Make Electronics

Chips, Ahoy! 187

Experiment 19: Learning Logic

essentIAls

Logic gate basics (continued)

If you have difficulty visualizing logic gates, a mechanical comparison may
help. You can think of them as being like sliding plates with holes in them, in a
bubblegum machine. Two people, A and B, can push the plates. These people
are the two inputs, which are considered positive when they are doing some-
thing. (Negative logic systems also exist, but are uncommon, so I’m only going
to talk about positive systems here.)

The flow of bubblegum represents a flow of positive current. The full set of pos-
sibilities is shown in Figures 4-58 through 4-63.

B

AND

B

A

A A

A

B B

NONO NO

NOYESNO

YESNO NO

YESYESYES

INPUT BINPUT AOUTPUT

Figure 4-58

NONO YES

NOYESYES

YESNO YES

YESYESNO

INPUT BINPUT AOUTPUT

NAND

B B

B A B A

A A

Figure 4-59