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.

Classic Car Weekly (UK) - - Living With Classics - FUZZ TOWN­SHEND CCW’S MAS­TER ME­CHANIC

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.

to Mer­cury switches are highly sen­si­tive ec­tive. move­ment – they’re small, but eff

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