While this dead horse is gently being flogged into the ground, I should
add my two or three cents. I have heard everything suggested, from
phony blinking lights to proximity sensors.
First, a note on how basic alarms work: The cheap kind (e.g., $30-$50
from a chain-store) generally have two mechanisms, which I'll discuss
in a moment. They are connected to any circuit that remains live when the
ignition is totally off (like the dome light), and draw minimal power
from the battery. There are only two wires to attach (ground and live),
and the unit consists of a combination sensor/siren that generally gets
mounted under the hood.
The two mechanisms mentioned above are 1) the shock sensor, and 2) the
electric monitor. The shock sensor consists of a single- or double-stage
switch (either mechanical or mercury) that "feels" motion. Banging on
the glass or fender with your fist elicits a series of warning chirps,
while slamming a door or having a big truck drive by results in the full
1-minute show (which everyone ignores as commonplace).
The second mechanism is the more important electrical sensor, which is
simply a circuit designed to detect a "dip" in the electrical system,
as would result when somebody opened the door, turned on the lights,
pushed in the cigarette lighter (or shuffled on wool carpet and touched
the side-mirror). I consider this mechanism more important because it
can be used to expand your system. You can add detectors, which need
only create a "dip" to set off the main alarm.
For those interested in simply keeping kids out of the car, blinking
lights might suffice (either a 555-timer circuit, a pre-fab blinker, or
the like). Or you might try this solution: From places like
"Radio Shack," you can purchase an infra-red alarm, which generally goes
under the guise of "travel security device." The ones I saw cost about
$20. This nifty unit detects infra red emmissions from moving bodies
(kids, theives, or the neighborhood cat looking for a nap). Such bodies
trigger a moderate-volume alarm (30db), which might be sufficient for
many.
By mounting the unit under the dash, you can shield it from the
outside world, so only those who actually breach the plane of the
interior will trigger sirens. If you want more noise, you can wire it to
the car's power supply, causing a "dip" and setting off the main alarm.
The result is a reasonably inexpensive "proximity sensor."
A note of caution, however. Hyper-sensitive alarm systems have been
known to provoke acts of violent rage from otherwise inoffensive
passers-by. (I once saw an unsavory type kick a dent in a car which had
announced as he was walking by, "Step away from the vehicle!")
More elaborate (and expensive) alarms generally become more electrically
intrusive, making your horn beep and your lights blink every time someone
brushes the vehicle. Many also disable the ingnition system. But a
$1000 alarm system seems silly on a car of equal value. Perhaps the
built in Lucas electrics are enough to foil the common theif (if you own
a '75 and can get the seatbelt switch working, nobody is going to get
away in your car!).
Finally, you can just buy (or make at the local copy store) a sticker for
your window or door that says something like, "This vehicle protected by
[insert name of favorite reptile] security systems." If you make it look
mean enough, there's probably no need for anything with which to back it
up!
(It's my lawyer-half that makes me so bloody long-winded).
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* ____ *
* Konstantinos Hennighausen, J.D., B.S.A.E. |____| *
* Department of Aerospace Engineering and Mechanics O \ / O *
* University of Minnesota //====|| *
* ('79 Midget) *
* *
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