KITMPVZ5004EVK【PDF 1/24 页】KIT EVAL PRESSURE SENS BOARD

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Freescale Semiconductor

Application Note

Water Level Monitoring

by: Michelle Clifford, Applications Engineer

Sensor Products, Tempe, AZ

AN1950

Rev 4, 11/2006

INTRODUCTION

Many washing machines currently in production use a

mechanical sensor for water level detection. Mechanical

sensors work with discrete trip points enabling water level

detection only at those points. The purpose for this reference

design is to allow the user to evaluate a pressure sensor for

not only water level sensing to replace a mechanical switch,

but also for water flow measurement, leak detection, and other

solutions for smart appliances. This system continuously

monitors water level and water flow using the temperature

compensated MPXM2010GS pressure sensor in the low cost

MPAK package, a dual op-amp, and the MC68HC908QT4,

eight-pin microcontroller.

SYSTEM DESIGN

Pressure Sensor

The pressure sensor family has three levels of integration

— Uncompensated, Compensated and

Integrated. For this design, the MPXM2010GS compensated

pressure sensor was selected because it has both

temperature compensation and calibration circuitry on the

measuring is between 0–40 cm. This corresponds to a

pressure range of 0–4 kPa. Therefore, the MPXM2010GS

was selected for this system. The sensor sensitivity is

2.5 mV/kPa, with a full-scale span of 25 mV at the supply

voltage of 10 V DC . The full-scale output of the sensor changes

linearly with supply voltage, so a supply voltage of 5 V will

return a full-scale span of 12.5 mV.

(V S actual / V S spec ) * V OUT full-scale spec = V OUT full-scale

(5.0 V/ 10 V) x 25 mV = 12.5 mV

Since this application will only be utilizing 40 percent of the

pressure range, 0–4kPa, our maximum output voltage will be

40 percent of the full-scale span.

V OUT FS * (Percent FS Range ) = V OUT max

12.5 mV * 40% = 5.0 mV

The package of the pressure sensor is a ported MPAK

package. This allows a tube to be connected to the sensor and

the tube is connected to the bottom of the tub. This isolates the

sensor from direct contact with the water. The small size and

low cost are additional features making this package a perfect

fit for this application.

silicon, allowing a simpler, yet more robust, system circuit

design. An integrated pressure sensor, such as the

MPXV5004G, is also a good choice for the design eliminating

the need for the amplification circuitry.

Figure 2. A Ported Pressure Sensor

Figure 1. Water Level Reference Design Featuring a

Pressure Sensor

The height of most washing machine tubs is 40 cm,

therefore the water height range that this system will be

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