Intruder alarms
Last updated: June 15, 2007.
There's a smash of glass and a
scampering of feet. Before you know where you are, someone's rifling
through your house, stealing your most precious possessions. If you're
miles away—at school, at work, or even in another country on
holiday—what can you do to stop thieves swiping everything you own?
That's what intruder alarms are for. These simple electrical circuits
are designed to raise the alarm the moment they detect anything out of
the ordinary. When thieves strike, intruder alarms can make them panic
and flee empty-handed.
So much for the theory, now what about the practice: have you ever
stopped to consider what's inside an alarm and how it works? Let's take
a closer look!
Photo: An intruder alarm (sometimes called a
burglar alarm) housing on the outside of a building has two jobs.
One is to protect the ringing bells of the alarm inside. (The black
slots at the top of each side panel let the noise travel out.)
The other is to serve as a deterrent to burglars: maybe this building
is too much effort
to attack?
An open and shut case
If you've read our article on electricity,
you'll know all about circuits. For
electricity to flow, there must be a path or circuit that it can travel
along and, as the name suggests, that path is usually a closed loop. An
electrical circuit is a bit like the loop of a toy train set. Unless
you make a continuous circuit of railroad track, the train can't go
around.
Just about the simplest circuit imaginable is the one that powers a
flashlight. When you flick the switch of a flashlight, you're shutting
a kind of trapdoor—making a continuous loop of metal so that electrons
(the tiny particles that carry electricity) can flow around it,
ferrying energy from the battery to the bulb so the lamp can light up.
There's an illustration of this in our article on batteries.
If you've experimented with electrical circuits at school, you can
probably already see how something as basic as a flashlight could be
modified to make a simple and relatively effective intruder alarm.
Suppose we replace the lamp in a flashlight with an electrical buzzer
and run a piece of string from the switch to the handle of a door. If
we can somehow hold the flashlight steady, maybe by taping it firmly to
the floor, what we've made here is a basic intruder alarm. If someone
comes in the door, the string will pull on the switch, complete the
electrical circuit, and trigger the buzzer. If you want to experiment
with this idea a bit more, try out the simple activity in the box below.
Although intruder alarms are much more complex than flashlights,
they are based around simple circuits and switches in just the same
way. That's why catching an intruder with an alarm is really an open
and shut case—a case of opening and shutting an electrical circuit.
To catch a thief
Whatever thieves may be, they're not stupid. So, obviously, the
switches we use in an intruder alarm have to be subtle and well
disguised. Although switches come in all shapes and sizes, there are
really only two kinds: ones that detect breakages and others that
detect motion.
The switches that detect breakages are usually fitted to doors and
windows. It's easy enough to make a switch that has two parts, one on
either side of the door frame. When the door is closed, the two parts
of the switch are close together and electricity can flow between them;
when the door is opened, the parts of the switch are forced apart. No
electricity can flow and the alarm is triggered. Similar switches can
be fitted to windows. Some buildings have metal tape or wires running
across or through the windows and carrying an electrical current. If a
window is broken, the metal tape or wire gets broken too and the
electrical current is interrupted, triggering the alarm.
If you've seen Tom Cruise dangling from the ceiling in the movie Mission Impossible, you'll know all about
switches that detect motion. They have two parts: a transmitter that
sends out an invisible beam of infrared radiation
(see our article on halogen hobs for
more details of infrared) or ultrasound
(sound of such high frequency that people cannot hear it) and a
receiver that detects the beam some distance away. Normally, the
receiver picks up a steady signal from the transmitter. But if someone
walks into the room and interrupts the beam, the receiver stops picking
up that signal. The circuit attached to it will notice that straight
away and set off the alarm.
Switches in reverse
Can you see something wrong here? Switches in an intruder alarm need
to work in the opposite way to a
conventional flashlight switch. In a flashlight, you close the switch
and complete the electrical path to make the lamp light up. But in an
intruder alarm switch, you need the circuit to work in reverse. You
need electricity to flow around the circuit normally when the switch is
closed, without activating the alarm. Then, when someone opens the
door, you need the alarm to sound when the circuit is interrupted. How
can we make a circuit that works when it's broken?
That turns out to be a fairly easy problem to solve with electronic
components like transistors. You
simply have two circuits connected together. The first circuit (which
we'll call the detector circuit) contains all the door and window or
motion switches positioned around your home. There can be as many
switches as you like; it takes only one of them to activate and trigger
the alarm. Normally this circuit is active: all the switches are
closed, the circuit is complete, and electricity flows around the
continuous loop. The detector circuit acts as the "input" to the second
circuit (which we'll call the alarm circuit). The alarm circuit is
normally switched off and no electricity flows around it. So the normal
situation looks like this:
However, if the detector circuit is suddenly switched off (by an
intruder opening a door and tripping one of our switches), the alarm
circuit spots that and switches on, immediately sounding the intruder
alarm:
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