Microcomputer Circuits and Processes

The second application, shown in figure 4.40(a), is a small weather
station. The measurements are temperature, pressure, and wind-speed,
and CONDAC must switch on an alarm siren if the combined
measurements indicate a serious storm. Temperature and pressure are
analogue signals; converted to voltage, they are both read and recorded
using a switched A-to-D converter. It is wind-speed that offers a chance
to think. You could connect the anemometer vanes to a dynamo, and
measure the output voltage, or current, with an A-to-D converter. But
their relation to speed is not simple. A better way is to attach a disk to
the vanes. This disk should be made of transparent plastic with opaque,
black stripes painted on - figure 4.40(b). Light is passed through the
disk and received by a photodetector device. If an opaque strip lies
between light and detector, the detector's output is zero. If a transparent
strip is in that position, the detector output is high, 5 V. As the vanes
and the disk rotate, the output of the detector is a square wave, as
alternate opaque and transparent strips cross the light path. The
frequency of this square wave must increase as the disk rotates faster,
and the increase is linear. So what do you do with the square wave?
feed it to a digital input of CONDAC, and write a program to
determine the frequency of the. square wave. The instrument is
calibrated by recording the frequency at one known wind speed. This is
a general method that can be used to measure the speed of rotating
shafts, motors, lathes, robot wheels, and so on.
The alarm signal is easy to generate, once the computer has
measured the three variables and been programmed to decide that a
serious storm exists. CONDAC outputs a binary number to the output
port, which then switches on the alarm siren.
In both of these examples the microprocessor is an ideal solution to the
how-to-make-the-measurement problem. In both cases measurements
have to be made infrequently over long periods of time. Also, the
system's response may have to be altered. The animal psychologist may
like to study how switching on light and heat with different time periods
affects the animal's behaviour, or to have a time lag between heat on
and light on. If the control circuit had been made using discrete logic
chips, many of the desired changes would have called for massive
rebuilding of the electronic circuit boards. With a microprocessor, a
simple change in software is all that is needed and the experimenter can
do this himself. A microprocessor is not always the best choice; simpler
or faster circuits can be made with discrete logic. But in applications
where the precise working of the circuit may need to be changed in the
future, the microprocessor is the natural choice.
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