How Stuff Works: Fuzz is feeling alarmed – car alarmed, that is
More annoying than someone else’s crying child – that’s car alarms, not your scribe, Fuzz Townshend.
Cars have been getting stolen since the late 19th century and so car owners have been paranoid about their charges for the best part of 125 years. All manner of anti-theft devices have been rigged up to cars in the intervening years, from illegal electrocution devices connected to door handles to simple and ever so slightly more gentle fuel cut-off taps and hidden ignition isolators.
Car alarms have become almost ubiquitous in the past 30 years and developed to quite a sophisticated degree, both as part of vehicle manufacturer’s specifications and as aftermarket items and so are found in classic cars, ancient and modern.
Back in the mid-1980s, I was working as a bus mechanic in the West Midlands. Allocated to Dudley garage, we had an old Daimler bus (fleet number 6550 if you’re interested), that had a particularly fruity exhaust note. A favourite pastime of mine was dropping the semi-automatic gearbox into top gear, while driving by rows of parked cars, where the resulting low bass exhaust rumble would set every alarm off. Priceless.
But I digress. The simplest designs of alarms are operated by utilising the courtesy light circuits and microswitches installed in most cars over the past 60 years. These can be armed by a simple hidden switch, which connects an audible siren or horn that is activated when a door is opened. A secondary power supply can help to thwart any attempted disconnection of the alarm. Of course, thieves can circumnavigate such devices simply by smashing a window to gain access, releasing the handbrake and towing the car away. Generally, more modern alarms are connected either to the car’s own electronic control unit (ECU), or their own specifically-designed control unit.
With both, more sensors can be utilised, often in multiples. Voltage drop sensors are activated as soon as a change in voltage is detected, such as activation of a courtesy light on opening a door, disconnection of a trailer, attempted ‘ hot-wiring’ of the ignition circuit, switching on of headlights, or disconnection of the battery. Here, a secondary power source is usually necessary in order to maintain sufficient power to the alarm system.
Air pressure sensors detect slight changes in the internal atmospheric pressure within the car itself, such as when a window is broken, or a door is opened. These use what is in essence a loudspeaker operating in reverse, so that when the air pressure changes, the cone of the speaker moves, sending a small, but detectable current to the control unit, which then sets off the alarm and immobiliser, if fitted. This is one of the types of sensor that can be activated by a passing vehicle’s exhaust note. A Daimler bus, for example. Microphonic sensors react to sound, sending a small current to the control unit in a similar fashion.
Motion sensors can utilise sensitive mercury switches, which complete an electrical circuit as soon as the car moves, even if buffeted by a strong wind caused by a passing vehicle. Similarly, less hazardous sensors use a steel ball that makes contact with a central contact pole and a series of fanned-out contact strips. Whenever contact between the two is lost, the voltage change promptly activates the alarm. These latter can detect between differing levels of movement, thus not being activated by sudden gusts of wind, but activating when a person enters or moves the car.