Classic Car Weekly (UK) - - Living With Classics -

1 Ba­sic alarm lay­out

A range of dif­fer­ent sen­sor types gives a greater range of in­for­ma­tion to re­duce false alarms. Each re­lays in­for­ma­tion to the con­trol unit, which is then able to as­cer­tain whether the car is be­ing sub­ject to un­wanted in­tru­sion. Radar tech­nol­ogy can also be used to de­ter thieves from even get­ting near to the car, by ac­ti­vat­ing a warn­ing ‘blip’ on the alarm.

2 Mer­cury switch

Mer­cury switches used to be used, on some older ve­hi­cles, to op­er­ate brake lights, which was a use­ful fea­ture be­cause the lights would be ac­ti­vated even when us­ing gears to slow down. How­ever, they have also a place within car alarm sys­tems and can help to de­tect any­thing from a per­son en­ter­ing a ve­hi­cle, to the slight­est ve­hi­cle move­ment.

3 Mo­tion sen­sor

The ba­sic, but clever ‘shock’ or mo­tion sen­sor uses noth­ing more tech­ni­cal than a metal ball in con­tact with a cen­tral ‘pole’ and a num­ber of fanned-out base con­tacts. If the ball is only slightly dis­turbed, such as by wind de­flec­tion from a pass­ing ve­hi­cle, the con­trol unit won’t ac­ti­vate the alarm, but more pro­nounced ag­i­ta­tion will set it off.

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