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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
Prepared by: C.S. Chua
Sensor Application Engineering
Singapore, A/P
INTRODUCTION
In the current design, whenever there is an incoming page,
the buzzer will “beep” until any of the buttons is depressed. It
can be quite annoying or embarrassing sometime when the
button is not within your reach. This application note describes
the concept of muting the “beeping” sound by tapping the
pager lightly, which could be located in your pocket or hand-
bag. This demo board uses an accelerometer, microcontroller
hardware/software and a piezo audio transducer. Due to the
wide frequency response of the accelerometer from d.c. to
400Hz, the device is able to measure both the static accelera-
tion from the Earth’s gravity and the shock or vibration from an
impact. This design uses a 40G accelerometer (Motorola P/N:
MMA1201P) which yields a minimum acceleration range of
–40G to +40G.
CONCEPT OF TAP DETECTION
To measure the tapping of a pager, the accelerometer must
be able to respond in the range of hundreds of hertz. During the
tapping of a pager at the top surface, which is illustrated in
Figure 1, the accelerometer will detect a negative shock level
between –15g to –50g of force depending on the intensity.
Similarly, if the tapping action comes from the bottom of the
accelerometer, the output will be a positive value. Normally, the
peak impact pulse is in the order of a few milliseconds. Figure 2
shows a typical waveform of the accelerometer under shock.
TAPPING ACTION
FRONT VIEW
PCB
Figure 1. Tapping Action of Accelerometer
–0.05
–0.03
–0.01
0.01
0.03
0.05
–70
–60
–50
–40
–30
–20
–10
0
10
20
30
0
TIME (seconds)
A
TAPPING OF
ACCELEROMETER
Figure 2. Typical Waveform of Accelerometer Under
Tapping Action
Therefore, we could set a threshold level, either by hard-
ware circuitry or software algorithm, to determine the tapping
action and mute the “beeping”. In this design, a hardware
solution is used because there will be minimal code added to
the existing pager software. However, if a software solution is
used, the user will be able to program the desire shock level.
HARDWARE DESCRIPTION AND OPERATION
Since MMA1201P is fully signal–conditioned by its internal
op–amp and temperature compensation, the output of the ac-
celerometer can be directly interfaced with a comparator. To
simplify the hardware, only one direction (tapping on top of the
sensor) is monitored. The comparator is configured in such a
way that when the output voltage of the accelerometer is less
than the threshold voltage or Vref (refer to Figure 3), the output
of the comparator will give a logic “1” which is illustrated in
Figure 4. To decrease the Vref voltage or increase the threshold
impact in magnitude, turn the trimmer R2 anti–clockwise.
SEMICONDUCTOR APPLICATION NOTE
REV 3
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Freescale Semiconductor, Inc.
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