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Practice 1 “Logic Gates”

Abstract— In this first practice of digital electronics we focus in Know the logic gates using the Nand with model 7400 and the Or gate with model serie 7402, also we learn how they work and the functions that they make, knowing also the connections basing on the datasheet of each logic gate.

I. OBJECTIVE

The student must apply the concepts learned in class in or-der to build the circuit correctly and prove the correct behavior of the logic circuit.

II. INTRODUCTION

Learn the best way to do a circuit with logic gates develop-ing the circuits looking the diagrams also using the datasheet in order to check which are the inputs and the outputs and finally apply the correct voltage to prove the logic of the cir-cuit in order to learn how it works.

III. DEVELOPMENT

The first step in the development of this practice is to do the diagram of each circuit with the table of truth in order to build physically the circuit, finally we had to check the correct operation of the logic gates with whit the switch looking when the led it is turned on or turned off in agreement with the table of truth.

IV. MATERIALS

The materials to realize practice were as follows:

• Power source of D.C voltage.

• Resistors of ½ watt.

• Proto board.

• Nippers

• Cable.

• Logic gates.

• Led.

• Switch.

V. THEORETICAL FRAMEWORK

A. Logic gates

1) Concept:

Logic gates are electronic circuits that can be used to im-plement the most elementary logic expressions, also known as Boolean expressions. The logic gate is the most basic building block of combinational logic. There are three basic logic gates, namely the OR gate, the AND gate and the NOT gate. Other logic gates that are derived from these basic gates are the NAND gate, the NOR gate, the EXCLUSIVEOR gate and the EXCLUSIVE-NOR gate.

Fig. 1. Physical example of a logic gate

B. Types of logic gates

1) Or gate:

An OR gate performs an ORing operation on two or more than two logic variables. The OR operation on two independ-ent logic variables A and B is written as Y = A+B and reads as Y equals A OR B and not as A plus B. An OR gate is a logic circuit with two or more inputs and one output. The output of an OR gate is LOW only when all of its inputs are LOW. For all other possible input combinations, the output is HIGH. This statement when interpreted for a positive logic system means the following. The output of an OR gate is a logic ‘0’ only when all of its inputs are at logic ‘0’. For all other possible input combinations, the output is a logic

Fig. 2. Shows the circuit symbol and the truth table of a two-input O gate.

Fig. 3 Datasheet 7432

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2) AND Gate :

An AND gate is a logic circuit having two or more inputs and one output. The output of an AND gate is HIGH only when all of its inputs are in the HIGH state. In all other cases, the output is LOW. When interpreted for a positive logic sys-tem, this means that the output of the AND gate is a logic ‘1’ only when all of its inputs are in logic ‘1’ state. In all other cases, the output is logic ‘0’.

Fig. 4. Shows the circuit symbol and the truth table of a AND.

Fig. 5 Datasheet 7408

3) NOT gate:

A NOT gate is a one-input, one-output logic circuit whose output is always the complement of the input. That is, a LOW input produces a HIGH output, and vice versa. When inter-preted for a positive logic system, a logic ‘0’ at the input pro-duces a logic ‘1’ at the output, and vice versa. It is also known as a ‘complementing circuit’ or an ‘inverting circuit’.

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Fig. 6. Shows the circuit symbol and the truth table of a NOT.

Fig. 7 Datasheet 7408

4) NAND gate:

The output is high when either of inputs A or B is high, or if neither is high. In other words, it is normally high, going low only if both A and B are high. The NAND gate and the NOR gate can be said to be universal gates since combinations of them can be used to accomplish any of the basic operations and can thus produce an inverter, an OR gate or an AND gate. The non-inverting gates do not have this versatility since they can't produce an invert.

Fig. 8. Shows the circuit symbol and the truth table of a NAND.

Fig. 9 Datasheet 7408

5) NOR gate:

The output is high only when neither A nor B is high. That is, it is normally high but any kind of non-zero input will take it low. The NOR gate and the NAND gate can be said to be universal gates since combinations of them can be used to accomplish any of the basic operations and can thus produce an inverter, an OR gate or an AND gate. The non-inverting gates do not have this versatility since they can't produce an invert.

Fig. 10. Shows the circuit symbol and the truth table of a NOR.

Fig. 11 Datasheet 7408

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