Intruder alarms

Last updated: December 7, 2009.

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: Left: 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? Right: Most alarms are placed high up on buildings where they're difficult to disable but still conscipuous.

Alarms are based on breaking electrical circuits

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.

Making intruder alarms invisible

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.

Switches like this can be made less conspicuous using magnetism detectors called reed switches. You have a magnet on one part of a door and a reed switch on the other. If the door opens, the magnet moves away from the reed switch, breaking the circuit and 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 and a receiver (usually a photocell) 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 straightaway and set off the alarm. Ultrasound beams (sounds of such high frequency that people cannot hear them) are sometimes used in place of infrared light.

Photo: A photocell (light-dependent resistor or LDR) like this is used to detect the beam in invisible, motion alarms.

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 relays and 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:

Further reading

Other articles you might like on our site:

• Smart homes: Why stop at automating the security? Why not automate the whole house?!
• Smoke detector: Now you know how to spot a burglar, but can you detect a fire?