Understanding Logic Gates
Understanding Logic Gates
Logic gates are the tiny decision-makers at the heart of every computer. Whether you're a curious beginner, a student, or someone trying to build a computer from scratch — this is your first real step into digital electronics.
In this post, we’ll explore:
- What logic gates are
- Why computers use only 0s and 1s
- How simple gates build the foundation of complex circuits
🔍 What Are Logic Gates and Why 0 & 1?
We often hear:
"Computers only understand 0 and 1."
As a kid, I used to wonder — if a computer can understand 0 and 1, why can't it just understand 2, 3, 4, 5... or even A, B, C? I mean, what’s the problem? 😅
But here’s the truth:
It’s not that computers can’t understand other things — it’s that they reduce everything down to just 0s and 1s.
Why? Because deep inside, computers are made of tiny electronic switches called logic gates.
What are these 0s and 1s, really?
Well, 0s and 1s aren’t magical numbers — they’re just representations of voltage levels.
In digital electronics, a 0 or 1 simply means a certain range of voltage. That's it.
⚡ Logic Levels: Voltage Ranges
TTL Logic (Transistor-Transistor Logic)
| Logic Level | Voltage Range | Description |
|---|---|---|
| 0 (LOW) | 0 V to 0.8 V | Interpreted as logic 0 |
| 1 (HIGH) | 2 V to 5 V | Interpreted as logic 1 |
⚠️ Typical operating voltage: Most TTL-based logic circuits use 5V, but this can vary depending on the technology: 3.3V, 1.8V, or even lower in modern low-power devices.
⚙️ How Do Logic Gates Use These Voltages?
Now that we know 0 and 1 are just voltage levels, the next question is:
How do logic gates use them to produce outputs?
The Role of Transistors
Logic gates are built from tiny components called transistors — basically electronic switches.
A transistor can:
- Allow current to pass (like a closed switch) → connected
- Block current (like an open switch) → disconnected
These switches respond to the input voltage (0 or 1), and control the output voltage.
Why It Matters
By connecting logic gates like NOT, AND, OR, and more:
- We can build circuits that make decisions
- We can store memory, do arithmetic, and even
- Build entire processors and computers
All of it — starting from simple voltage-based switches that only understand 0 and 1.
Types of Logic Gates and Their Working
Logic gates can be broadly categorized into the following types:
Essential (Fundamental) Logic Gates
These are the basic building blocks of all digital circuits:
NOT Gate (Inverter)
Example: A NOT gate is like a "toggle switch" — if ON, turn OFF. It simply flips the input.
A → Y
0 → 1
1 → 0
AND Gate
Example: "You can go out IF you finish your homework AND clean your room." Only when both inputs are 1 → output is 1.
A B → Y
0 0 → 0
0 1 → 0
1 0 → 0
1 1 → 1
OR Gate
Example: "You'll get dessert if you finish your homework OR help in the kitchen." If any input is 1 → output is 1.
A B → Y
0 0 → 0
0 1 → 1
1 0 → 1
1 1 → 1
Derived Universal Gates
These gates are combinations of the basic ones — and they are powerful enough to build any digital circuit.
AND + NOT = NAND
Example: It's like a strict parent who says no only if you do both things. Only when both inputs are 1 → output is 0.
A B → Y
0 0 → 1
0 1 → 1
1 0 → 1
1 1 → 0
OR + NOT = NOR
Example: A gate that only opens if no one asks for it. Only when both inputs are 0 → output is 1.
A B → Y
0 0 → 1
0 1 → 0
1 0 → 0
1 1 → 0
Other Common (Composite) Gates
These are built from combinations of the basic or universal gates and used frequently in real-world logic designs:
XOR Gate (Exclusive OR)
Example: “You’ll get a reward only if exactly one of you does the task — not both.” Output is 1 only when inputs are different.
A B → Y
0 0 → 0
0 1 → 1
1 0 → 1
1 1 → 0
XNOR Gate (Exclusive NOR)
Example: “You get a bonus only if both answers match.” Output is 1 only when inputs are the same.
A B → Y
0 0 → 1
0 1 → 0
1 0 → 0
1 1 → 1
📊 Truth Tables Explained
In digital logic, each input can only be 0 or 1.
So for a gate with n inputs:
Total combinations = 2ⁿ
| Gate Type | Inputs | Combinations |
|---|---|---|
| NOT | 1 | 2 |
| AND, OR, etc. | 2 | 4 |
| 3-input gate | 3 | 8 |
This is why we use truth tables — to see how a logic gate behaves for every possible combination of inputs.
Conclusion & What’s Next
- Logic gates use voltage to represent 0 and 1.
- They are built using transistors.
- With just a few types of gates, we can create everything from simple adders to CPUs.
Up Next: Building a Half Adder
In the next post, we'll begin building circuits using these gates, starting with a Half Adder — the building block of arithmetic.
Stay tuned!