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Introduction to the fundamental building blocks of digital circuits and boolean logic.
Every time you unlock your phone or send a text, billions of tiny 'decision-makers' are working together to process your commands. What if you could see the simple 'Yes/No' rules that power every computer on Earth?
Computers don't understand words or pictures; they only understand binary, which consists of two states: (On/True) and (Off/False). Logic gates are the physical components that process these signals. The simplest gate is the NOT gate, also known as an inverter. It takes a single input and flips it to the opposite value. If you send a , it outputs a . If you send a , it outputs a . This is the foundation of computer 'thinking'—the ability to change a state based on a rule.
Let's look at how a NOT gate behaves using a truth table. 1. If Input , then Output . 2. If Input , then Output . Mathematically, this is written as .
Quick Check
If a NOT gate receives a signal of 0, what will be the output signal?
Answer
1
Most logic gates take two inputs. The AND gate is like a strict security guard: it only outputs a if both Input A AND Input B are . If either is , the output is . The OR gate is more relaxed: it outputs a if Input A OR Input B (or both) are . It only outputs a if both inputs are completely off. Think of it like this: To go to the movies, you need money AND a ride. To stay dry in the rain, you need an umbrella OR a raincoat.
Imagine a home security system. The alarm () should only go off if the system is armed () AND a window is opened (). 1. System Disarmed (), Window Open (): (No Alarm). 2. System Armed (), Window Closed (): (No Alarm). 3. System Armed (), Window Open (): (ALARM!).
Quick Check
In an OR gate, if Input A is 1 and Input B is 0, what is the output?
Answer
1
In the real world, engineers combine these gates to create complex logic. We can represent these combinations using Boolean Algebra. For an AND gate, we use the dot symbol: . For an OR gate, we use the plus symbol: . By nesting these, we can build circuits that handle multiple conditions at once. For example, a car might chime if (Door is Open OR Lights are On) AND (Key is in Ignition).
A bank vault requires two keys ( and ) to be turned, but the vault won't open if the 'Lockdown' switch () is active. 1. The keys use an AND gate: . 2. The Lockdown uses a NOT gate: . 3. The final output uses another AND gate to combine them: . If and , the output is . The vault stays closed because of the lockdown!
Which gate is represented by the Boolean expression ?
If an AND gate has inputs and , what is the output?
A NOT gate can have two or more inputs.
Review Tomorrow
In 24 hours, try to draw the truth tables for AND, OR, and NOT from memory without looking at your notes.
Practice Activity
Look at a household appliance (like a microwave or washing machine) and try to write down one 'IF-THEN' logic statement that governs how it works using AND or OR.