Make Electronics

Chips, Ahoy! 207

Experiment 21: Race to Place

Button

Blocker

Latch

Button

Blocker

Latch

Button

Blocker

Latch

Figure 4-89. If a latch is added below each button, it can
retain one input and then block all inputs from all buttons.
This simplifies the concept.

Button

Blocker

Latch

Reset

Play Button

Blocker

Latch

Figure 4-90. A quizmaster switch will be needed to

activate the buttons initially and then reset the circuit

after a winning input has been recorded.

Now I have to deal with a logic problem in the diagram. The way I’ve drawn
it, after the output from the lefthand latch goes up to the “button blockers,” it
can also run down the wire to the other half of the circuit (against the direc-
tion to the arrows), because everything is joined together. In other words, if
the lefthand LED lights up, the righthand LED will light up, too. How can I stop
this from happening?

Well, I could put diodes in the “up” wires to block current from running down
them. But I have a more elegant idea: I’ll add an OR gate, because the inputs to
an OR gate are separated from each other electrically. Figure 4-91 shows this.

Usually an OR gate has only two logical inputs. Will this prevent me from adding
more players? No, because you can actually buy an OR that has eight inputs. If
any one of them is high, the output is high. For fewer than eight players, I can
short the unused inputs to ground, and ignore them.

Looking again at Figure 4-91, I’m getting a clearer idea of what the thing I’ve
called a “button blocker” should actually be. I think it should be another logic
gate. It should say, “If there’s only one input, from a button, I’ll let it through.
But if there is a second input from the OR gate, I won’t let it through.”

That sounds like a NAND gate, but before I start choosing chips, I have to de-
cide what the latch will be. I can buy an off-the-shelf flip-flop, which flips “on” if
it gets one signal and “off” if it gets another, but the trouble is, chips containing
flip-flops tend to have more features than I need for a simple circuit like this.
Therefore I’m going to use 555 timers again, in flip-flop mode. They require
very few connections, work very simply, and can deliver a good amount of cur-
rent. The only problem with them is that they require a negative input at the
trigger pin to create a positive output. But I think I can work with that.

Button

Blocker

Latch

Reset

Play Button

Blocker

Latch

OR

Figure 4-91. To prevent the output from one

latch feeding back around the circuit to the

output from another latch, the outputs can

be combined in an OR gate.