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. 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