Make: Electronics by Charles Platt (classic books to read TXT) 📗

it to cycle fast enough so that when the user presses and releases a button, the count will stop at an unforeseeable number.

The button starts and stops the 555 timer by applying and releasing power to the timing circuit only. This is the equivalent of shaking and then throwing the die.

While the counter is running fast, the LEDs are flashing so fast that all of them will seem to be on at once. At the same time, the circuit charges a new 68 µF capacitor, which I have added between the pushbutton and ground. When you release the button, this capacitor discharges itself through the 1K timing resistor. As the charge dissipates, the timing capacitor will take longer and longer to charge, and discharge, and the frequency of the 555 will gradually diminish. Consequently the LED display will also flash slower, like the reel on a Las Vegas slot machine gradually coming to a stop. This increases the tension as players can see the die display counting to the number that they’re hoping for—and maybe going one step beyond it.

Note that to maximize this effect, the button has to be held down for a full second or more, so that the 68 µF capacitor becomes fully charged before the button is released.

Figure 4-110. The electronic dice schematic applied to a breadboard, with a pushbutton at the top to start and stop the counter, and 7 LEDs at the bottom to display the output.

So, this circuit now fulfills the original goal. But can it be better? Of course it can.

Enhancements

The main thing I want to improve is the brightness of the LEDs. I could add a transistor to amplify the current to each one, but there’s a simpler alternative: a TTL “open collector” inverter.

I want to use an inverter because in the world of TTL, as I mentioned earlier, we can sink much more power into the output pin of a chip than we can source from it. So, I’m going to turn each LED the other way around and connect their load resistors to the positive side of the power supply. This way, they’ll sink their power into the outputs of the inverter.

And the great advantage of an “open collector” version of the inverter chip is that it is designed to sink much more current than a normal TTL logic chip. It is rated for 40mA per pin. The only disadvantage is that it cannot source any current at all; instead of its output going high, it just behaves like an open switch. But that’s OK for this circuit.

So the next and final schematic, in Figure 4-111, includes the 74LS06 inverter, which has also been added to the breadboarded version shown in Figure 4-112. I suggest that you put aside the little low-current LEDs and substitute some normal-size ones. Using Kingbright “standard” WP15031D 5mm LEDs, I find that each draws almost exactly 20mA with a voltage drop of about 2V with a 120 ohm series resistor. Because each output pin from the 74LS06 inverter powers no more than two LEDs at a time, this is exactly within its specification. I suggest that if you build this circuit, you check the consumption of your particular choice of LEDs and adjust the resistors if necessary.

Remember: to measure the voltage drop across an LED, simply touch the probes of your meter across it while it is illuminated. To measure the current, disconnect one side of the LED and insert the meter, in milliamp mode, between the leg of the LED and the contact that it normally makes in the circuit.

For a really dramatic display, you can get some 1 cm diameter LEDs (Figure 4-113). Check the specification, and you should find that many of these don’t use more power than the usual 5 mm type. But whatever kind you use, don’t forget to turn them around so that their negative sides face toward the inverter, and their positive sides face the resistors, which are connected to the positive side of the power supply.

One last detail: I had to add two 10K resistors to this version of the circuit. Can you see why? Diodes D1 through D4 are designed to transmit positive voltage through to the inverter when appropriate, but they prevent the inputs of the inverter from “seeing” the negative side of the power supply when the counter outputs are low. These inverter inputs require pull-down resistors to prevent them from “floating” and producing erroneous results.

Figure 4-111. If open-collector inverters are added to the dice schematic, it can drive full-size LEDs with up to 40mA, as long as the LEDs are turned around to sink current into the TTL output stage instead of trying to source current from it.

Figure 4-112. The completed circuit using an open-collector inverter to drive full-size LEDs.

The final enhancements are up to you. Most obviously, you can add a second die, as many games require two dice. The 74LS27 chip still has a couple of spare NOR gates in it, one of which you can make use of, but you will need an additional 555 timer, running at a significantly different speed to ensure randomness, and it will have to drive a second counter.

After you get your dice up and running, you may want to test them for randomness. Because the pulses from a 555 timer are of equal length, every number has an equal chance of coming up; but the longer you hold down the Start button, the better your odds are of interrupting the counting process at a truly random moment. Anyone using your electronic dice should be told that “shaking” them for a full second is mandatory.

Of course, I could have simulated dice more easily by writing a few lines of software to generate random numbers on a screen, but even a fancy screen image cannot have the same appeal as a well-made piece of hardware. Figure 4-113 shows white 1 cm LEDs mounted in a sanded polycarbonate enclosure