Experiment 15: Intrusion Alarm Revisited
142 Chapter 3
Installing the Switches
If you bought a project box from RadioShack, it may have come with two op-
tional top panels: one made of metal, the other made of plastic. I’ll assume that
you’ll use the plastic one, as you’ll have more trouble drilling holes in metal.
The type of plastic used by RadioShack is ABS, which is very easy to shape with
the tools that I have recommended.
You have to choose a layout for the switches and other components that will
share the top panel of the project box. I like a layout to be neat, so I take the
trouble to draw it using illustration software, but a full-size pencil sketch is
almost as good. Just make sure there’s room for the components to fit to-
gether, and try to place them similarly to the schematic, to minimize the risk
of confusion.
Tape your sketch to the inside of the top panel, as shown in Figure 3-107, and
then use a sharp pointed tool, such as a pick, to press through and mark the
plastic at the center of each hole. The indentations will help to center your bit
when you drill the holes. Remember that you’ll need to make multiple holes to
vent the sound from the loudspeaker, which will be beneath the top panel of
the box. The result is shown in Figure 3-108.
I placed all the components on the top panel, with the exception of the power
input jack, which I positioned at one end of the box. Naturally, each hole has to
be sized to fit its component, and if you have calipers, they’ll be very useful for
taking measurements and selecting the right drill bit. Otherwise, make your
best guess, too small being better than too large. A deburring tool is ideal for
slightly enlarging a hole so that a component fits snugly. This may be neces-
sary if you drill 3/16-inch holes for your 5-inch mm LEDs. Fractionally enlarge
each hole, and the LEDs should push in very snugly.
If your loudspeaker lacks mounting holes, you’ll have to glue it in place. I used
five-minute epoxy to do this. Be careful not to use too much. You don’t want
any of the glue to touch the speaker cone.
Drilling large holes in the thin, soft plastic of a project box can be a problem.
The drill bit tends to dig in and create a mess. You can approach this problem
in one of three ways:- Use a Forstner drill bit if you have one. It creates a very clean hole.
- Drill a series of holes of increasing size.
- Drill a smaller hole than you need, and enlarge it with a deburring tool.
Regardless of which approach you use, you’ll need to clamp or hold the top
panel of the project box with its outside surface face-down on a piece of scrap
wood. Then drill from the inside, so that your bit will pass through the plastic
and into the wood.
Finally, mount the components in the panel, as shown in Figure 3-109, and
turn your attention to the underneath part of the box.
Figure 3-107. A printed layout for the
switches, LEDs, and other components
has been taped to the underside of the
lid of the project box. An awl is pressed
through the paper to mark the center of
each hole to be drilled in the lid.
Figure 3-108. The exterior of the panel after
drilling. A small handheld cordless drill
can create a neat result if the holes were
marked carefully.