MPXM2102AS datasheet(544/670 Pages) MOTOROLA

544 / 670 page

AN1950

3–398

Motorola Sensor Device Data

www.motorola.com/semiconductors

Table 2: Parts List

Ref

Qty.

Description

Value

Vendor

Part No.

Pressure Sensor

Motorola

MPXM2010GS

Vcc Cap

0.1uF

Generic

Op–Amp Cap

0.1uF

Generic

Shift Register Cap

0.1uF

Generic

Red LED

Generic

Green LED

Generic

S2,S3

Pushbuttons

Generic

Quad Op–Amp

ADI

AD8544

Voltage Regulator

Fairchild

LM78L05ACH

Microcontroller

8pin

Motorola

MC68HC908QT4

¼ W Resister

22K

Generic

¼ W Resister

2.4K

Generic

R3,R6

¼ W Resister

1.2M

Generic

R4,R5

¼ W Resister

1.5K

Generic

R7,R8

¼ W Resister

10K

Generic

R9,R10

¼ W Resister

Generic

LCD (Optional)

16x2

Seiko

L168200J000

Shift Register

Texas Instruments

74HC164

Smart Washer Software

This application note describes the first software version

that was available. However updated software versions may

be available with further functionality and menu selections.

Software User Instructions

When the system is turned on or reset, the microcontroller

will flash the select LED and display the program title on the

LCD for 5 seconds or until the select (SEL) button is pushed.

Then the menu screen is displayed. Using the select (SEL)

pushbutton, the user can scroll through the menu options for

a software program. To run the water level program, use the

select button to highlight the “Water Level” option, then press

the enter (ENT) pushbutton. The Water Level program will

display current water level, the rate of flow, a message if the

container is “FILLING”, “EMPTYING”, “FULL”, or “EMPTY”,

and a scrolling graphical history displaying data points

representing the past forty level readings.

The Water Level is displayed by retrieving the digital voltage

from the internal A/D Converter. This voltage is converted to

pressure in millimeters of water and then displayed on the

LCD.

Calibration and Calibration Software

To calibrate the system, a two–point calibration is

performed. The sensor will take a calibration point at 0mm and

at 40mm of water. Hold down both the SEL and ENT buttons

on system power–up to enter calibration mode. At this point,

the calibration menu will be displayed with the previously

sampled offset voltage. To recalibrate the system, expose the

sensor to atmospheric pressure and press the SEL button

(PB1). At this point, the zero offset voltage will be sampled and

saved to a location in the microcontroller memory. To obtain

the second calibration point, place the end of the plastic tube

from the pressure sensor to the bottom of a container holding

40mm of water. Then press the ENT button (PB2). The

voltage output will be sampled, averaged and saved to a

location in memory. To exit the calibration mode, press the

SEL (PB1) button.

Figure 4. Water level system set–up for demonstration

–40 cm

–35 cm

–30 cm

–25 cm

–20 cm

–15 cm

–10 cm

– 5 cm

Converting Pressure to Water Level

Hydrostatic Pressure that we are measuring is the pressure

at the bottom of a column of fluid caused by the weight of the

fluid and the pressure of the air above the fluid. Therefore, the

hydrostatic pressure depends on the air pressure, the fluid

density and the height of the column of fluid.

P= Pa +

ρ g ∆h

where P = pressure

Pa = pressure

ρ = mass density of fluid

g = 9.8066 m/s^2

h = height of fluid column

To calculate the water height, we can use the measured

pressure

with

the

following

equation,

assuming

the

atmospheric pressure is already compensated for by the

selection of the pressure sensor being gauge:

∆h = P \ ρ g

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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