Getting Somewhat More Serious 143Experiment 15: Intrusion Alarm RevisitedThe circuit board will sit on the bottom, held in place with four #4-size ma-
chine screws (bolts) with washers and nylon-insert locknuts. You need to use
locknuts to eliminate the risk of a nut working loose and falling among com-
ponents where it can cause a short circuit.
You’ll have to cut the perfboard to fit, taking care not to damage any of the
components on it. Also check the underside the board for loose fragments of
copper traces after you finish cutting.
Drill bolt holes in the board, if necessary, taking care again not to damage any
components. Then mark through the holes to the plastic bottom of the box,
and drill the box. Countersink the holes (i.e., bevel the edges of a hole so that
a flat-headed screw will fit into it flush with the surrounding surface), push
the little bolts up from underneath, and install the circuit board. Be extremely
careful not to attach the circuit board too tightly to the project box. This can
impose bending stresses, which may break a joint or a copper trace on the
board.
I like to include a soft piece of plastic under the board to absorb any stresses.
Because you’re using locknuts, which will not loosen, there’s no need to make
them especially tight.
Test the circuit again after mounting the circuit board, just in case.
Soldering the Switches
Figure 3-110 shows how the physical switches should be wired together. Re-
member that S1 is a toggle switch and S2 is a DPDT pushbutton. Your first step
is to decide which way up they should be. Use your meter to find out which
terminals are connected when the switch is flipped, and when the button is
pressed. You’ll probably want the switch to be on when the toggle is flipped
upward. Be especially careful with the orientation of the pushbutton, because
if you wire it upside-down, it will constantly have the alarm in “test” mode,
which is not what you want.
Remember, the center terminal of any double-throw switch is almost always
the pole of the switch, connecting with the terminals immediately above it
and below it.
Stranded wire is appropriate to connect the circuit board with the compo-
nents in the top panel, because the strands flex easily and impose less stress
on solder joints. Twisting each pair of wires together helps to minimize the
mess.
Remember to install the LEDs with their short, negative wires connected with
the resistor. This will entail some wire-to-wire soldering. You may want to pro-
tect some of these bare leads and joints with thin heat-shrink tubing, to mini-
mize the risk of short circuits when you push all the parts into the box.
When you connect wires or components with the lugs on the switches, your
pencil-style soldering iron probably won’t deliver enough heat to make good
joints. You can use your higher-powered soldering iron in these locations, but
you absolutely must apply a good heat sink to protect the LEDs when you
Figure 3-109. Components have been
added to the control panel of the project
box (seen from the underside). The loud-
speaker has been glued in place. Spare
glue was dabbed onto the LEDs, just in
case. The SPDT on/off switch is at the top
right, the DPDT pushbutton is at top left,
and the binding posts, which will con-
nect with the network of magnetic sensor
switches, are at the bottom.