Chips, Ahoy! 181Experiment 19: Learning LogicSumming Up
This project demonstrated how a counter chip can be controlled, how counter
chips can be chained together, and three different functions for 555 timers. It
also showed you how chips can communicate with each other, and introduced
you to the business of calibrating a circuit after you’ve finished building it.
Naturally, if you want to get some practical use from the circuit, you should
build it into an enclosure with heavier-duty pushbuttons—especially the but-
ton that stops the count. You’ll find that when people’s reflexes are being test-
ed, they are liable to hit the stop button quite hard.
Because this was a major project, I’ll follow it up here with some quicker, easier
ones as we move into the fascinating world of another kind of integrated circuit:
logic chips.
Experiment 19: Learning Logic
You will need:
- Assorted resistors and capacitors.
- 74HC00 quad 2-input NAND chip, 74HC08 quad 2-input AND chip, and
LM7805 voltage regulator. Quantity: 1 of each. - Signal diode, 1N4148 or similar. Quantity: 1.
- Low-current LED. Quantity: 1.
- SPST tactile switches. Quantity: 2.
You’re going to be entering the realm of pure digital electronics, using “logic
gates” that are fundamental in every electronic computing device. When you
deal with them individually, they’re extremely easy to understand. When you
start chaining them together, they can seem intimidatingly complex. So let’s
start with them one at a time.
Logic gates are much fussier than the 555 timer or the 4026 counter that you
used previously. They demand an absolutely precise 5 volts DC, with no fluc-
tuations or “spikes” in the flow of current. Fortunately, this is easy to achieve:
just set up your breadboard with an LM7805 voltage regulator, as shown in
the schematic in Figure 4-44 and the photograph in Figure 4-45. The regula-
tor receives 9 volts from your usual voltage supply, and reduces it to 5 volts,
with the help of a couple of capacitors. You apply the 9 volts to the regulator,
and distribute the 5 volts down the sides of your breadboard instead of the
unregulated voltage that you used previously. Use your meter to verify the
voltage, and make sure you have the polarity clearly marked.
After installing your regulator, take a couple of tactile switches, two 10K resis-
tors, a low-current LED, and a 1K resistor, and set them around a 74HC00 logic
chip as shown in Figure 4-46. You may notice that many of the pins of the chip
are shorted together and connected to the negative side of the power supply.
I’ll explain that in a moment.
LM78050.33uF9v DC5v DC0.1uFFigure 4-44. This simple circuit is essential
to provide a regulated 5V DC supply for
logic chips.Figure 4-45. The voltage regulator and its
two capacitors can fit snugly at the top of
a breadboard. Remember to apply the 9V
input voltage at the left pin of the regula-
tor, and distribute the 5V output down the
sides of the breadboard.