Fig. G: After installation in this arcade-style
mini-cabinet, Zap-a-Mole allows you to whack
an illuminated button into submission, using
a suitably downsized mallet.
low voltage until the button overwhelms the resistor
with positive voltage. The only pins you can leave
unconnected are labeled “NC” (meaning “no connection”) in the schematic.
When you power up the game, if the score counter
doesn’t show 00, press the Score Reset button.
Then press the Game Start button. The Game Over
LED goes out (if you wired it the right way), and you
can start playing.
All the LEDs come on simultaneously as the Flash
Counter cycles them, and then one stays lit until
you hit the button identified with it. After about a
minute, the Game Over light comes on and your
score freezes. Another game? Press the Score Reset
and the Game Start button again. Happy zapping!
Lessons to Learn
Figure E (previous page) shows the pulses emitted by chips and switches in the schematic. This
provides a clearer idea of what’s actually happening.
If you design a circuit of your own, drawing this kind
of diagram will help you get the chips to talk to each
other. Figure F shows pinouts for the chips; compare
their functions with the way they’re wired in Figure B.
The Flash Counter is a decade counter with decoded outputs, meaning that the counter responds
to each positive input pulse by moving a positive
voltage from one output pin to the next. After it
counts from 0 to 9, the counter goes back to 0. In
Zap-a-Mole, I have only 5 LEDs, so I connected a
jumper wire from pin 1 (the “ 5 output”) to pin 15
(the “reset”). This forces the chip to go back to 0
144 Make: Volume
23
after counting from just 0 to 4.
We’ve talked about the 555 timer in MAKE before
(see Volume 10, “The Biggest Little Chip”), but this
game emphasizes the use of its reset pin. Remember: the timer’s output goes from low to high when
the input pin goes from high to low (assuming the
reset pin is held constantly high). But the output also
goes from low to high if the reset pin goes from low
to high (assuming the input pin is held constantly
low). Check Online Resources for more information.
Going Further
Instead of a battery, try using a 12-volt AC adapter
(which has a DC output), and passing it through a
9-volt voltage regulator such as the LM7809.
A problem with this game is that it waits for you
to press a button, no matter how long you take. In
the real Whac-A-Mole, if you don’t whack the mole
promptly, it drops back down and you miss your
chance. Can you figure out how to add this feature
using logic chips? A microcontroller would handle
it much more easily. I’ll show you how in the More
Modern Version, in the next issue.
I have mixed feelings about microcontrollers.
They’re powerful and versatile, but you lose the aesthetics of pure hardware. When you open the back of
your Zap-a-Mole cabinet and look at those vintage
CMOS chips trading voltage pulses, I think they
inspire a fascination that software can never equal.
Online Resources
General primer on logic gates and counters: »
makezine.com/go/logicgates
555 timer overview, circuits, and simple math: »
doctronics.co.uk/555.htm
Fairchild LM7809 voltage regulator datasheet: »
fairchildsemi.com/ds/LM/LM7809.pdf
(See datasheet page 22 for schematic.)
4017 decade counter datasheet: »
focus.ti.com/lit/ds/symlink/cd4017b.pdf
4017 tutorial with many examples: »
doctronics.co.uk/4017.htm
4026 counter datasheet: »
focus.ti.com/lit/ds/symlink/cd4026b.pdf
8-input OR/NOR gate datasheet: »
focus.ti.com/lit/ds/symlink/cd4078b.pdf
Charles Platt is the author of Make: Electronics, an introductory guide for all ages. A contributing editor to MAKE, he
designs and builds medical equipment prototypes in Arizona.
