MPXM2202GS datasheet(113/670 Pages) MOTOROLA

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Motorola Sensor Device Data

www.motorola.com/semiconductors

AN1612

Shock and Mute Pager Applications

Using Accelerometer

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

TIME (seconds)

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.

MOTOROLA

SEMICONDUCTOR APPLICATION NOTE

REV 3

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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