3–216
Motorola Sensor Device Data
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DESIGN CONSIDERATIONS
In this type of an application the design challenge is how to
interface a sensor with the bar graph output. MPX5100
Sensors and LM3914 Bar Graph Display drivers fit together so
cleanly that having selected these two devices the rest of the
design is quite straight forward.
A block diagram that appears in Figure 4 shows the
LM3914’s internal architecture. Since the lower resistor in the
input comparator chain is pinned out at RLO, it is a simple
matter to tie this pin to a voltage that is approximately equal
to the MPX5100’s zero pressure output voltage. In Figure 2,
this is accomplished by dividing down the 5 volt regulator’s
output voltage through R1, R4, and adjustment pot R6. The
voltage generated at the wiper of R6 is then fed into RLO which
matches the sensor’s zero pressure voltage and zeros the bar
graph.
The full scale measurement is set by adjusting the upper
comparator’s reference voltage to match the sensor’s output
at full pressure. An internal regulator on the LM3914 sets this
voltage with the aid of resistors R2, R3, and adjustment pot R5
that are shown in Figure 2.
The MPX5100 requires 5 volt regulated power that
is supplied by an MC78L05. The LED’s are powered
directly from LM3914 outputs, which are set up as current
sources. Output current to each LED is approximately
10 times the reference current that flows from pin 7 through
R2, R5, and R3 to ground. In this design it is nominally
(4.5 V/4.9K)10 = 9.2 mA.
Over a zero to 85
°
C temperature range accuracy for both
the sensor and driver IC are
±
2.5%, totaling
±
5%. Given a 10
segment display total accuracy is approximately
±
(10 kPa
+5%).
CONCLUSION
Perhaps the most noteworthy aspect to the bar graph
pressure gauge described here is how easy it is to design. The
interface between an MPX5100 sensor, LM3914 display
driver, and bar graph output is direct and straight forward. The
result is a simple circuit that is capable of measuring pressure,
vacuum, or differential pressure; and will also send an analog
signal to other control circuitry.
Figure 2. MPX5100 Pressure Gauge
S1
ON/OFF
1
2
3
4
5
6
7
8
9
18
17
16
15
14
13
12
11
10
LED
LED
LED
GND
B+
RLO
SIG
RHI
REF
ADJ
MOD
U1
LM3914
TP2(FULL SCALE CALIBRATION)
TP1(ZERO CALIBRATION)
GND
FULL SCALE CALIBRATION
D1
D2
D3
D4
D5
D6
D7
D8
D9
D10
R2
1.2 k
R5
1 k
R3
2.7 k
R1
100
R6
100
U2
MPX5100
1
2
3
1
2
3
I
G
O
GND
ANALOG OUT
+12 V
C1
0.1
μ
F
C2
1
μ
F
U3
MC78L05ACP
ZERO
CAL.
R4
1.3K
LED
LED
LED
LED
LED
LED
LED
F
Freescale Semiconductor, Inc.
n
.