Page 26 - DCAP108_DIGITAL_CIRCUITS_AND_LOGIC_DESIGNS

Unit 2: Logic Gates

A letter, also known as variable, represents a binary number. So “A” can be “0” or “1”. So, if A Notes
is connected to a switch, A will be “0” when the switch is turned off and “1” when the switch
is turned on. A line drawn right above the variable name means that the variable is inverted.
For example, if A = 0, /A will be “1”, and if A = 1, /A will be “0”. On text processors we need
to substitute the line drawn above the letter with a slash because it is hard to draw a line above
the letter. So here we will be using this kind of notation due to a limitation on our text processor.

2.1 Types of Logic Gates

There are many types of logic gates, are illustrated with drawings of their relay-and-switch
implementations:
2.1.1 The NOT (Inverter) Gate

As the name implies, inverter will invert the number entered. If you enter ”0“, you will get a
”1“on its output, and if you enter a ”1“, you will get a ”0“ on its output. The inverter symbol you
can see in Figure 2.1. Inverter gate is also known as NOT and its output is Y = /A.
Figure 2.1: Inverter

On the truth table below you can see a summary of how this circuit works.
Table 2.1: Truth Table

A (Input) Y (Output)
0 1
1 0

On logic circuits, an “o” symbol is a short for inverter. You will see that on logic gates like NAND,
NOR and XNOR.
The most famous inverter integrated circuit is 7404 and you see its pinout in Figure 2.2. It has six
inverters inside. To make this integrated circuit work, you need to connect it to a 5 V power supply.
Figure 2.2: 7404 Integrated Circuit Provides Six Inverters

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