MPXM2102AS datasheet(429/670 Pages) MOTOROLA

429 / 670 page

AN1322

3–283

Motorola Sensor Device Data

www.motorola.com/semiconductors

MICROCOMPUTER BAR GRAPH

Microcomputers with internal A/D converters such as an

MC68HC05B5 lend themselves to easily creating bar graphs.

Using the A/D converter to measure the sensor’s analog

output voltage and output ports to individually switch LED’s

makes a relatively straightforward pressure gauge. This type

of design is facilitated by a new MDC4510A gated current sink.

The MDC4510A takes one of the processor’s logic outputs

and switches 10 mA to an LED. One advantage of this

approach is that it is very flexible regarding the number of

segments that are used, and has the availability through

software to independently adjust scaling factors for each

segment. This approach is particularly useful for process

monitoring in systems where a microprocessor is already in

place.

Figure 4 shows a direct connection from an MPX5100

sensor to the microcomputer. Similar to the previous example,

an MPX2000 series sensor with the op amp interface that is

shown in Figure 1 can be substituted for the MPX5100. In this

case the op amp interface’s output at pin 7 ties to port PD5,

and its supply needs to come from a source greater than

6.5 V.

PROCESS MONITOR

For applications where an inexpensive HIGH-LOW-OK

process monitor is required, the circuit in Figure 5 does a good

job. It uses an MC33161 Universal Voltage Monitor and the

same analog interface previously described to indicate high,

low or in-range pressure.

A block diagram of the MC33161 is illustrated in Figure 6.

By tying pin 1 to pin 7 it is set up as a window detector.

Whenever input 1 exceeds 1.27 V, two logic ones are placed

at the inputs of its exclusive OR gate, turning off output 1.

Therefore this output is on unless the lower threshold is

exceeded. When 1.27 V is exceeded on input 2, just the

opposite occurs. A single logic one appears at its exclusive

OR gate, turning on output 2. These two outputs drive LED’s

through MDC4010A 10 mA current sources to indicate low

pressure and high pressure.

Returning to Figure 5, an in-range indication is developed

by turning on current source I1 whenever both the high and

low outputs are off. This function is accomplished with a

discrete gate made from D1, D2 and R7. Its output feeds the

input of switched current source I1, turning it on with R7 when

neither D1 nor D2 is forward biased.

Thresholds are set independently with R8 and R9. They

sample the same 4.0 V full scale span that is used in the other

examples. However, zero pressure offset is targeted for 1.3 V.

This voltage was chosen to approximate the 1.27 V reference

at both inputs, which avoids throwing away the sensor’s

analog output signal to overcome the MC33161’s input

threshold. In addition, R10 and R11 are selected such that at

full scale output, ie., 5.3 V on pin 7, the low side of the pots is

nominally at 1.1 V. This keeps the minimum input just below

the comparator thresholds of 1.27 V, and maximizes the

resolution available from adjustment pots R8 and R9. When

level adjustment is not desired, R8 – R11 can be replaced by

a simpler string of three fixed resistors.

CONCLUSION

The circuits that have been shown here are intended to

make simple, practical and cost effective bar graph pressure

gauges. Their application involves a variety of trade-offs that

can be matched to the needs of individual applications. In

general, the most important trade-offs are the number of

segments required and processor utilization. If the system in

which the bar graph is used already has a microprocessor with

unused A/D channels and I/O ports, tying MDC4510A current

sources to the unused output ports is a very cost effective

solution. On a stand-alone basis, the MC33161 based

process monitor is the most cost effective where only 2 or 3

segments are required. Applications that require a larger

number of segments are generally best served by one of the

circuits that uses a dedicated bar graph display.

REFERENCES

1. Alberkrack, Jade, & Barrow, Stephen; “Power Supply

Monitor IC Fills Voltage Sensing Roles,”

Power Conver-

sion & Intelligent Motion, October 1991.

2. Lucas, William, “An Evaluation System for Direct Inter-

face of the MPX5100 Pressure Sensor with a Micropro-

cessor,” Motorola Application Note AN1305.

3. Schultz, Warren, “Integrated Sensor Simplifies Bar

Graph Pressure Gauge,” Motorola Application Note

AN1304.

4. Schultz, Warren, “Compensated Sensor Bar Graph

Pressure Gauge,” Motorola Application Note AN1309.

Freescale Semiconductor, Inc.

For More Information On This Product,

Go to: www.freescale.com

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