Make Electronics

Getting Somewhat More Serious 145

Experiment 15: Intrusion Alarm Revisited

Final Test

When you’ve completed the circuit, test it! If you don’t have your network of
magnetic sensor switches set up yet, you can just use a piece of wire to con-
nect the two binding posts. Make sure that S1 is in its Off position, then solder
the appropriate plug to your 12-volt power source, and plug it into the power
jack. When you press the button, the green LED should light up to show con-
tinuity between the two binding posts. Now disconnect the wire between the
binding posts, press the button again, and the green LED should remain dark.

Reconnect the binding posts, flip S1 to its On position, and the red LED should
light up. Press the button, and the alarm should start. Reset it by turning S1
off and then on again; then disconnect the wire between the binding posts.
Again, the alarm should start, and it should continue even if you reconnect
the wire.

If everything works the way it should, it’s time to screw the top of the box in
place, pushing the wires inside. Because you’re using a large box, you should
have no risk of metal parts touching each other accidentally, but still, proceed
carefully.

Alarm Installation

Before you install your magnetic sensor switches, you should test each one by
moving the magnetic module near the switch module and then away from it,
while you use your meter to test continuity between the switch terminals. The
switch should close when it’s next to the magnet, and open when the magnet
is removed.

Now draw a sketch of how you’ll wire your switches together. Always remem-
ber that they have to be in series, not in parallel! Figure 3-112 shows the con-
cept in theory. The two terminals are the binding posts on top of your control
box (which is shown in green), and the dark red rectangles are the magnetic
sensor switches on windows and doors. Because the wire for this kind of instal-
lation usually has two conductors, you can lay it as I’ve indicated but cut and
solder it to create branches. The solder joints are shown as orange dots. Note
how current flows through all the switches in series before it gets back to the
control box.

Figure 3-113 shows the same network as you might actually install it in a situ-
ation where you have two windows and a door. The blue rectangles are the
magnetic modules that activate the switch modules.

You’ll need a large quantity of wire, obviously. The type of white, stranded
wire that is sold for doorbells or furnace thermostats is good. Typically, it is
20-gauge or larger.

After you install all the switches, clip your meter leads to the wires that would
normally attach to the alarm box. Set your meter to test continuity, and open
each window or door, one at a time, to check whether you’re breaking the
continuity. If everything is OK, attach the alarm wires to the binding posts on
your project box.

Figure 3-112. Dual-conductor, white

insulated wire can be used to connect the

terminals on the alarm control box with

magnetic sensors (shown in dark red).

Because the sensors must be in series, the

wire is cut and joined at positions marked

with orange dots.

Figure 3-113. In an installation involving two

windows and a door, the magnetic com-

ponents of the sensors (blue rectangles)

could be placed as shown, while the

switches (dark red) are located alongside

them.