compare this with the schematic in Figure E.
Include a tactile switch between the positive supply and pin 1 of the 4026 counter, with a 10K resistor
to keep the input to the 4026 counter negative until
the button is pressed. Make sure all your positives
and negatives are correct, and turn on the power.
You should find that when you tap the tactile
switch lightly, the counter advances the numeric
display from 0 through 9 and then begins all over
again from 0. You may also find that the chip
sometimes misinterprets your button-presses, and
counts two or even three digits at a time. I’ll deal
with this problem a little later on.
The LED segments won’t be glowing very brightly,
because the 1K series resistors deprive them of the
power they would really like to receive. Those resistors are necessary to avoid overloading the outputs
from the counter.
Assuming that you succeed in getting your counter to drive the numeric display, you’re ready to add
two more counters, which will control the remaining
two numerals. The first counter will count in ones,
the second in tens, and the third in hundreds.
In Figure G (following page), I’ve used arrows
and numbers to tell you which pins of the counters
should be connected to which pins of the numeric
display. Otherwise, the schematic would be a confusing tangle of wires crossing each other.
At this point, you can give up in dismay at the
number of connections — but really, using a breadboard, it shouldn’t take you more than half an hour
to complete this phase of the project. I suggest you
give it a try, because there’s something magical
about seeing a display count from 000 through 999
“all by itself,” and I chose this project because it also
has a lot of instructional value.
S1 is attached to the “clock disable” pin of IC1, so
that when you hold down this button, it should stop
that counter from counting. Because IC1 controls
IC2, and IC2 controls IC3, if you freeze IC1, the other
two will have to wait for it to resume. Therefore you
won’t need to make use of their “clock disable”
features.
S2 is connected to the “reset” pins of all three
counters, so that when you hold down this button, it
should set them all to zero.
S3 sends positive pulses manually to the “clock
input” pin of the first counter.
S1, S2, and S3 are all wired in parallel with 1K
resistors connected to the negative side of the
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Fig. D: When building circuits around chips, it’s
convenient to have a positive and negative power
supply down each side of your breadboard. For the
reaction timer circuit, a 9V supply with a 100μF
smoothing capacitor can be set up like this. If your
breadboard doesn’t color-code the columns of holes
on the left and right sides, I suggest you do that
yourself with a permanent marker.
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Fig. E: IC3 is a 4026 counter. IC4 is a triple 7-segment
display chip. The arrows tell you which pins on the LED
display should be connected to the pins on the counter.
99 Make:
