Make: Electronics by Charles Platt (read me a book .TXT) 📕

to showing just two players, to minimize the clutter of lines and boxes, but the concept is still easily expandable.

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.

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 direction 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.

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.

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 decide 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 current. 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.

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 components around a little to minimize wire crossovers, but you can see that logically, it’s the same basic idea.

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.

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 including 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 negative 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 trigger, 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.

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.

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 question, 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