Make Electronics

Experiment 19: Learning Logic

182 Chapter 4

10K

10K

1K

74HC00

NAND

gate

LM7805

0.33uF

0.1uF

9v DC

Figure 4-46. By observing the LED when you press either, both, or neither of the buttons,

you can easily figure out the logical function of the NAND gate.

When you connect power, the LED should light up. Press one of the tactile

switches, and the LED remains illuminated. Press the other tactile switch, and

again the LED stays on. Now press both switches, and the light should go out.

Pins 1 and 2 are logic inputs for the 74HC00 chip. Initially they were held at

negative voltage, being connected to the negative side of the power supply

through 10K resistors. But each pushbutton overrides its pull-down resistor

and forces the input pin to go positive.

The logic output from the chip, as you saw, is normally positive—but not if

the first input and the second input are positive. Because the chip does a “Not

AND” operation, it’s known as a NAND logic gate. You can see the breadboard

layout in Figure 4-47. Figure 4-48 is a simplified version of the circuit. The U-

shaped thing with a circle at the bottom is the logic symbol for a NAND gate.

No power supply is shown for it, but in fact all logic chips require a power sup-

ply, which enables them to put out more current than they take in. Anytime

you see a symbol for a logic chip, try to remember that it has to have power

to function.

FundAmentAls

Voltage regulators

The simplest versions of these

little semiconductors accept a

higher DC voltage on one pin

and deliver a lower DC voltage

on another pin, with a third pin

(usually in the middle) serving as

a common negative, or ground.

You should also attach a couple of

capacitors to smooth the current,

as shown in Figure 4-46.

Typically you can put a 7.5-volt or

9-volt supply on the “input” side of

a 5-volt regular, and draw a precise

5 volts from the “output” side. If

you’re wondering where the extra

voltage goes, the answer is, the

regulator turns the electricity into

heat. For this reason, small regula-

tors (such as the one in Figure 4-8)

often have a metal back with a

hole in the top. Its purpose is to

radiate heat, which it will do more

effectively if you bolt it to a piece

of aluminum, since aluminum

conducts heat very effectively. The

aluminum is known as a heat sink,

and you can buy fancy ones that

have multiple cooling fins.

For our purposes, we won’t be

drawing enough current to require

a heat sink.

Figure 4-47. This breadboard layout is
exactly equivalent to the schematic in
Figure 4-46.