack offers a kit that can turn just
about anyone into a designer of printed circuit
boards.
The Fine Print
A printed circuit is actually quite simple (but very
cleverly conceived) in both design and execution. Its basis
is a sheet of insulating material — usually either fiberglass
or phenolic plastic — over which a very thin layer of copper
is bonded. In order to turn the metal surface into a
circuit, a design is placed on the copper . . . and any
unwanted material is removed, using a chemical etching
process.
To begin, the pattern that the electricity will eventually
follow is drawn on the metal with an ink that’s resistant
to the etchant’s action. And after the process is
completed, a copper network that duplicates the original
inked design will remain. These thin metal paths serve as
the connecting “wires” for the components that make up the
final product. Now printed circuits may sound
exotic, but the builder who isn’t familiar with electronic
construction will find them perfectly suited to his or her
lack of expertise . . . because, once the board’s been
properly laid out, there’s little chance of making a wiring
error.
Radio Shack’s Printed Circuit Kit (ask for part number
276-1576) contains everything you’ll need to make a pair of printed circuit boards: two 3″ X 4 1/2″ printed circuit boards, one
resist ink pen, one six-ounce bottle of etchant, a two
ounce bottle of resist ink solvent, a 1/16″ drill bit,
and a plastic box to serve as the etchant tub. (The boards
come coated with a protective lacquer that must be removed
prior to use by scrubbing with water and an abrasive pad .
. . which is included in the kit.)
Laying Out a Circuit
Before you can begin constructing the board, you’ll have to
lay out the design. If you’re making up your own circuit,
first sketch out the pattern on a piece of paper. When
you’re satisfied with your arrangement, simply redraw the
lines on the board with the resist ink pen.
If you’re working from an article, though, it will probably
feature a photograph or drawing of-the board that’s needed
to assemble the project. (The picture in the Image Gallery should be the same
size as the board you’re going to prepare.) In such a case,
transfer that design to your own board.
If you have sharp eyes and a steady hand, you may be able
to freehand the circuit, using the photo or drawing from the Image Gallery as a
guide (personally, I’m not quite good enough to do it this
way). Actually, I find that the task is easier if I place a
sheet of clear plastic over the magazine pattern and trace
the design onto the transparent material. Then I use tape
to hinge the top edge of the plastic to my new copper-clad
board. By drawing on the metal and checking my progress and
accuracy frequently with my see-through overlay, I can
reproduce a pattern pretty faithfully. As an additional
guide, I often use a straight pin to poke a few strategic
holes through the plastic (and into the foil). If you try
this approach, be careful to make punctures only in areas
that will later be removed, to avoid damaging the
circuit.
When drawing the circuit lines, make absolutely sure that
you lay down a heavy mark with the resist ink pen.
Any breaks or thin spots could permit the etchant to reach
the bare metal. I have the best luck by starting with
small, light strokes . . . then I build the line up,
allowing each layer to dry before adding the next.
Rub-on Decals
Yet another way to place a circuit pattern onto a blank
board is with dry transfer decals. Radio Shack sells a package of four for under $3.00
(request part number 276-1577). Every sheet has an
assortment of integrated circuit patterns — each with the
proper hole spacings — as well as a selection of straight
lines and curves of different radii.
To apply a decal, remove its backing and then position the
chosen pattern on the copper-clad board. Next, connect the
components, cut out the desired number of straight lines,
and lay them onto the board in the appropriate locations.
Now, using a ballpoint pen, trace over the circuit paths to
transfer them to the board. Press gently at first,
gradually increasing pressure, to prevent the decal from
distorting or cracking. Finally, burnish the transferred
pattern by laying the backing paper across the design and
rubbing it firmly with the tip of the pen.
Etching
Your board is now ready for etching. Pour half the contents
of the etchant bottle into the bottom part of the plastic
box, saving the remainder of the liquid for your next
project. Then put the printed circuit into the tray, face
down, and gently agitate the mixture every so often.
When you’re working with the etchant, it’s a good idea to
use tongs or wear gloves. The liquid isn’t terribly
dangerous, but it is corrosive and can irritate sensitive
skin. (Besides, it always leaves a horrible
yellowish brown stain on your skin that takes days to wear
off.)
The exact length of time required to remove all the
unwanted copper will depend on the temperature, the amount
of metal to be removed, the frequency of agitation, etc.
But after 20 minutes, you can lift up the board for
inspection. If any exposed copper remains, return
the project to the solution.
After the board is completely etched, rinse it thoroughly
under cold running water for at least two minutes. The used
liquid should be discarded, but I don’t recommend flushing
it down the sink. Again, though the solution isn’t very
dangerous, it’s probably better left out of our sewage
systems. A decent burial would be a fitting end for
it.
Now, all that remains is to strip off the resist ink (use
the solvent supplied with the kit), or to remove the
transfer decals. But don’t put the solvent in the
etching tray: It’ll melt the plastic!
Mount parts to the finished board by drilling holes where
component leads must pass. Then solder the components in
place, using a small iron to prevent the foil from
lifting off the board. And what if you’ve made a mistake?
Easy. Open the circuits by scratching them with an
X-acto knife .. . and bridge gaps with a short piece of
wire.
By now, you’re probably itching to put your new knowledge
to work, so why not start by building the power-failure
system shown here?
MOTHER EARTH NEWS Power-Failure Alarm
Here’s a chance to exercise your newly learned skill while
putting together a very useful device. This
alarm is a simple circuit that monitors your AC
(alternating current) power and alerts you if it’s
interrupted. Not only could such a warning save a
freezer full of food from damage, it
might also help you rescue a brood of day-old chicks that
would otherwise freeze to death overnight.
Basically, the AC power (from a cord plugged into a
wall outlet) holds one of the alarm’s relays engaged, which
prevents the buzzer from sounding. When the utility power
fails, however, the relay drops out, the contacts close,
and a 9-volt battery supplies power to a warning alarm.
When power is restored, the device automatically resets and
waits for the next power failure.
Construction begins (of course) with the production of
a printed circuit board. Using the photo of the pattern
shown in the Image Gallery, fabricate a board according to the technique
described in the main article. (The original for this
project was built using only the contents of the Radio
Shack Printed Circuit Kit, so you should have little
trouble duplicating it.)
Once the board is finished, drill a hole through the
center of each of the circular pads with the 1/16″ drill
bit. Now, mount and solder all the components to the board,
but watch their orientations carefully. . . a mix up here
could do irreversible damage.
Finally, test the unit by inserting the battery into
its clip and holder. The buzzer should sound, and — when you
plug the alarm into the wall — the noise should stop. If the
buzzer fails to sound, or if it doesn’t switch
off when it should, recheck your work far
errors.
With the power-failure alarm plugged in (and a healthy
battery in place), you won’t be caught unaware even by a
midnight black out. And, if you’d like to develop
a more ambitious power failure system, you’ll find that the
relay is hefty enough to control an emergency lighting
setup, too!
