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A Primer on Security Systems


All security systems may not be equal in capability, but all have basic features and work pretty much the same.

Today's alarm systems come with a remote control transmitter which communicates a code via radio frequency. A receiver captures the signal which is then decoded by the alarm's main control module or brain. All alarms also include a siren and sensor, and some alarms include pinswitches which tie the doors, hood, and trunk into the system.

In this article we will explore the basic elements of an alarm system as well as how they work.

Under the hood
Just like any other electronic product, an alarm system needs a power source. Commonly, it is the vehicle's battery. Since all cars sold in the United States operate on 12-volt negative ground, today's alarm systems sold in this country are designed for 12 volt negative ground electrical systems. If you are a classic car buff and collect old 1950s style cars, you may be interested in knowing that your classic car features a 6 volt electrical system. A few large trucks also do not work on a 12 volt negative ground electrical system, but rather feature a 24-volt system. The vintage '50s cars and the trucks that have 24-volt electrical systems will not accept 12 volt alarm systems.

The device that is designed to gain your attention as well as the attention of other passersby is, of course, the siren. Commonly placed under the hood, the siren wails out a massive level of decibal sound. It varies as to how loud the noise is from manufacturer to manufacturer. Typically, the level is between 110 dB and 130 dB. Moreover, manufacturers program their sirens to sound different tones. Some sirens will play a number of tones during an alarm condition. Some sirens allow the user to program what tones are played. This way you can customize the sound of your siren to better allow you to distinguish your car from a parking lot full of cars. The siren will chirp usually once to indicate when the alarm is armed and twice when the alarm is disarmed. If the alarm was triggered while it was armed, it may chirp three times to alert you to that fact.

Many manufacturers offer as an option back-up battery sirens. These sirens are equipped with their own battery. If the standard siren is somehow shut down by a thief, the back up battery siren wails. Usually, this siren can only be turned off by a special key.

Inside the Car
Moving inside the car, today's alarm systems commonly include a Light Emitting Diode, also called an LED. The LED flashes red when the alarm is armed. It flashes a different pattern when the alarm is in valet. Of course, it doesn't flash at all when the alarm is off. The LED is also used to provide information about the alarm to the user. If, for example, your alarm system had been triggered while you were away from your car, the LED will flash to indicate that fact. The LED will also flash a number of times to tell you what zone had been attacked to cause the alarm to trip. There are usually four zones to the system -- the hood, the trunk, the doors, and the sensor. LEDs drain very little current from the battery so there is no chance it will drain a battery.

Another important feature of alarm systems is the starter disabler. This is a relay that cuts off the wire that feeds electricity to the starter. This relay can also be put on the line that provides electricity to the ignition, or fuel injectors. The starter is most commonly the one that is chosen for this feature. Should something go wrong, you won't be able to start your car. If the ignition is the one chosen to cut, then if something goes wrong, the ignition could be cut while you are driving causing your car to die. This feature prevents a thief from hot wiring the car to start it. Even if he should get a hold of the key, he won't be able to start the car as long as the alarm is armed. The starter disable relay can be configured in two fashions -- normally open and normally closed. What this means is that when the relay is at rest -- that is, it is not being fed any energy -- the relay either cuts the wire (normally open) or connects the wire (normally closed). If you wish to have a normally open relay for your starter disabler, you should also include an override switch so that you can start your car if the relay malfunctions.

Every alarm comes with a sensor. Moreover, there are a variety of sensors you can use for your system. Most alarms come with a shock sensor. Obviously, these sensors detect an impact on the vehicle. There are single stage shock sensors and dual stage shock sensors. When a single stage shock sensor senses an impact it trips the system into a full alarm condition. A dual stage shock sensor detects a light impact on the car and will cause the siren to chirp a number of times to warn away the intruder. A harder impact will cause the sensor to trip the system into a full alarm condition.

Other sensors include a digital tilt sensor, microwave or radar sensor, a motion sensor, an audio discrimination sensor, and doppler sensor.

