74LS02 (7402) Logic Gate - NOR, Pinout, and Applications

The 74LS02 (also known as 7402) is a TTL integrated circuit that contains four independent two-input NOR (Negated-OR) logic gates. It is powered with +5V (4.75–5.25V), and each gate performs the Boolean operation Y = (A+B)'. The output is "1" (HIGH) only when both inputs are "0" (LOW). If at least one input is "1", the output will be "0".

Like the NAND gate, the NOR gate is a "universal gate," which means it can be used to build any other logic function (AND, OR, NOT, etc.), making it extremely versatile in digital circuit design.

What is the 74LS02?

The 74LS02 is a quad low-power NOR gate (LS series). Its main features are:

Four independent two-input NOR gates.
Low power consumption and high speed.
Compatible with standard TTL logic levels (5V).
It is a universal gate, a foundation for building other logic circuits.

Basic Operation and Truth Table

Each NOR gate in the 74LS02 functions as an OR gate followed by an inverter (NOT). The output is the negation of the result of an OR operation.

Truth table for each NOR gate:

Input A	Input B	Output Y
0	0	1
0	1	0
1	0	0
1	1	0

Pinout and Connections

Important: The pinout of the 74LS02 is different from other quad gates like the 7400 or 7408. The arrangement of inputs and outputs is unique.

Pin 7: GND (Ground)
Pin 14: VCC (+5V)
Gates organized as follows:
- Gate 1: Output (Pin 1), Inputs (Pin 2, 3)
- Gate 2: Output (Pin 4), Inputs (Pin 5, 6)
- Gate 3: Output (Pin 10), Inputs (Pin 8, 9)
- Gate 4: Output (Pin 13), Inputs (Pin 11, 12)

Detailed Technical Specifications

Specification	Parameter (74LS02)
Logic Function	NOR (Negated-OR)
Number of gates	4 (Quad)
Inputs per gate	2
Supply Voltage	4.75 – 5.25 V
Technology	TTL 74LS family (LS-TTL)
Output Current (I_OL)	up to 8 mA (low level)
Output Current (I_OH)	–0.4 mA (high level)
Typical Delay (t_pd)	~10 ns (propagation)
Operating T_range	0 °C to 70 °C

Component Variants

SN7402/SN74LS02: TTL versions.
SN74HC02: High-speed CMOS version, operates from 2V to 6V.
SN74HCT02: CMOS version with TTL-compatible inputs.

Practical Applications and Universality

The NOR gate is universal, allowing you to create any other logic function:

Inverter (NOT): By connecting the two inputs of a NOR gate together.
OR Gate: Using a NOR gate followed by an inverter (another NOR with its inputs tied together).
AND Gate: By inverting both inputs with two NOR gates and connecting their outputs to a third NOR gate.

Use cases:

Control logic: Implementing any Boolean function using only one type of IC.
Memory circuits: Base for building SR flip-flops.
Safety systems: Activating an output only if all safety conditions are "0" (e.g., neither sensor A nor sensor B is active).

Practical Resources

Datasheet SN74LS02 (Texas Instruments): [Quad 2-Input NOR Gates]
Datasheet SN74HC02 (Texas Instruments): [Quad 2-Input NOR Gates - CMOS]
Simulations: Tinkercad, Falstad, and other simulators are perfect for exploring the universality of the NOR gate.

Frequently Asked Questions (FAQ)

What gate is the 7402? The 7402 (or 74LS02) is an integrated circuit containing four two-input NOR logic gates.

What is the truth table for the 7402? The output is 1 only if both inputs are 0. If any of the inputs is 1, the output is 0.

What is the pinout of the 7402? The pinout is unique: VCC on pin 14, GND on pin 7. The outputs are on pins 1, 4, 10, and 13, with their respective inputs next to them.

What does the 7402 integrated circuit do? It performs the NOR (negated OR) logic operation. It is a universal gate, allowing any other logic function to be built from it.

What voltage does the 74LS02 operate at? It operates at a standard TTL voltage of 5V (range 4.75–5.25V).

What do SN74LS02, 74HC02, etc., mean? They are different logic families: 74LS02 (TTL Low-Power Schottky), 74HC02 (High-speed CMOS), and 74HCT02 (CMOS with TTL-compatible inputs). All perform the NOR function but with different electrical characteristics.

References: Information is based on official datasheets from Texas Instruments and digital electronics technical guides.