MadSci Network: Engineering

Re: How do the keyless entry systems on cars work?

Date: Sat Dec 9 07:41:50 2000
Posted By: Karl Kolbus, Staff, Data processing, Mequon Consulting Corp.
Area of science: Engineering
ID: 975719896.Eg

Hi Katherine!

Basically, there are two type of keyless entry systems in common use. One 
uses a radio signal, the other has a series of numbered buttons on the car 
door. Let's concentrate on the former; the radio signal. The small keypad 
(people usually attach them to their keychains) has a button which, when 
pressed, activates a small radio transmitter, not unlike a radio station 
sending out the latest "Back Street Boys" recording. Just as you tune your 
radio receiver to a station of your choosing, a receiver in your car is 
tuned to the particular transmitting frequency of your sending unit. When 
the receiver "hears" the transmitter, it activates a special circuit which 
activates a solenoid attached to the locking/unlocking mechaninism of the 
door. A solenoid is basically an circular electromagnet, in the center of 
which is an iron core which moves either in or out, depending on the 
polarity of the voltage applied to the electromagnet. 

But, wait a minute! What if someone else has a transmitter that sends out 
the same frequency signal? Wouldn't it also open your car's doors? Yup.
But those guys took care of that problem. Just as you favorite radio 
station sends out a signal at a very specific frequency, it 
then "modulates" it with the "Back Street Boys" song. Your little 
transmitter does the same thing except, rather than using the "Back Street 
Boys", it modulates the signal with a digital "code", a very specific 
pattern of ones and zeros. The receiver in your car is set up to respond 
to only that particular pattern of ones and zeros, and will ignore all 
other codes that someone elses transmitter might send out. Usually there 
are about 64 thousand different codes that could be used, so the chances 
of someone in your area having the same code are pretty small. Also, the 
frequency of the transmitter must match your receiver, or it won't open 
the doors either!

I hope this helps.

Your not-so-mad scientist,

Karl Kolbus   

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