MPXM2102AS datasheet(659/670 Pages) MOTOROLA

659 / 670 page

AN1690

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Motorola Sensor Device Data

www.motorola.com/semiconductors

ALARM THRESHOLD ADJUSTMENTS

The alarm trigger point (alarm threshold) is set externally to

any voltage level with a simple voltage divider connected to

pin 13. For instance, to connect the Alarm IC to a sensor that

has an output of 1.0 V during a no alarm condition and 4.0 V

during an alarm condition, the alarm threshold voltage could

be set to 3.0 V using a 2 M

Ω and a 1 MΩ resistor connected

between VDD and ground (See Figure 3). Pin 13 connects

internally to the negative input of the Detect Comparator.

Based on the input impedance of the Detect Comparator the

maximum suggested total resistance for the threshold voltage

divider is 10 M

Ω.

Figure 3. Alarm Threshold Voltage Divider

VDD

2 M

1 M

PIN 13

OSCILLATOR

The master clock frequency for the MC14600 is determined

by the external components Rbias (pin 7) and Cosc (pin 12).

This RC network provides the timing for the various functions

conducted by the IC. The oscillator timing affects the period

between LED pulses, alarm signal sampling, and the horn out-

put pulses and power consumption. A standard RC network

for the MC14600 oscillator uses an 8.2 M resistor (Rbias) con-

nected from VDD to pin 7 and a 0.1 uF capacitor (Cosc) con-

nected from pin 12 to ground. This configuration will provide

a period of approximately 1.65 sec in standby and 41.67 msec

in alarm. A change in oscillator speed is accomplished by

changing the resistor and capacitor values previously stated.

Changing the oscillator timing will not change the horn pattern

but it will change the speed at which it’s delivered. The table

below lists examples of RC values and measured sampling

periods achieved with those values (deviation from theoretical

values are due to tolerance in components).

Table 1. Oscillator Period vs. Rbias and Cosc Value

Rbias

Cosc

Period (no alarm)

Period (alarm)

5.6 M

Ω

0.01

µF

93 msec

2.3 msec

8.2 M

Ω

0.01

µF

142 msec

3.4 msec

10 M

Ω

0.01

µF

172 msec

3.9 msec

5.6 M

Ω

0.1

µF

1.4 sec

32 msec

8.2 M

Ω

0.1

µF

2.2 sec

50 msec

10 M

Ω

0.1

µF

2.7 sec

60 msec

8.2 M

Ω

1.0

µF

20.1 sec

456 msec

PIEZO HORN INTERFACE

The MC14600 contains on–board horn driver circuitry to

drive three leaded piezo horns. A three leaded horn is consid-

ered self–driven, having a feedback pin that is connected to

a closed loop oscillation circuit. The MC14600 uses pin 8

(Horn Feedback), pin 10 (Horn Out 1) and pin 11 (Horn Out 2)

to interface to a piezo horn and achieve the drive circuit. Pin

10 and pin 11 alternate their output providing the oscillation for

the horn. Three external components are required to interface

a piezo horn to the Alarm IC: R1, C1 and R2 (Figure 4). R1 is

usually around 1.5 M

Ω and is the least critical component as

it only biases the horn. R2 and C1 are critical to achieve maxi-

mum horn output. The two components must be set so that the

value of 1/(R2*C1) is close to the resonant frequency of the

horn being used. Table 2 lists a common horn frequency and

potential external components that can be used for R2 and C1.

Figure 4. Piezo Horn Interface to MC14600

FDBK

OUT 2

ALARM LOGIC

OUT 1

Table 2. External Components for a 3.4 kHz

Three Leaded Piezo Horn

Horn Osc.

Frequency

1/(R2*C1)

3.4

" 0.4 kHz

1.5 M

Ω

820 k

Ω

1.5 M

Ω

1.5 M

Ω

200 k

Ω

200 k

Ω

120 k

Ω

100 k

Ω

1.5 nF

2.2 nF

3.33 kHz

3.79 kHz

4.55 kHz

LOW BATTERY THRESHOLD ADJUSTMENTS

The Alarm IC has a typical internal low battery reference

voltage of 6 V. An internal resistor divider string provides a volt-

age of 80% of VDD which is compared to the 6 V reference volt-

age (See Figure 5). This results in a low battery condition and

horn chirp if the VDD level is decreased to approximately 7.5 V.

The percentage of VDD that is compared can be changed by

adding a resistor to pin 3. A resistor from pin 3 to VDD will lower

the percentage while a resistor from pin 3 to GND will increase

the percentage. The low battery comparator information will

be latched only during the LED pulse. Testing of the voltage

at pin 3 should be done during the LED pulse for confirmation.

It should also be measured through a high impedance buffer

to avoid altering the voltage level.

ALARM LATCHING APPROACHES

There are detection applications where the event that trig-

gers the alarm can be instantaneous, such as shock or motion.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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