MPXM2102AS datasheet(362/670 Pages) MOTOROLA

362 / 670 page

AN1304

3–216

Motorola Sensor Device Data

www.motorola.com/semiconductors

DESIGN CONSIDERATIONS

In this type of an application the design challenge is how to

interface a sensor with the bar graph output. MPX5100

Sensors and LM3914 Bar Graph Display drivers fit together so

cleanly that having selected these two devices the rest of the

design is quite straight forward.

A block diagram that appears in Figure 4 shows the

LM3914’s internal architecture. Since the lower resistor in the

input comparator chain is pinned out at RLO, it is a simple

matter to tie this pin to a voltage that is approximately equal

to the MPX5100’s zero pressure output voltage. In Figure 2,

this is accomplished by dividing down the 5 volt regulator’s

output voltage through R1, R4, and adjustment pot R6. The

voltage generated at the wiper of R6 is then fed into RLO which

matches the sensor’s zero pressure voltage and zeros the bar

graph.

The full scale measurement is set by adjusting the upper

comparator’s reference voltage to match the sensor’s output

at full pressure. An internal regulator on the LM3914 sets this

voltage with the aid of resistors R2, R3, and adjustment pot R5

that are shown in Figure 2.

The MPX5100 requires 5 volt regulated power that

is supplied by an MC78L05. The LED’s are powered

directly from LM3914 outputs, which are set up as current

sources. Output current to each LED is approximately

10 times the reference current that flows from pin 7 through

R2, R5, and R3 to ground. In this design it is nominally

(4.5 V/4.9K)10 = 9.2 mA.

Over a zero to 85

°C temperature range accuracy for both

the sensor and driver IC are

±2.5%, totaling ±5%. Given a 10

segment display total accuracy is approximately

±(10 kPa

+5%).

CONCLUSION

Perhaps the most noteworthy aspect to the bar graph

pressure gauge described here is how easy it is to design. The

interface between an MPX5100 sensor, LM3914 display

driver, and bar graph output is direct and straight forward. The

result is a simple circuit that is capable of measuring pressure,

vacuum, or differential pressure; and will also send an analog

signal to other control circuitry.

Figure 2. MPX5100 Pressure Gauge

ON/OFF

LED

GND

B+

RLO

SIG

RHI

REF

ADJ

MOD

LM3914

TP2 (FULL SCALE CALIBRATION)

TP1 (ZERO CALIBRATION)

GND

FULL SCALE CALIBRATION

D10

1.2 k

1 k

2.7 k

100

MPX5100

GND

ANALOG OUT

+12 V

0.1

µF

MC78L05ACP

ZERO

CAL.

1.3K

LED

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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