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Motorola Sensor Device Data
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R1
100 k
+5.0 V
4
156
7
8
3
2
+5.0 V
R2
100 k
2
1
C3
1.0
+
+
–
U1
LM311N
VOUT
VIN
VREF
Figure 3. Comparator Circuitry
For instance, if the threshold level is to be set to –20g, this
will correspond to a Vref voltage of 1.7 V.
V
REF
V
OFFSET
V
G
G
THRESHOLD
2.5
(0.04
[
20])
1.7 V
Under normal condition, Vin (which is the output of the
accelerometer) is at about 2.5V. Since Vin is higher than Vref,
the output of the comparator is at logic “0”. During any shock
or impact which is greater than –20g in magnitude, the output
voltage of the accelerometer will go below Vref. In this case,
the output logic of the comparator changes from “0” to “1”.
When the pager is in silence mode, the vibrator produces an
output of about
±
2g. This will not trigger the comparator.
Therefore, even in silence mode, the user can also tap the
pager to stop the alert. Refer to Figure 5 for the vibrator
waveform.
–0.05
–0.03
–0.01
0.01
0.03
0.05
0
1.0
2.0
3.0
4.0
5.0
6.0
0
TIME (seconds)
V
(
O
Figure 4. Comparator Output Waveform
–0.025
–0.015
–0.005
0.005
0.015
0.025
–2.0
–1.5
–1.0
–0.5
0
1.5
2.0
0
TIME (seconds)
V
0.5
1.0
Figure 5. Vibrator Waveform
F
Freescale Semiconductor, Inc.
n
.