Re: A4 alarm (triggering motion sensors)

[ScharfR@aol.com]

What follows is abridged from some Audi technical information I came across.

The Audi A4 has a fairly conventional theft alarm, plus a pair of special
sensors to monitor glass breakage and movement within the interior of the
car. There is also an override switch on the lower "B" pillar on the driver's
side to turn off the sensors. This permits the owner to leave a pet in the
locked car without the risk of setting off the alarm.

Unfortunately, demonstrating the system it isn’t as simple as locking
yourself in the car, then moving around to set off the alarm.  A brief
explanation of the technical function will help understand why.

The motion detectors are actually ultrasound sensors. They are located behind
small grill openings at the top of the "B" pillar. Locking the car activates
the alarm, and for the next 30 seconds, the sensors monitor interior motion
to establish a baseline level of "noise". Once the baseline noise level is
established, the sensors are "armed" to trigger the alarm when that level is
exceeded by a specific amount.

When you lock a test candidate in the car, even breathing produces a
sufficient level of noise to cause the sensors to "set" for a very high
baseline level. The incremental noise required to trigger the alarm is then
quite high. Usually, you  can’t produce sufficient movement to trigger the
alarm. (Breaking a window probably will, so may a barking dog left in the
car.)  Similarly, locking the car with a window or the sunroof open results
in a such a high level of ultra sound noise that the increment necessary to
set off the alarm is too high. So you can’t always trigger the alarm by
reaching your hand into the open window.

However, for a simple and effective demonstration, all you need is a roll of
tape and an Audi A4.

· Completely cover each of the four small grills at the top of the "B" pillar
with a piece of tape.
· Leave one of the windows open, then lock the car.
· Wait 35-40 seconds while the sensors establish a baseline noise level, then
tear off one of the pieces of tape.
· Turn off the alarm.

When the grill openings were covered by tape, the ultrasound sensors were
only able to "learn" the noise profile of the small area within the "B"
pillar. Removing the tape after the 30 second learning period exposed the
sensors to a much higher nose level, thereby triggering the alarm.