Make Electronics

Chips, Ahoy! 171

Experiment 18: Reaction Timer

Check Figure 4-33 showing the Kingbright display, and you’ll see I have an-
notated 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 ap-
propriate 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 this is a rather old-fashioned
CMOS chip (meaning, Complementary Metal Oxide Semiconductor) and is thus
sensitive to static electricity. Check the caution on page 172 before continuing.

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 pow-
er on both sides. See Figure 4-34 for details. If your breadboard does not 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 dis-
play 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 4-35 shows how the pins of the 4026 chip should be con-
nected. The arrows tell you which pins on the display should be connected with
pins on the counter.

Figure 4-36 shows the “pinouts” (i.e., the functions of each pin) of a 4026 coun-
ter chip. You should compare this with the schematic in Figure 4-35.

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 un-
til 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 misin-
terprets your button-presses, and counts two or even three digits at a time. I’ll
deal with this problem a little further on.

The LED segments will not be glowing very brightly, because the 1K series
resistors deprive of them of the power they would really like to receive. Those
resistors are necessary to avoid overloading the outputs from the counter.

1e^1

2 3 4 5 6 7 8 9

10

11

12

13

14

28

27

26

25

24

23

22

21

20

19

18

17

16

15

1d

1c

1h

2e

2d

2c

2h

3e

3d

3g

3c

3h

1g

1f

1a

1 1

2

3

1b

2f

2g

2a

2b

3f

3a

3b

g a

d

b

f

c

e

g a

d

b

f

c

e

g a

d

b

f

c

e

h

h

h

Numeral 3

Numeral 2

Numeral 1

Figure 4-33. This Kingbright unit incor-

porates three seven-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.

100uF

9v

DC

Figure 4-34. 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 100 μF

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.