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Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
By Michelle Clifford
Applications Engineer
Sensor Products Division
Tempe, AZ
INTRODUCTION
Many washing machines that are currently in production
use a mechanical sensor for water level detection.
Mechanical sensors work with discrete trip points that enable
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, 8–pin microcontroller.
SYSTEM DESIGN
PRESSURE SENSOR
The Motorola 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 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.
The height of most washing machine tubs is 40cm,
therefore the water height range that this system will be
measuring is between 0–40cm. This corresponds to a
pressure range of 0 – 4 kPa. Therefore, the MPXM2010GS
was selected for this system. The sensor sensitivity is
2.5mV/kPa, with a full–scale span of 25mV at the supply
voltage of 10 Vdc. The full–scale output of the sensor changes
linearly with supply voltage, so a supply voltage of 5V will
return a full–scale span of 12.5 mV.
(Vs actual / Vs spec) x Vout full–scale spec = Vout full–scale
(5 V/ 10 V) x 25 mV = 12.5 mV
Since this application will only be utilizing 40% of the
pressure range, 0–4kPa, our maximum output voltage will be
40% of the full–scale span.
Vout FS x (Percent FS Range) = Vout max
12.5 mV x 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;
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 that make this package a
perfect fit for this application.
Figure 1. MPXM2010GS/GST1
Case 1320A
SEMICONDUCTOR APPLICATION NOTE
REV 0
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Freescale Semiconductor, Inc.
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