Q
&
A
certain amount of time.
— Christopher Rust
Maple Grove, MN
AThis brings me back to my
childhood, where we used
voice commands to operate
a model railroad. STOP!
(One syllable.) GO FORWARD! (Two
syllables.) NOW BACK UP! (Three
syllables.) Back then, we used a
microphone, tube amp, and a
ratcheting relay to make the train obey
the “voice” commands. Thankfully,
today we have the 4017 decade
counter — which is a lot quieter and
uses less power. All you need to do
is turn on the 4017 for five seconds
and count the number of pulses that
arrive in that time.
The 4017 will need some help,
though, in the way of sequencing
timers. See Figure 8 for this discussion. We first assume that the 4017
is on standby, in that the Reset (pin
15) and Clock Enable (pin 13) inputs
are both high. For this circuit, I’m
using a push-button switch — but any
negative-going pulse will do. Push
the button once, and the 15-minute
and five-second timer go high and
turn on the 4017 through the 4049
inverter. This sets the stage for
counting — but the clock pulse (pin
14) can’t arrive until the 4017 has
stabilized. Hence, the 1 ms timer. All
clock pulses are delayed by one
millisecond.
Once the Reset and Clock
Enable inputs are established, let
the counting begin. After five
seconds, the Clock Enable line
goes high and freezes the count.
Fifteen minutes later, the Reset input
goes high and clears the output,
readying the circuit for the next
pulse train. As for the outputs —
you’re on your own. Note that the
brightness of the lamps will ripple
from dim to bright as the count
progresses. If that’s not acceptable,
use the five-second output for
an inhibit signal. Of course, the
15-minute and five-second timers
can be adjusted to any periods you
wish — and can be reduced to a
single chip using a 556 IC. But leave
the 1 ms timer as it is.
