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QUESTIONS & ANSWERS
; FIGURE 2.
Matthew, told me to use the scan
word. Eureka! That works and leaves
six address lines for 64 different
patterns. You could have 128
different patterns by using a 2716
EPROM. The TTL does not have
much drive capability, so I am using
a 74HCT240 inverting buffer and a
74HCT244 non-inverting buffer to
get up to 20 mA drive to the LEDs;
150 ohms in series ensures that the
current will not be excessive.
The pattern needs to be
displayed multiple times in order for
the brain to see it, so the seven-stage
counter (IC10) increments the
address after 128 scans (about four
seconds’ time). You can decode a
different number from IC10 for a
smaller time or use a 12-stage
counter (CD4040) for a longer time.
FAN SPEED CONTROL
QI need to power-up and control a 12V squirrel cage blower motor from an automobile. The blower is
going into a segment of my heating
system and I want to control its
speed based on ambient temperature
— in a linear fashion from about 60°F
to 75°F — so it’s higher speed at
The blower draws 12 amps at
12 volts; I will need a power supply
for that. I would like to use a
microprocessor if you can do it.
— David Abineri
; FIGURE 3.
September 2012 21