74LS08 (7408) Logic Gate - Specifications, Pinout, and Application

The 74LS08 (also known as 7408) is a TTL integrated circuit that contains four independent two-input AND logic gates. It is typically powered with +5V (4.75–5.25V), and each gate performs the Boolean operation Y = A·B: the output is "1" only if both inputs are "1". In any other case, the output is "0".

This component from the 74LS (Low-power Schottky) series is known for its ease of use in breadboards and digital projects involving Arduino, microcontrollers, and telecommunication systems where combined AND logic functions are needed.

What is the 74LS08?

The 74LS08 is a quad low-power AND gate (LS series). Its main features include:

Four independent two-input AND gates.
Low power consumption.
High switching speed (~35 MHz).
Ability to sink up to ~16 mA on a low output.
Compatible with standard TTL levels (5V).

This allows it, for example, to drive LEDs or simple loads directly from the output, making it ideal for basic digital electronics projects.

Basic Operation and Truth Table

Each AND gate inside the 74LS08 operates with a simple logic: the output is "1" only when both inputs are "1". This Boolean operation is expressed as Y = A·B.

Truth table for each AND gate:

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

Internal operation: Each gate is based on low-power Schottky TTL technology, using bipolar transistors and Schottky devices to switch quickly with a typical delay of ~10 ns.

Pinout and Connections

The 74LS08 comes in a 14-pin DIP package with the following configuration:

Pin 7: GND (ground)
Pin 14: VCC (+5V)
Gates organized as follows:
- Gate 1: inputs A (pin 1), B (pin 2), output Y (pin 3)
- Gate 2: inputs A (pin 4), B (pin 5), output Y (pin 6)
- Gate 3: inputs A (pin 9), B (pin 10), output Y (pin 8)
- Gate 4: inputs A (pin 12), B (pin 13), output Y (pin 11)

Note: The full pin diagram is shown in the datasheet from the previous section.

Detailed Technical Specifications

Specification	Parameter (74LS08)
Logic Function	AND
Number of AND 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 16 mA (low level)
Output Current (I_OH)	–0.8 mA (high level)
Max. Speed (f_max)	~35 MHz
Typical Delay (t_pd)	~10 ns (propagation)
Operating T_range	0 °C to 70 °C

Key Features:

High output current: Can sink up to 16 mA in a low state, ideal for LEDs and light loads.
TTL-LS Technology: Offers high noise immunity and compatibility with 5V systems.
Four independent gates: Allows for multiple simultaneous AND operations.
Weak high-state outputs: Only -0.8 mA, so it is recommended to connect loads to VCC.

Component Variants

In addition to the 74LS08 (TTL LS), there are variants compatible with different technologies:

SN7408/SN74LS08: Original TTL versions for 5V.
SN74HC08: High-performance CMOS, operates from 2V to 6V with lower consumption (5.2 mA per output).
SN74HCT08: CMOS with TTL-compatible inputs, operates at 4.5–5.5V with low consumption (4 mA).
Other variants: 74ALS08 (Advanced High-Speed), 74AC08 (Advanced CMOS), 74F08 (Very High-Speed).

Practical Applications

Use cases with Arduino and microcontrollers

The 74LS08 is very useful in digital prototypes where hardware logic combinations are required:

Arduino UNO (5V): Each AND input can be connected to digital I/O signals, using the output to turn on LEDs or trigger other circuits.
3.3V Microcontrollers (ESP32, etc.): It is more appropriate to use the 74HC08/HCT08 version for logic level compatibility.
Digital automation: Activating alarms only when two conditions are met simultaneously.
Motor control: Enabling movement only when safety signals are present.

Basic circuit example

Example circuit: Two push-buttons (S1, S2) with pull-down resistors (R1, R2) control the A and B inputs of the gate. The Y output drives an LED with a resistor (R3). The LED only turns on when both buttons are pressed.

Circuit connections:

VCC = +5V to pin 14
GND to pin 7
10kΩ resistors keep inputs at "0" when buttons are open.
Pressing both buttons simultaneously causes the 74LS08 to produce a "1" at the output.
The LED lights up with a 1kΩ resistor.

Important Considerations for Use

Voltage and Power

Correct voltage: The 74LS08 must operate near 5V (4.75–5.25V). Do not apply more voltage or use it directly with 3.3V (use the HCT variant for 3.3V).
Power connections: Always connect VCC to pin 14 and GND to pin 7.

Current Handling

Low outputs: Can sink up to 16 mA, recommended for LEDs connected to VCC.
High outputs: Limited to -0.8 mA, should not drive heavy loads in high mode.
Fan-out: Can drive up to 10 TTL loads simultaneously.

TTL Inputs

Do not leave floating: Always use pull-up or pull-down resistors on unconnected inputs.
Logic levels: "HIGH" is recognized above 2V (TTL).
3.3V compatibility: Directly connecting a 3.3V microcontroller pin is marginal; it's better to use a 74HCT08.

Comparisons with Other TTL Gates

74LS04: 6 inverters (NOT) in a DIP-14 package, does not require two inputs.
74LS32: 4 OR gates with 2 inputs, output is "1" if any input is "1".
74LS08 vs 74S08: The 74S08 is faster (~5 ns) but consumes more power.
CMOS families (74HC, 74HCT): Operate over a wider voltage range (2–6V) and are more efficient.

Practical Resources

Datasheet SN74LS08 (Texas Instruments): [Quadruple 2-Input Positive-AND Gates] - Includes complete electrical specifications.
Datasheet SN74HC08 (Texas Instruments): [Quadruple 2-Input AND Gates] - For the 74HC CMOS variant.
Datasheet SN74HCT08 (Texas Instruments): [AND Gates with TTL-compatible inputs] - For use with 5V/3.3V systems.
Simulations: Tinkercad or Falstad for testing AND gates with the 7408.
Software: Proteus, Multisim include the 7408 family.

Frequently Asked Questions (FAQ)

What gate is the 7408? The 7408 (also 74LS08) is an integrated circuit with four two-input AND gates. It contains four independent AND logic gates.

What is the truth table for the 74LS08? Each 7408 AND gate follows the table: A=0,B=0 → Y=0; 0,1→0; 1,0→0; 1,1→1. It only outputs "1" when A=1 and B=1.

What voltage does the 74LS08 operate at? It operates at a standard TTL voltage, typically 5V (range 4.75–5.25V). It does not work directly at 3.3V. For 3.3V, it's better to use a 74HCT08.

Can I use a 74HC08 instead of a 74LS08? Yes. The 74HC08 is CMOS and works between 2V and 6V. In a 5V system, it will function as a functional equivalent, although it provides less output current (5.2 mA) compared to the LS version (16 mA).

How do I connect the 74LS08 on a breadboard with Arduino? Connect VCC=+5V (pin 14) and GND=0V (pin 7). Use the outputs to turn on LEDs or trigger Arduino pins, and connect inputs to buttons or digital signals from the Arduino. Add pull-down/up resistors if you leave inputs unconnected.

What do SN74LS08, 74HC08, etc., mean? The prefix indicates the logic family: 74LS08 is TTL Low-Power Schottky; 74HC08 is High-speed CMOS; 74HCT08 is CMOS with TTL-compatible inputs. All perform the same AND function but differ in voltage, speed, and consumption.

References: Information is based on official datasheets from Texas Instruments and technical sources to ensure accuracy.