Not Gates Digital Logic

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Building a NOT Gate (74LS04)

One of the simplest yet most important digital logic gates is the NOT gate (also called an inverter). It flips whatever signal you feed into it: if the input is HIGH, the output goes LOW, and vice versa.

In this post, we’ll build one using the 74LS04 TTL IC and see it in action with use of an LED.

What You’ll Need

  • 1 × 74LS04 IC

  • Breadboard + jumper wires

  • 1 × 330 Ω resistor

  • 1 × 10 kΩ resistor

  • 2 × LEDs (red/green work great)

  • 5 V power supply (or your Arduino’s 5 V pin)

  • 1 × push button or jumper wire for input

How the NOT Gate Works

A NOT gate has one input and one output. It simply inverts the logic level:

Input Output
0 (LOW) 1 (HIGH)
1 (HIGH) 0 (LOW)

Inside the 74LS04 are six NOT gates (that’s why it’s called a hex inverter). The pinout for 74LS04 is shown below: -

You can use any of them for example, input on pin 1, output on pin 2.

Circuit Diagram

Wiring It Up

  1. Power the chip:
     Connect pin 14 to +5 V and pin 7 to GND.
    (Add a small 0.1 µF capacitor between them to smooth any noise.)
  2. Connect your input (push button):
     One side of the push button goes to +5 V.
    The other side connects to pin 1 (the NOT gate input).
    Then add a 10 kΩ resistor from pin 1 to GND — this acts as a pull-down resistor so the input stays LOW when the button isn’t pressed.
  3. Output LED:
     Connect pin 2 (the NOT gate output) through a 330 Ω resistor to an LED, then to GND.
    When the input is HIGH, the LED will turn OFF.
    When the input is LOW, the LED will turn ON.
  4. Optional power LED:
    From +5 V → 1 kΩ resistor → LED → GND, so you can easily see when the circuit is powered.
Connect the power rails of the breadboard to your bench power supply and set the bench power supply to 5V. When turned on the LED should be on. With the button pressed the LED should be off reflecting our truth table above.

Why It Matters

The NOT gate is the foundation of all digital logic. Combine it with AND, OR, and XOR gates, and you can build everything from flip-flops to microprocessors.

This little chip (74LS04) is often used for clock signal shaping, buffering, and even simple oscillators. 

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