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Chips, Ahoy! 205

Experiment 21: Race to Place

Experiment 21: Race to Place

The next project is going to get us deeper into the concept of feedback, where
the output is piped back to affect the input—in this case, blocking it. It’s a small
project, but quite subtle, and the concepts will be useful to you in the future.

You will need:

• 74HC32 chip containing four OR gates. Quantity: 1.


• 555 timers. Quantity: 2.


• SPDT switch. Quantity: 1.


• SPST tactile switches. Quantity: 2.


• Various resistors.


• 5V supply using power regulator as before.

The Goal

On quiz shows such as Jeopardy, contestants race to answer each question.
The first person who hits his answer button automatically locks out the other
contestants, so that their buttons become inactive. How can we make a circuit
that will do the same thing?

If you search online, you’ll find several hobby sites where other people have
suggested circuits to work this way, but they lack some features that I think are
necessary. The approach I’m going to use here is both simpler and more elabo-
rate. It’s simpler because it has a very low chip count, but it’s more elaborate in
that it incorporates “quizmaster control” to make a more realistic game.

I’ll suggest some initial ideas for a two-player version. After I develop that idea,
I’ll show how it could be expanded to four or even more players.

A Conceptual Experiment

I want to show how this kind of project grows from an idea to the finished
version. By going through the steps of developing a circuit, I’m hoping I may
inspire you to develop ideas of your own in the future, which is much more
valuable than just replicating someone else’s work. So join me in a conceptual
experiment, thinking our way from a problem to a solution.

First consider the basic concept: two people have two buttons, and whoever
goes first locks out the other person. I always find it helps me to visualize this
kind of thing if I draw a sketch, so that’s where I’ll begin. In Figure 4-87, the
signal from each button passes through a component that I’ll call a “button
blocker,” activated by the other person’s button. I’m not exactly sure what the
button blocker will be or how it will work, yet.

Now that I’m looking at it, I see a problem here. If I want to expand this to three
players, it will get complicated, because each player must activate the “button
blockers” of two opponents. Figure 4-88 shows this. And if I have four players,
it’s going to get even more complicated.

Anytime I see this kind of complexity, I think there has to be a better way.

Button

Blocker

Button

Blocker

Figure 4-87. The basic concept of the quiz

project is that the output from one button

should feed back to intercept the output

of another button. At this point, the way in

which the “button blocker” circuit works

has not been figured out.