Make Electronics

Experiment 21: Race to Place

208 Chapter 4

So now, finally, here’s a simplified schematic, in Figure 4-92. I like to show the

pins of the 555 timers in their correct positions, so I had to move the compo-

nents around a little to minimize wire crossovers, but you can see that logi-

cally, it’s the same basic idea.

Before you try to build it, just run through the theory of it, because that’s the

final step, to make sure there are no mistakes. The important thing to bear in

mind is that because the 555 needs a negative input on its trigger pin to create

its output, when any of the players presses a button, the button has to create a

negative “flow” through the circuit. This is a bit counterintuitive, so I’m includ-

ing a three-step visualization in Figure 4-93, showing how it will work.

In Step 1, the quizmaster has asked a question and flipped his switch to the

right, to supply (negative) power to the players’ buttons. So long as no one

presses a button, the pull-up resistors supply positive voltage to OR2 and OR3.

An OR gate has a positive output if it has any positive input, so OR2 and OR3

keep the trigger inputs of the 555 timers positive. Their outputs remain low,

and nothing is happening yet.

In Step 2, the lefthand player has pressed his button. Now OR2 has two nega-

tive inputs, so its output has gone low. But IC1 hasn’t reacted yet.

In Step 3, just a microsecond later, IC1 has sensed the low voltage on its trig-

ger, so its output from pin 3 has gone high, lighting the LED. Remember, this

555 timer is in flip-flop mode, so it locks itself into this state immediately.

Meanwhile its high output also feeds back to OR1. Because OR1 is an OR gate,

just one high input is enough to make a high output, so it feeds this back to

OR2 and OR3. And now that they have high inputs, their outputs also go high,

and will stay high, regardless of any future button-presses.

1 8

IC1

1 8

IC2

OR1 OR2 OR3

S1

S2 S3

R2

R4

R1

R3

1 8

IC1

1 8

IC2

OR1 OR2 OR3

S1

S2 S3

R2

R4

R1

R3

1 8

IC1

1 8

IC2

OR1 OR2 OR3

S1

S2 S3

R2

R4

R1

R3

Figure 4-93. These three schematics show the prevalence of higher and lower voltages

(red and blue lines) through the quiz circuit when a pushbutton is pressed.

Because OR2 and OR3 now have high inputs and outputs, IC1 and IC2 cannot be

triggered. But IC1 is still locked into its “on” state, keeping the LED illuminated.

1 8

555

timer

1 8

OR1 OR2 OR3

S1

S2 S3

R2

R4

R1

R3

555

timer

Figure 4-92. Now that the basic concept
of the quiz circuit has been roughed out,
specific components can be inserted, with
compatible inputs and outputs.