Make Electronics

Chips, Ahoy! 209

Experiment 21: Race to Place

The only way to change IC1 is if the quizmaster flips his switch back to the left.
That applies negative power to the reset pins of both the timers. Consequently
their outputs go low, the LED goes out, and the circuit goes back into the same
state as where it started. Having reset it, the quizmaster can ask another ques-
tion, but the players’ buttons are not activated until the quizmaster flips the
switch back to the right again.

There’s only one situation that I haven’t addressed: what if both players press
their buttons absolutely simultaneously? In the world of digital electronics,
this is highly unlikely. Even a difference of a microsecond should be enough
time for the circuit to react and block the second button. But if somehow both
buttons are pressed at the same instant, both of the timers should react, and
both of the LEDs will light up, showing that there has been a tie.

In case you feel a little uncertain about the way in which a two-player circuit
can be upgraded to handle extra players, I’ve included a simplified three-player
schematic in Figure 4-94.

Breadboarding It

Now it’s time to create a schematic that’s as close to the breadboard layout
as possible, so that you can build this thing easily. The schematic is shown in
Figure 4-95 and the actual components on a breadboard are in Figure 4-96.
Because the only logic gates that I’ve used are OR gates, and there are only three
of them, I just need one logic chip: the 74HC32, which contains four 2-input OR
gates. (I’ve grounded the inputs to the fourth). The two OR gates on the left
side of the chip have the same functions as OR2 and OR3 in my simplified
schematic, and the OR gate at the bottom-right side of the chip works as OR1,
receiving input from pin 3 of each 555 timer. If you have all the components,
you should be able to put this together and test it quite quickly.

You may notice that I’ve made one modification of the previous schematic. A
0.01 μF capacitor has been added between pin 2 of each 555 timer (the Input)
and negative ground. Why? Because when I tested the circuit without the ca-
pacitors, sometimes I found that one or both of the 555 timers would be trig-
gered simply by flipping S1, the quizmaster switch, without anyone pressing
a button.

At first this puzzled me. How were the timers getting triggered, without any-
one doing anything? Maybe they were responding to “bounce” in the quiz-
master switch. Sure enough, the small capacitors solved the problem. They
may also slow the response of the 555 timers fractionally, but not enough to
interfere with slow human reflexes.

As for the buttons, it doesn’t matter if they “bounce,” because each timer locks
itself on at the very first impulse and ignores any hesitations that follow.

You can experiment building the circuit, disconnecting the 0.01 μF capacitors,
and flipping S1 to and fro a dozen times. If you have a high-quality switch, you
may not experience any problem. If you have a lower-quality switch, you may
see a number of “false positives.” I’m going to explain more about “bounce,”
and how to get rid of it, in the next experiment.

1 8

IC1

1 8

IC2

OR2 OR3

S1

S2 S3

R2

R1

R3

R4

1 8

IC3

S4

OR1 OR4

Figure 4-94. The two-player schematic in

can be easily upgraded to a three-player

version, as shown here, provided the first

OR gate can handle three inputs.