AN1950 Datasheet, PDF (1/24 Pages) Freescale Semiconductor, Inc – Water Level Monitoring

Freescale Semiconductor
Application Note
AN1950
Rev 4, 11/2006
Water Level Monitoring
by: Michelle Clifford, Applications Engineer
Sensor Products, Tempe, AZ
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
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.
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.
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
© Freescale Semiconductor, Inc., 2006. All rights reserved.