sensor, microcontroller, and LCD. The battery is mounted on
the back of the board using a space saving spring battery clip.
OTHER
This system is designed to run on a 9.0 V battery. It
contains a 5.0 V regulator to provide 5.0 V to the pressure
Table 2. Parts List
Ref.
Qty
Description
Value
Vendor
Part No.
U2
1
Pressure Sensor
1
Freescale
MPXM2010GS
C1
C2
1
1
1
1
1
2
1
1
1
1
1
2
2
2
2
1
1
Vcc Cap
Op-Amp Cap
Shift Register Cap
Red LED
0.1µf
0.1µf
0.1µf
—
Generic
Generic
Generic
Generic
Generic
Generic
ADI
—
—
C3
—
D1
—
D2
Green LED
—
—
S2, S3
U1
Pushbuttons
Quad Op-Amp
Voltage Regulator
Microcontroller
¼ W Resistor
¼ W Resistor
¼ W Resistor
¼ W Resistor
¼ W Resistor
¼ W Resistor
LCD (Optional)
Shift Registor
—
—
—
AD8544
U3
5.0 V
8-pin
22 K
2.4 K
1.2 M
1.5 K
10 K
1.0 K
16 x 2
—
Fairchild
Freescale
Generic
Generic
Generic
Generic
Generic
Generic
Seiko
LM78L05ACH
U4
MC68HC908QT4
R1
—
R2
—
R3, R6
R4, R5
R7, R8
R9, R10
U6
—
—
—
—
L168200J000
74HC164
U5
Texas Instruments
buttons on system power-up enters the calibration mode. At
this point, the calibration menu is 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
40 mm 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.
Smart Washer Software
This application note describes the first software version
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 selected LED and display the program title on the
LCD for five seconds, or until the select (SEL) button is
pushed. Then the menu screen is displayed. Using the select
(SEL) pushbutton, it is easy to 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
Figure 5. Water Level System Set-Up
for Demonstration
To calibrate the system, a two-point calibration is
performed. The sensor will take a calibration point at 0 mm
and at 40 mm of water. Depressing both the SEL and ENT
AN1950
Sensors
4
Freescale Semiconductor
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