A 555 timer is ideal for creating a stream of pulses
that drive a counter chip. Figure H, page 103, shows
how to connect these chips to the positive and
negative rails on your breadboard. Also I’m showing
the connection between pins 2 and 6 in the way that
you’re most likely to make it, via a wire that loops
over the top of the chip.
For the current experiment, I’m suggesting initial
component values that will generate only four
pulses per second. Any faster than that, and you
won’t be able to verify that your counters are counting properly.
Install IC5 and its associated components on your
NOTE: In the course of researching the book, author
Charles Platt ran across the home phone number of
Hans Camenzind, inventor of the venerable 555 chip. On
a lark, he decided to dial the number. You can read about
what happened next at Make: Online ( makezine.com/go/
Counters and Seven-Segment Displays
Most counters accept a stream
of pulses and distribute them to
a series of pins in sequence. The
4026 decade counter is unusual in
that it applies power to its output
pins in a pattern that is just right
to illuminate the segments of a
7-segment numeric display.
Some counters create positive
outputs (they “source” current)
while others create negative
outputs (they “sink” current).
Some 7-segment displays require
positive input to light up the numbers. These are known as “common
cathode” displays. Others require
negative input and are known as
“common anode” displays. The
4026 delivers positive outputs and
requires a common cathode display.
Check the data sheet for any
counter chip to find out how much
power it requires, and how much
it can deliver. CMOS chips are
becoming dated, but they are very
useful to hobbyists, because they
tolerate a wide range of supply
voltage — from 5 to 15 volts in the
case of the 4026. Other types of
chips are much more limited.
Most counters can source or sink
only a few milliamps of output
power. When the 4026 is running on
a 9-volt power supply, it can source
about 4mA of power from each pin.
This is barely enough to drive a
You can insert a series resistor
between each output pin of the
counter and each input pin of the
numeric display, but a simpler,
quicker option is to use just one
series resistor for each numeral,
between the negative-power pin
and ground. The experiment that
I’m describing uses this shortcut.
Its disadvantage is that digits that
require only a couple of segments
(such as numeral 1) will appear
brighter than those that use many
segments (such as numeral 8).
If you want your display to look
bright and professional, you really
need a transistor to drive each
segment of each numeral. An alternative is to use a chip containing
multiple “op amps” to amplify the
When a decade counter reaches 9
and rolls over to 0, it emits a pulse
from its “carry” pin. This can drive
another counter that will keep track
of tens. The carry pin on that coun-
ter can be chained to a third counter
that keeps track of hundreds,
and so on. In addition to decade
counters, there are hexadecimal
counters (which count in 16s), octal
counters (in 8s), and so on.
Why would you need to count in
anything other than tens? Consider
that the four numerals on a digital
clock each count differently. The
rightmost digit rolls over when it
reaches 10. The next digit to the left
counts in sixes. The first hours digit
counts to 10, gives a carry signal,
counts to 2, and gives another carry
signal. The leftmost hours digit is
either blank or 1, when displaying
time in 12-hour format. Naturally
there are counters specifically
designed to do all this.
Counters have control pins such
as “clock disable;” which tells the
counter to ignore its input pulses
and freeze the display; “enable
display,” which enables the output
from the chip; and “reset,” which
resets the count to zero.
The 4026 requires a positive input
to activate each control pin. When
the pins are grounded, their features are suppressed.
To make the 4026 count and
display its running total you must
ground the “clock disable” and
“reset” pins (to suppress their function) and apply positive voltage to
the “enable display” pin (to activate
the output). See Figure F to see
these pins identified.