3–283
Motorola Sensor Device Data
For More Information On This Product,
Go to: www.freescale.com
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.
F
Freescale Semiconductor, Inc.
n
.