AN1304 Integrated Sensor Simplifies Bar Graph Pressure Gauge

Freescale Semiconductor
Application Note
AN1304
Rev 2, 05/2005
Integrated Sensor Simplifies Bar Graph
Pressure Gauge
by: Warren Schultz
Discrete Applications Engineering
INTRODUCTION
Integrated semiconductor pressure sensors such as the
MPX5100 greatly simplify electronic measurement of
pressure. These devices translate pressure into a 0.5 to
4.5 volt output range that is designed to be directly compatible
with microcomputer A/D inputs. The 0.5 to 4.5 volt range also
facilitates interface with ICs such as the LM3914, making Bar
Graph Pressure Gauges relatively simple. A description of a
Bar Graph Pressure Sensor Evaluation Board and its design
considerations are presented here.
Figure 1. DEVB129 MPX5100 Bar Graph Pressure Gauge
(Board No Longer Available)
© Freescale Semiconductor, Inc., 2005. All rights reserved.
EVALUATION BOARD DESCRIPTION
Pin-by-Pin Description
A summary of the information required to use evaluation
board number DEVB129 is presented as follows. A discussion
of the design appears under the heading Design
Considerations.
Function
The evaluation board shown in Figure 1 is designed to
provide a 100 kPa full scale pressure measurement. It has two
input ports. P1, the pressure port is on the top side of the
MPX5100 sensor, and P2, a vacuum port, is on the bottom
side. These ports can be supplied up to 100 kPa (15 psi)1 of
pressure on P1 or up to 100 kPa of vacuum on P2, or a
differential pressure up to 100 kPa between P1 and P2. Any
of these sources will produce the same output.
The primary output is a 10 segment LED bar graph, which
is labeled in increments of 10 kPa. If full scale pressure is
adjusted for a value other than 100 kPa the bar graph may be
read as a percent of full scale. An analog output is also
provided. It nominally supplies 0.5 volts at zero pressure and
4.5 volts at 100 kPa. Zero and full scale adjustments are made
with potentiometers so labeled at the bottom of the board.
Both adjustments are independent of each other.
Electrical Characteristics
The following electrical characteristics are included to
describe evaluation board operation. They are not
specifications in the usual sense and are intended only as a
guide to operation.
Characteristic
Power Supply Voltage
Full Scale Pressure
Overpressure
Analog Full Scale
Analog Zero Pressure
Offset
Symbol
MIn
Typ
Max
Units
B+
6.8
—
13.2
Volts
PFS
—
—
100
kPa
PMAX
—
—
700
kPa
VFS
—
4.5
—
Volts
VOFF
—
0.5
—
Volts
Analog Sensitivity
SAOUT
—
40
—
mV/kPa
Quiescent Current
ICC
—
20
—
mA
Full Scale Current
IFS
—
140
—
mA
Content
Board contents are described in the following parts list,
schematic, and silk screen plot. A pin-by-pin circuit description
follows in the next section.
B+
Input power is supplied at the B+ terminal. Minimum input
voltage is 6.8 volts and maximum is 13.2 volts. The upper limit
is based upon power dissipation in the LM3914 assuming all
10 LED's are lit and ambient temperature is 25°C. The board
will survive input transients up to 25 volts provided that power
dissipation in the LM3914 does not exceed 1.3 watts.
OUT
An analog output is supplied at the OUT terminal. The
signal it provides is nominally 0.5 volts at zero pressure and
4.5 volts at 100 kPa. This output is capable of sourcing 100 µA
at full scale output.
GND
There are two ground connections. The ground terminal on
the left side of the board is intended for use as the power
supply return. On the right side of the board, one of the test
point terminals is also connected to ground. It provides a
convenient place to connect instrumentation grounds.
TP1
Test point 1 is connected to the zero pressure reference
voltage and can be used for zero pressure calibration. To
calibrate for zero pressure, this voltage is adjusted with R6 to
match the zero pressure voltage that is measured at the
analog output (OUT) terminal.
TP2
Test point 2 performs a similar function at full scale. It is
connected to the LM3914's reference voltage which sets the
trip point for the uppermost LED segment. This voltage is
adjusted via R5 to set full scale pressure.
P1, P2
Pressure and Vacuum ports P1 & P2 protrude from the
MPX5100 sensor on the right side of the board. Pressure port
P1 is on the top and vacuum port P2 is on the bottom. Neither
is labeled. Either one or a differential pressure applied to both
can be used to obtain full scale readings up to 100 kPa
(15 psi). Maximum safe pressure is 700 kPa.
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
1. 100 kPa = 14.7 psi, 15 psi is used throughout the text for convenience.
AN1304
2
Sensors
Freescale Semiconductor
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
issupplied by an MC78L05. The LED's are powered
directlyfrom LM3914 outputs, which are set up as current
sources.Output current to each LED is approximately 10times
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.
S1
+12 V
ON/OFF
D1
D2
D3
D4
1
2
3
4
5
6
7
8
9
LED
GND
B+
RLO
SIG
RHI
REF
ADJ
MOD
D5
D6
D7
D8
D9
D10
C2
1 µF
U3
MC78L05ACP
3
I
U1
C1
0.1 µF
O
1
R4
G
2
1.3K
3
GND
1
2
R2
1.2 k
U2
MPX5100
ZERO
CAL.