The microwave or radar sensor bathes the interior of the vehicle with microwaves. If the microwaves are disturbed, the sensor detects it and trips the alarm. Microwaves are not affected by air movement, so these sensors are ideal for use in a convertible car. In addition, the microwaves can pass through just about any material except metal. So the sensor can be hidden behind cloth, wood, plastic, etc. As is the case for the shock sensor, the radar or microwave sensor comes in single stage or dual stage. The single stage radar sensor bathes only the interior of the vehicle with microwaves. So someone actually has to enter the vehicle to trip the sensor. A dual stage microwave sensor bathes the interior with microwaves and it also creates a "bubble of protection" for some distance outside the car. This is possible because microwave can penetrate through glass. When an intruder penetrates the bubble outside the vehicle, the sensor will detect it and cause the siren to chirp a number of times to provide a warning for the intruder. If the intruder enters the vehicle, the sensor will trip the system into a full alarm condition.

A motion sensor detects movement of the vehicle. At first, these sensors featured a mercury glob which moved when the car moved. However, there were many false alarms caused when the car was parked on a hill. Today's motion sensors use more sophisticated electronics and even infrared technology to avoid false alarms.

Audio discriminators sense sound. A commonly used audio discrimination sensor is the glass breaking detection sensor. The sensor includes a microphone which is listening for the sound frequency of breaking glass or metal striking metal.

The doppler sensor is similar to the microwave sensor. However, instead of bathing the car with microwaves, the doppler sensor uses ultrasonic sound waves. However, wind and weather can affect the performance of this sensor. So it is not recommended for use in convertibles.

There are other types of sensing features used on alarm systems. All alarms allow for the inclusion of pinswitches. Pinswitches are used to incorporate the door, trunk, and hood into the alarm system. Usually, these sensors feature a plunger like screw. When the door, hood, or trunk is closed the plunger pinswitch is depressed. When a door, hood, or trunk is opened, the plunger pinswitch springs up and causes the alarm to trip. Car manufacturers include pinswitches on the doors and perhaps the hood and trunk of their cars. These switches can easily be incorporated into an alarm system. Other switch sensors include mercury switches and magnetic switches. Mercury switches can prove ideal for hatchbacks. Magnetic switches can be used to protect the back window of pickup trucks.

There are also alarms available today which include voltage (also called current) sensing. This sensing feature detects the drop of voltage at the battery or fuse box. This sensing feature will trip the alarm when the dome light goes on as a result of a door opening or senses any other item that can draw current. However, it may false alarm if the fan in the engine compartment or a car's computer goes on and if the dome light is burned out, it will not trip the alarm when a door is opened.

Many alarms also include a closed loop trigger. This is a wire that is commonly connected to an accessory that can be removed from the car. When the accessory is removed, the wire of the closed loop trigger is cut and the trigger trips the alarm. The loop can be connected to a cellular phone, the radio, or other accessory.

There are a variety of other features commonly found on alarms. For example, most alarms today permit the user to work it passively or actively. What does this mean? Passive means that the alarm will arm automatically. It will arm when the last door is closed as you leave the car. This feature usually includes an exit delay of 30, 45, or 60 seconds. The clock begins to run when you shut off the ignition. On some alarms this feature also includes an entry delay that allows you to get into the car and turn on the ignition to disarm the alarm. The entry delay is usually 12 seconds, but some of the better alarms allow you to adjust the delay. Active arming means that you arm and disarm the alarm using the remote transmitter.  

Most if not all alarms also include a valet mode. This feature when activated will permit who ever is using the car to enter and exit, drive it, and open the hood. It prevents the alarm from being accidentally armed or triggered so a mechanic or parking a lot attendant can do their job with the car.

All alarms also include an emergency override circuit. This is commonly a small button that is hidden in the car. This provides a manner in which you can disarm the alarm if you lose your remote. In order to work this feature, you need to turn on the ignition before pushing the override button.

Finally, most if not all alarms that include a remote transmitter has a feature called remote panic. When you hold down a button on the transmitter for three seconds, the siren will sound just as if the alarm was triggered. However, this will only work when you are within normal arm and disarm range of the vehicle.