they may be specified that way, but in practice they aren't.  remember,
the detectors have to work reliably over a wide temperature range, and
the flash sources have to work over a wide temperature range as well,
from a poorly regulated vehicle power supply and the flash is usually
generated by strobe lights.  i'd be very, very suprised if they actually
had only a 1% tolerance on the frequency in terms of what the detectors
will actually recognize.  also consider that this system has been around
for a long time, i think it was around when i started driving around
1980, and stoplight controllers etc. were not terribly sophisticated
then but these systems were available as add ons.  considering they are
put at all the major intersections they have to be reasonable priced.

given the abuse emergency vehicles have to take, and that they are
already expensive i doubt very much that the strobes are crystal
controlled, though they may at least be synchronized with each other.
also remember the value of disinformation to the manufacturers, they
want and need to deter people from cracking the system to cheat, and to
try and discourage competitors from making thier own compatible systems.
 bogus specs that are widely available are a good way to send people
down the wrong path.  there's also the matter of how many correctly
timed flashes have to be detected, if it's only 3 or 4, or 4 out of 6
with correct timing you could do it by hand.

you can dial a pulse telephone by hand by banging on the hook, i've done
it and dialed 2 digits of a number for a computer from a terminal that
had a lock on it's phone so you "couldn't" dial 9 to get off campus and
make long distance calls.  phone pulses are at 10hz, and the dials have
governors to regulate the speed.  it's a pain to dial by hand, and it
would take real practice to dial 7 digits by hand, but i've done 2 out
of 7 with fair repeatability.  so if the frequency is 10 or 15 hz, doing
it by hand should be possible.  there may also be old systems and newer
systems with the newer systems being more sophisticated and harder to
cheat, they may even require 2 frequencies that have a phase locked
relationship.  but remember, cost and reliability are big issues, if
they didn't care about cost it could be done much more securely with rf
and much more consistantly (though it might be harder to spot cheaters,
but proving it would be a lot easier when you had thier magic box in
hand).

on the other hand, there is probably some need for the new systems to
work with the old ones, particularly when one cities
fire/police/ambulance services help out anothers.  i suspect it's not
that hard to cheat, though the person who claimed they could do it by
hand may have been lying or just mistaken (after all, the light could
have been about to change anyway).

best not to mess with this system unless you have a good reason, most
juries and judges would rightly hang you out to dry if you were caught.

David VanHorn wrote:
>
> At 05:56 PM 6/15/2004 -0600, Philip Stortz wrote:
>
> >yeah, you can trip them by flashing your brights during the day and
> >possibly at night,
>
> The activation frequency is very narrow, I seriously doubt it.
-------

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads