brett...
The transmitter in the remote and the receiver in the door
opener are simple transmitters and receivers that work much
like a simple walkie-talkie, on a single frequency.
From your question, I get the impression that you think it's
only the frequency of the remote and the receiver at the
door that must match, and that determines whether the door
will open.
While this was the case in very early models, it was quickly
realized that this didn't provide enough security. Since they
*all* used the identical frequency, you could accidentally
(or purposely) open a neighbor's garage door by clicking your
remote while driving by!
This was initially addressed by adding 8 dip-switches in
both the transmitter and receiver, which were set to the
same code, and which would be transmitted and recognized
by your door unit. This provided 256 unique combinations
of code which could be sent, and all the remotes could
still use the same frequency. The code consisted of a
series of 1s an 0s determined by the on-off positions of
the dip switches.
This worked for awhile, but thieves soon figured out that
a radio scanner could be used to determine the frequency
of your transmitter and the code it sent when you used it,
and it could be easily duplicated.
Though it's still possible to find garage door opener units
that use a greater number of switches to create a unique
code, the more modern units provide much better security.
Modern units work much the way a car's keyless entry system
works, preventing code capture by generating a 'rolling' or
'hopping' code at random. Every time the transmitter is used
it opens the door and simultaneously creates a new code, using
a pseudo-random number generator which is identical to one in
the receiver. The identical pseudo-random number generators in
both the transmitter and receiver use the previous unique code
to produce another unique one. The random number generators are
synchronized by the interaction, so the numbers will match when
next used. This can all be done on the same frequency, which is
around 300 or 400 MHZ, but the codes will be unique to the degree
of one in a trillion.
Clever, huh?
The How Stuff Works website has a thorough explanation of
garage door openers and remote entry in general here:
http://auto.howstuffworks.com/remote-entry1.htm
You can learn more about psuedo-random number generators
on this page from their site:
http://computer.howstuffworks.com/question697.htm
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