NOR Gate (74LS02)

-

Building a NOR Gate (74LS02)

If you’ve been following the digital logic series, you’ve seen the NOT, AND, and OR gates in action. Now let’s explore the NOR gate, a gate that only goes HIGH when none of its inputs are HIGH.

In this post, we’ll build and test one using the 74LS02 TTL IC.

What You’ll Need

  • 1 × 74LS02 IC (Quad 2-input NOR gates)

  • Breadboard + jumper wires

  • 2 × push buttons (or jumper wires for manual input)

  • 2 × 10 kΩ resistors (pull-down resistors)

  • 1 × 330 Ω resistor (for LED)

  • 1 × LED (any colour)

  • 5 V power supply (or Arduino 5 V pin)

How the NOR Gate Works

A NOR gate combines the logic of OR and NOT:
it outputs HIGH only when all inputs are LOW.

Think of it as:

“If neither A nor B is pressed, the output is ON.
If either (or both) are pressed, the output turns OFF.”

Inside the 74LS02

The 74LS02 contains four independent 2-input NOR gates.
Each one can be used separately, perfect for experimenting or chaining logic.

Pinout (74LS02):

Use pin 7 for ground (GND) and pin 14 for +5 V (VCC).
Each gate follows the pattern: A input → B input → F output.

Circuit Diagram

Wiring the Circuit

  1. Place the 74LS02 in the middle of your breadboard. Connect pin 14 to +5 V and pin 7 to GND.

  2. Add two push buttons for A and B. Connect one side of each to +5 V. Connect the other side to pins 2 and 3 

  3. Connect 10 kΩ resistors to GND (pull-down). between the push buttons and pins 2 and 3 

  4. Connect pin 1 (output) through a 330 Ω resistor to the LED then to GND.

Understanding It Visually

  • No buttons pressed: both inputs LOW → LED ON

  • Only A pressed: one HIGH → LED OFF

  • Only B pressed: one HIGH → LED OFF

  • Both pressed: both HIGH → LED OFF

That’s the NOR gate in action, an inverted OR!

Why NOR Matters

The NOR gate is functionally complete, meaning you can build any logic circuit (AND, OR, NOT, XOR, etc.) using just NORs.
It’s often used in control systems, simple CPUs, and TTL logic designs.


Comments

Post a Comment

Popular posts from this blog

Math Behind Logic Gates

-
Image
Understanding the Real Electronics Behind Logic Gates (The Practical Math Behind Digital Circuits) When we build circuits with logic chips like the 74LS series, it’s easy to think only in 1s and 0s. But under the hood, every logic gate follows the same rules as any other circuit, Ohm’s Law, current limits, and voltage thresholds. So yes, there is real-world math behind those blinking LEDs and logic tables.  1. Logic Levels — The Digital View At the logic level, everything is simple: 0 V → LOW (logic 0) 5 V → HIGH (logic 1) We don’t care about exact voltages here just whether a signal crosses a defined threshold. For TTL (Transistor-Transistor Logic) chips like the 74LS86 XOR gate , these thresholds are: This means a logic gate “decides” based on whether the voltage is above or below that boundary, which is how binary logic becomes real voltage levels. 2. Inside the Circuit — Real Electrical Math Even though logic circuits process bits, they still obey Ohm’s Law ...
Read more

6502 - Part 2 Reset and Clock Circuit

-
Image
6502 Retro Console Part 2 Add a Reset and Clock Circuit In Part 1 we powered up the 6502 and watched it running a NOP test with just the CPU and LEDs on the address bus. Now we’ll give it a clock and a clean start every time you power on, rather than relying on a function generator. In this post we will cover a simple 555 Timer Clock circuit and reset circuit. 555 Timer The 6502 needs a steady pulse on its Φ0 (pin 37) to step through instructions. A NE555 timer is perfect for low-speed testing reliable, simple, and easy to wire. Parts List NE555 timer 1 kΩ resistor 10 kΩ resistor 100 nF capacitor (timing) 0.1 µF capacitor (decoupling) Breadboard + jumper wires Circuit Diagram How to Wire It Add a 0.1 µF decoupling cap across the +5V and GND rails stability. Typical speed: ≈ 7–10 kHz perfect for a visible NOP test. Use larger resistors or capacitors to slow it down. Tip: Swap the timing capacitor for 1 µF and the 10 kΩ for 100 kΩ to step slowly enough ...
Read more

Building a 6502 NOP Test

-
Image
 The Simplest Working 8-bit CPU Circuit Before we start building a full retro console, let’s do something beautifully simple get a 6502 CPU to run,  no RAM, no ROM, no glue logic, just the chip, a few LEDs, and a clock. This is called a NOP test (No Operation). It’s the digital equivalent of “does it breathe?” What You’ll Need Part Purpose MOS 6502 CPU The heart of our system Bench power supply (5 V) Power for the CPU Function generator Clock source 8 × LEDs To watch address lines change 8 × 470 Ω resistors Current-limiters for the LEDs 1 × 0.1 µF capacitor Decoupling between VCC and GND Breadboard and jumper wires For connections MOS 6502 Cpu Pin Out Check your IC and its datasheet to ensure that you have the correct pin out map as the steps below may vary slightly. Step 1 — Power and Decoupling Give the 6502 a clean 5 V supply: VCC (pin 8) → +5 V VSS (pins 1 and 21) → Ground Place the 0.1 µF capacitor between VCC and VSS close to the chip. That tiny capacitor...
Read more