PROJECTS: REACTION TIMER
Fig. B: The most basic and common digital numeral
consists of seven LED segments identified by letters,
as shown here, plus an optional decimal point.
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Fig. C: This Kingbright unit incorporates three
7-segment numeric displays in one package, and can
be driven by three chained 4026 decade counters.
The pin numbers are shown close to the chip. Segments
a through g of numeral 1 are identified as 1a through
1g. Segments a through g of numeral 2 are identified as
2a through 2g. Segments a through g of numeral 3 are
identified as 3a through 3g.
98 Make: Volume 21
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a decimal point, and although we won’t be using it,
I’ve identified it with the letter h.
Check Figure C showing the Kingbright display,
and you’ll see I have annotated each pin with its
function. You can step down the display with the
positive wire from your power supply, making sure
that each pin lights an appropriate segment.
Incidentally, this display has two pins, numbered
3 and 26, both labeled to receive negative voltage
for the first of the digits.
Why two pins instead of one? I don’t know. You
need to use only one, and as this is a passive chip, it
doesn’t matter if you leave the unused one unconnected. Just take care not to apply positive voltage
to it, which would create a short circuit.
A numeric display has no power or intelligence of
its own. It’s just a bunch of light-emitting diodes. It’s
not much use, really, until we can figure out a way to
illuminate the LEDs in appropriate groups — which
will be the next step.
Step 2: Counting
Fortunately, we have a chip known as the 4026,
which receives pulses, counts them, and creates
an output designed to work with a seven-segment
display so that it shows numbers 0– 9. The only
problem is that the 4026 is a rather old-fashioned
CMOS chip (meaning, Complementary Metal Oxide
Semiconductor) and is thus sensitive to static
electricity.
Switch off your power supply and connect its
wires to the top of the breadboard, noting that for
this experiment, we’re going to need positive and
negative power on both sides. See Figure D for
details. If your breadboard doesn’t already have the
columns of holes color-coded, I suggest you use
Sharpie markers to identify them, to avoid polarity
errors that can fry your components.
The 4026 counter chip is barely powerful enough
to drive the LEDs in our display when powered by
9 volts. Make sure you have the chip the right way
up, and insert it into the breadboard immediately
above your three-digit display, leaving just one row
of holes between them empty.
The schematic in Figure E shows how the pins of
the 4026 chip should be connected. The arrows tell
you which pins on the display should be connected
with pins on the counter.
Figure F (page 100) shows the “pinouts” (i.e., the
functions of each pin) of a 4026 chip. You should
