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

buttons are pressed at the same instant, both of the timers should react, and both of the LEDs will light up, showing that there has been a tie.

In case you feel a little uncertain about the way in which a two-player circuit can be upgraded to handle extra players, I’ve included a simplified three-player schematic in Figure 4-94.

Figure 4-94. The two-player schematic in can be easily upgraded to a three-player version, as shown here, provided the first OR gate can handle three inputs.

Breadboarding It

Now it’s time to create a schematic that’s as close to the breadboard layout as possible, so that you can build this thing easily. The schematic is shown in Figure 4-95 and the actual components on a breadboard are in Figure 4-96. Because the only logic gates that I’ve used are OR gates, and there are only three of them, I just need one logic chip: the 74HC32, which contains four 2-input OR gates. (I’ve grounded the inputs to the fourth). The two OR gates on the left side of the chip have the same functions as OR2 and OR3 in my simplified schematic, and the OR gate at the bottom-right side of the chip works as OR1, receiving input from pin 3 of each 555 timer. If you have all the components, you should be able to put this together and test it quite quickly.

You may notice that I’ve made one modification of the previous schematic. A 0.01 µF capacitor has been added between pin 2 of each 555 timer (the Input) and negative ground. Why? Because when I tested the circuit without the capacitors, sometimes I found that one or both of the 555 timers would be triggered simply by flipping S1, the quizmaster switch, without anyone pressing a button.

At first this puzzled me. How were the timers getting triggered, without anyone doing anything? Maybe they were responding to “bounce” in the quizmaster switch. Sure enough, the small capacitors solved the problem. They may also slow the response of the 555 timers fractionally, but not enough to interfere with slow human reflexes.

As for the buttons, it doesn’t matter if they “bounce,” because each timer locks itself on at the very first impulse and ignores any hesitations that follow.

You can experiment building the circuit, disconnecting the 0.01 µF capacitors, and flipping S1 to and fro a dozen times. If you have a high-quality switch, you may not experience any problem. If you have a lower-quality switch, you may see a number of “false positives.” I’m going to explain more about “bounce,” and how to get rid of it, in the next experiment.

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 inside 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.

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.

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, paper, and an eraser can still be quicker, initially, than circuit-design software or graphic-design software, in my opinion.

Experiment 22: Flipping and Bouncing

I mentioned in the previous experiment that “bounce” from the buttons in the circuit wouldn’t be a problem, because the buttons were activating 555 timers that were wired in bistable, flip-flop mode. As soon as the timer receives the very first pulse, it flips into its new state and flops there, ignoring any additional noise in the circuit. So can we debounce a switch or a button using a flip-flop? And as some 74HCxx chips are available containing flip-flops, can we use them?

The answers are yes, and yes, although it’s not quite as simple as it sounds.

You will need:

74HC02 logic chip containing 4 NOR gates. 74HC00 logic chip containing 4 NAND gates. Quantity: 1 of each.

SPDT switch. Quantity: 1.

LEDs, low-current. Quantity: 2.

10K resistors and 1K resistors. Quantity: 2 of each.

Assemble the components on your breadboard, following the schematic shown in Figure 4-98. When you apply power (through your regulated 5-volt supply), one of the LEDs should be lit.

Figure 4-98. A simple circuit to test the behavior of two NOR gates wired as a simple flip-flop that retains its state after an input pulse ceases.

Now I want you to do something odd. Please disconnect the SPDT switch by taking hold of the wire that connects the positive power supply to the pole of the switch, and pulling the end of the wire out of the breadboard. When you do this, you may be surprised to find that the LED remains lit.

Push the wire back into the breadboard, flip the switch, and the first LED should go out, while the other LED should become lit.