BUILD A SOUND-ACTIVATED
By Paulo Oliveira
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Many methods to assist the “parking challenged” — a group the author
proudly belongs to — have been developed over the years. These methods
range from the prosaic “hanging tennis ball” to more sophisticated
ultrasound-based proximity detectors. In this article, we show an approach
built around two ordinary low power laser pointers repurposed as parking
assist beams. The lasers in this project are activated by the sound generated
while the garage door is opening.
Figure 1 shows the block diagram for this project. The
garage door sensor detects when the garage is opened (or
closed). Perhaps the simplest way to implement such a
detector would be to use a simple mechanical limit switch
or even a magnetic switch coupled to the garage door
rails. Another option is to use a signal derived from the
garage opener box such as a light sensor (most garage
door openers turn a lamp ON while opening the garage
door). In this project, we opted for a sound detector.
While this type of detector might not be the simplest, it
has the advantage that it requires no extra wiring towards
the garage door. Furthermore, because this sound
40 May 2012
detector has no moving parts, it can be expected to be
more reliable than solutions built around mechanical
switches. It is also much more fun to build!
One obvious disadvantage is that the sound trigger is
not only activated when the garage opens, but also when
it closes. This is not an issue, though, as the laser beams
can be ignored if not needed.
The signal from the garage door sensor — in this case,
the output from an electret microphone — is amplified and
low-pass filtered before driving a threshold comparator.
Recording the sound produced by the garage door opener
when in use and then running it through audio FFT
spectrum analysis software resulted in the plot in Figure 2
(x axis is frequency while y axis is amplitude). The plot