Make Electronics

Experiment 21: Race to Place

210 Chapter 4

1 8

555

1 8

555

74HC32

5V DC regulated

10K

10K

OR2

OR3

OR1

10K

S2

S3

S1

330

0.01

0.01

330

Figure 4-95. Applying the simplified schematic to a breadboard inevitably entails
a wiring layout that is less intuitively obvious and appears more complex. The
connections are the same, though.

Figure 4-96. The quiz schematic applied to a

breadboard, to test the concept prior to full-scale

implementation.

Enhancements

After you breadboard the circuit, if you proceed to build a permanent version,

I suggest that you expand it so that at least four players can participate. This

will require an OR gate capable of receiving four inputs. The 74HC4078 is the

obvious choice, as it allows up to eight. Just connect any unused inputs to

negative ground.

Alternatively, if you already have a couple of 74HC32 chips and you don’t want

to bother ordering a 74HC4078, you can gang together three of the gates in-

side a single 74HC32 so that they function like a four-input OR. Look at the

simple logic diagram in Figure 4-97 showing three ORs, and remember that

the output from each OR will go high if at least one input is high.

And while you’re thinking about this, see if you can figure out the inputs and

output of three ANDs in the same configuration.

For a four-player game, you’ll also need two additional 555 timers, of course,

and two more LEDs, and two more pushbuttons.

As for creating a schematic for the four-player game—I’m going to leave that

to you. Begin by sketching a simplified version, just showing the logic symbols.

Then convert that to a breadboard layout. And here’s a suggestion: pencil, pa-

per, and an eraser can still be quicker, initially, than circuit-design software or

graphic-design software, in my opinion.

Figure 4-97. Although a four-input OR gate
is not manufactured, its functionality can
be achieved easily by linking three 2-input
OR gates together.