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| MOTOROLA SEMICONDUCTOR APPLICATION NOTE Freescale Semiconductor, Inc. Order this document by AN1304/D Integrated Sensor Simplifies Bar Graph Pressure Gauge Prepared by: Warren Schultz Discrete Applications Engineering AN1304 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. Freescale Semiconductor, Inc... Figure 1. DEVB129 MPX5100 Bar Graph Pressure Gauge REV 1 Motorola Sensor Device Data (c) Motorola, Inc. 1997 For More Information On This Product, Go to: www.freescale.com 1 AN1304 Freescale Semiconductor, Inc. EVALUATION BOARD DESCRIPTION PIN-BY-PIN DESCRIPTION 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 25C. 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 A 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. 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)* 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 Analog Sensitivity Quiescent Current Full Scale Current Symbol B+ PFS PMAX VFS VOFF SAOUT ICC IFS Min 6.8 -- -- -- -- -- -- -- Typ -- -- -- 4.5 0.5 40 20 140 Max 13.2 100 700 -- -- -- -- -- Units Volts kPa kPa Volts Volts mV/kPa mA mA Freescale Semiconductor, Inc... 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. * 100 kPa = 14.7 psi, 15 psi is used throughout the text for convenience 2 For More Information On This Product, Go to: www.freescale.com Motorola Sensor Device Data Freescale Semiconductor, Inc. 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. AN1304 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 85C 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%). Freescale Semiconductor, Inc... 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 D5 D6 D7 D8 D9 D10 C2 1 F U3 I MC78L05ACP G 3 O 2 3 1 C1 0.1 F U1 1 2 3 4 5 6 7 8 9 LED GND B+ RLO SIG RHI REF ADJ MOD LED LED LED LED LED LED LED LED LED 18 17 16 15 14 13 12 11 10 R4 1.3K 1 U2 MPX5100 ZERO CAL. R2 1.2 k R6 100 R1 100 R5 1k LM3914 TP2 (FULL SCALE CALIBRATION) TP1 (ZERO CALIBRATION) GND 2 GND FULL SCALE CALIBRATION R3 2.7 k ANALOG OUT Figure 2. MPX5100 Pressure Gauge Motorola Sensor Device Data For More Information On This Product, Go to: www.freescale.com 3 AN1304 Freescale Semiconductor, Inc. MPX5100 PRESSURE GAUGE MOTOROLA DISCRETE APPLICATIONS kPa 100 90 80 PRESSURE MV57164 70 60 50 40 30 20 LM3914 MPX5100 Freescale Semiconductor, Inc... 10 C2 C1 B+ U3 OUT R2 GND GND R3 TP1 TP2 DEVB129 ON R6 R5 OFF ZERO FULL SCALE Figure 3. Silk Screen 2X Table 1. Parts List Designators C1 C2 D1-D10 R1 R2 R3 R4 R5 R6 S1 U1 U2 U3 -- -- -- -- Quant. 1 1 1 1 1 1 1 1 1 1 1 1 1 1 3 4 4 Description Ceramic Cap Ceramic Cap Bar Graph LED 1/4 W Film Resistor 1/4 W Film Resistor 1/4 W Film Resistor 1/4 W Film Resistor Trimpot Trimpot On/Off Switch Bar Graph IC Pressure Sensor Voltage Regulator Terminal Block Test Point Terminal Nylon Spacer 4-40 Nylon Screw 100 1.2K 2.7K 1.3K 1K 100 Rating 0.1 F 1 F GI MV57164 Manufacturer Part Number Bourns Bourns NKK National Motorola Motorola Augat Components Corp. 12SDP2 LM3914 MPX5100 MC78L05ACP 25V03 TP1040104 3/8 1/4 Note: All resistors have a tolerance of 5% unless otherwise noted. Note: All capacitors are 50 volt ceramic capacitors with a tolerance of 10% unless otherwise noted. 4 For More Information On This Product, Go to: www.freescale.com Motorola Sensor Device Data Freescale Semiconductor, Inc. LM3914 RHI 6 1k - + - + - + - + - + - + - + - + - + - + V+ FROM PIN 11 CONTROLS TYPE OF DISPLAY, BAR OR SINGLE LED LED V+ COMPARATOR 1 of 10 10 AN1304 11 1k 12 1k 13 1k 14 Freescale Semiconductor, Inc... REF OUT THIS LOAD DETERMINES LED BRIGHTNESS REF ADJ 7+ REFERENCE VOLTAGE SOURCE 1.25 V - 1k 15 1k 16 8 1k 17 1k 18 V+ 3 1k 1 1k RLO 4 MODE SELECT AMPLIFIER - BUFFER SIG IN 5 20 k + 9 2 V- Figure 4. LM3914 Block Diagram Motorola Sensor Device Data For More Information On This Product, Go to: www.freescale.com 5 AN1304 Freescale Semiconductor, Inc. Freescale Semiconductor, Inc... Motorola reserves the right to make changes without further notice to any products herein. Motorola makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does Motorola 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 which may be provided in Motorola 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. Motorola does not convey any license under its patent rights nor the rights of others. Motorola 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 Motorola product could create a situation where personal injury or death may occur. Should Buyer purchase or use Motorola products for any such unintended or unauthorized application, Buyer shall indemnify and hold Motorola 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 Motorola was negligent regarding the design or manufacture of the part. Motorola and are registered trademarks of Motorola, Inc. Motorola, Inc. is an Equal Opportunity/Affirmative Action Employer. Mfax is a trademark of Motorola, Inc. How to reach us: USA / EUROPE / Locations Not Listed: Motorola Literature Distribution; P.O. Box 5405, Denver, Colorado 80217. 303-675-2140 or 1-800-441-2447 JAPAN: Nippon Motorola Ltd.; Tatsumi-SPD-JLDC, 6F Seibu-Butsuryu-Center, 3-14-2 Tatsumi Koto-Ku, Tokyo 135, Japan. 81-3-3521-8315 MfaxTM: RMFAX0@email.sps.mot.com - TOUCHTONE 602-244-6609 ASIA/PACIFIC: Motorola Semiconductors H.K. Ltd.; 8B Tai Ping Industrial Park, - US & Canada ONLY 1-800-774-1848 51 Ting Kok Road, Tai Po, N.T., Hong Kong. 852-26629298 INTERNET: http://motorola.com/sps 6 For More Information On This Product, Go to: www.freescale.com AN1304/D Motorola Sensor Device Data |
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