R6
100
R1
100
R5
1k
LED
LED
LED
LED
LED
LED
LED
LED
LED
18
17
16
15
14
13
12
11
10
LM3914
TP2 (Full Scale Calibration)
TP1 (Zero Calibration)
Full Scale
Calibration
GND
R3
2.7 k
ANALOG OUT
Figure 2. MPX5100 Pressure Gauge
AN1304
Sensors
Freescale Semiconductor
3
MPX5100 Pressure Gauge
kPa
100
Freescale
Discrete
Applications
Pressure
90
60
50
LM3914
70
MV57164
80
MPX5100
40
30
20
C2
10
C1
B+
TP2
R3
U3
OUT
TP1
R2
GND
DEVB129
ON
R6
R5
Zero
Full Scale
GND
OFF
Figure 3. Silk Screen 2X
Table 1. Parts List
Designators
C1
C2
Quant.
1
1
Description
Ceramic Cap
Ceramic Cap
Rating
Manufacturer
Part Number
0.1 µF
1 µF
D1-D10
1
Bar Graph LED
R1
R2
R3
R4
R5
R6
1
1
1
1
1
1
1/4 W Film Resistor
1/4 W Film Resistor
1/4 W Film Resistor
1/4 W Film Resistor
Trimpot
Trimpot
GI
S1
1
On/Off Switch
NKK
12SDP2
U1
U2
U3
1
1
1
Bar Graph IC
Pressure Sensor
Voltage Regulator
National
Freescale
Freescale
LM3914
MPX5100
MC78L05ACP
—
—
—
—
1
3
4
4
Terminal Block
Test Point Terminal
Nylon Spacer
4-40 Nylon Screw
Augat
25V03
Components Corp. TP1040104
100
1.2K
2.7K
1.3K
1K
100
MV57164
Bourns
Bourns
3/8″
1/4″
Notes: All resistors have a tolerance of 5% unless otherwise noted.
All capacitors are 50 volt ceramic capacitors with a tolerance of 10% unless otherwise noted.
AN1304
4
Sensors
Freescale Semiconductor
LM3914
RHI
6
1k
11
–
+
12
–
+
13
1k
–
+
14
1k
–
+
15
1k
–
+
16
1k
–
+
17
1k
–
+
18
1k
–
+
1
1k
V+
1k
7+
This load
determines
LED
brightness
REF
ADJ
V+
RLO
Reference
Voltage
Source
1.25 V
–
8
3
4
10
–
+
1k
REF
OUT
Comparator
1 OF 10
–
+
LED
V+
From
Pin 11
Mode
Select
Amplifier
9
–
Buffer
SIG
IN
5 20 k
V–
Controls
type of
display bar
or single
LED
2
+
Figure 4. LM3914 Block Diagram
AN1304
Sensors
Freescale Semiconductor
5
NOTES
AN1304
6
Sensors
Freescale Semiconductor
NOTES
AN1304
Sensors
Freescale Semiconductor
7
How to Reach Us:
Home Page:
www.freescale.com
E-mail:
[email protected]
USA/Europe or Locations Not Listed:
Freescale Semiconductor
Technical Information Center, CH370
1300 N. Alma School Road
Chandler, Arizona 85224
+1-800-521-6274 or +1-480-768-2130
[email protected]
Europe, Middle East, and Africa:
Freescale Halbleiter Deutschland GmbH
Technical Information Center
Schatzbogen 7
81829 Muenchen, Germany
+44 1296 380 456 (English)
+46 8 52200080 (English)
+49 89 92103 559 (German)
+33 1 69 35 48 48 (French)
[email protected]
Japan:
Freescale Semiconductor Japan Ltd.
Headquarters
ARCO Tower 15F
1-8-1, Shimo-Meguro, Meguro-ku,
Tokyo 153-0064
Japan
0120 191014 or +81 3 5437 9125
[email protected]
Asia/Pacific:
Freescale Semiconductor Hong Kong Ltd.
Technical Information Center
2 Dai King Street
Tai Po Industrial Estate
Tai Po, N.T., Hong Kong
+800 2666 8080
[email protected]
For Literature Requests Only:
Freescale Semiconductor Literature Distribution Center
P.O. Box 5405
Denver, Colorado 80217
1-800-441-2447 or 303-675-2140
Fax: 303-675-2150
[email protected]
AN1304
Rev. 2
05/2005
Information in this document is provided solely to enable system and software
implementers to use Freescale Semiconductor products. There are no express or
implied copyright licenses granted hereunder to design or fabricate any integrated
circuits or integrated circuits based on the information in this document.
Freescale Semiconductor reserves the right to make changes without further notice to
any products herein. Freescale Semiconductor makes no warranty, representation or
guarantee regarding the suitability of its products for any particular purpose, nor does
Freescale Semiconductor assume any liability arising out of the application or use of any
product or circuit, and specifically disclaims any and all liability, including without
limitation consequential or incidental damages. “Typical” parameters that may be
provided in Freescale Semiconductor data sheets and/or specifications can and do vary
in different applications and actual performance may vary over time. All operating
parameters, including “Typicals”, must be validated for each customer application by
customer’s technical experts. Freescale Semiconductor does not convey any license
under its patent rights nor the rights of others. Freescale Semiconductor products are
not designed, intended, or authorized for use as components in systems intended for
surgical implant into the body, or other applications intended to support or sustain life,
or for any other application in which the failure of the Freescale Semiconductor product
could create a situation where personal injury or death may occur. Should Buyer
purchase or use Freescale Semiconductor products for any such unintended or
unauthorized application, Buyer shall indemnify and hold Freescale Semiconductor and
its officers, employees, subsidiaries, affiliates, and distributors harmless against all
claims, costs, damages, and expenses, and reasonable attorney fees arising out of,
directly or indirectly, any claim of personal injury or death associated with such
unintended or unauthorized use, even if such claim alleges that Freescale
Semiconductor was negligent regarding the design or manufacture of the part.
Freescale™ and the Freescale logo are trademarks of Freescale Semiconductor, Inc.
All other product or service names are the property of their respective owners.
© Freescale Semiconductor, Inc. 2005. All rights reserved.
Similar pages