4–68
Motorola Sensor Device Data
www.motorola.com/semiconductors
Go to: www.freescale.com
Figure 5. Low Battery Detection Circuitry
In this case the Alarm IC would alarm for the brief moment that
the event occurred and then stop. This is not always desirable,
in particular during events where safety is of concern.
A latch can be implemented using the concept of
hystere-
sis
to alter the alarm threshold level and therefore remain in
an alarm condition. It is very simple as it requires only one
resistor, R3, connected to pin 1 (Detect Comp. Out.) and
added in series to the alarm threshold voltage divider, R1 and
R2, on pin 13 (See Figure 6). During a no alarm condition pin
1 is high which makes the alarm threshold voltage divider look
like it would without R3 connected, keeping the alarm thresh-
old at the initial desired point. When an alarm condition occurs
pin 1 goes low, which in turn dramatically lowers the threshold
voltage into the alarm comparator. When the alarm signal
ends and the input voltage into pin 15 decreases, the alarm
condition does not end because the alarm threshold has been
lowered to below a standby voltage level. The MC14600 will
continue in an alarm condition until the unit is RESET or pin 15
receives a signal below this alarming threshold. A RESET is
implemented by connecting a switch to pin 1 that will toggle to
VDD through a resistor. This solution has the possibility that it
will not latch on to the alarm condition indefinitely. As
described above it is essentially just lowering the alarm
threshold voltage so if the output from the sensor during a no
alarm condition is below this threshold the latch will not work.
+
–
VDD
LOW V COMP. OUT
4
VDD
3
LOW V SET
OSC AND
TIMING
SAMPLE DETECTION INPUTS
The MC14600 is a versatile device because its high imped-
ence input pin allows it to be connected to a variety of systems
and input signals. All that is required for an input is a device
Figure 6. Latch Using Resistor in Series with
Threshold Divider
or circuit that will produce a change in voltage that corre-
sponds to an environmental change. For example, a simple
circuit around a thermistor could cause the MC14600 to alarm
when the temperature gets too high. A phototransistor could
be connected to cause an alarm for either the absence or exis-
tence of light.
Motorola also has sensors, specifically accelerometers
and pressure sensors, that could be used as the input to the
MC14600. An accelerometer, such as Motorola’s
MMA1201P, could be used to sense a shock or vibration. A
possible solution is shown in Figure 7. The MC7805 is a volt-
age regulator that provides the 5 V supply required by the
MMA1201P. Since the output of the MMA1201P resulting from
a shock or vibration is very short some simple peak detection
circuitry is required to keep the signal high long enough for the
MC14600 to latch onto the alarm condition.
+
–
ALARMDETECT
INPUT (PIN 15)
ALARM
THRESHOLD
R2
R1
13
1
DETECT COMP. OUT
R3
VDD
Internal to MC14600
100
VDD
RESET
SWITCH
Figure 7. Shock and Vibration Detection Circuit
OUTPUT TO
PIN 15 (ALARM
DETECT INPUT)
10 M
1.0
μ
F
D1
MMA1201P
7805
5 V
F
Freescale Semiconductor, Inc.
n
.