MPXM2102AS datasheet(660/670 Pages) MOTOROLA

660 / 670 page

AN1690

4–68

Motorola Sensor Device Data

www.motorola.com/semiconductors

Figure 5. Low Battery Detection Circuitry

–

VDD

LOW V COMP. OUT

VDD

LOW V SET

OSC AND

TIMING

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.

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

–

ALARM DETECT

INPUT (PIN 15)

ALARM

THRESHOLD

DETECT COMP. OUT

VDD

Internal to MC14600

100

Ω

VDD

RESET

SWITCH

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.

accelerometer,

such

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.

Figure 7. Shock and Vibration Detection Circuit

OUTPUT TO

PIN 15 (ALARM

DETECT INPUT)

10 M

1.0

µF

MMA1201P

7805

5 V

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

MPXM2102AS Datasheet(PDF) 660 Page - Motorola, Inc

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