Programming · Grade 7 · Chapter 2
Logic Gates → Python Thinking
Bridging physical switches & Truth Tables into real Python boolean expressions.
Chapter Goals
What We Will Master Today
- Hardware Logic: Connect binary (1/0) state to hardware switches.
- Truth Tables: Evaluate AND, OR, and NOT gate truth tables.
- Scratch to Python: Translate Scratch green boolean blocks to Python text syntax.
- Python Logical Operators: Master `and`, `or`, and `not` precedence.
Here's a problem
From Switches to Code Sentences
Computers don't have brains — they have billions of microscopic transistors that act like light switches (ON = 1, OFF = 0). How do switches make decisions?
Discuss before revealing: How can simple ON/OFF switches calculate math or run video game rules?
Core Concept 1
The Binary World (Bits & Signals)
Every piece of computer logic relies on Boolean values: True (1) or False (0).
High Voltage (1 / True)
Switch CLOSED, current flows, light turned ON, button pressed.
Low Voltage (0 / False)
Switch OPEN, current stopped, light OFF, button released.
Physical Circuits
Series vs Parallel Circuits
Series Circuit (AND Logic)
Two switches in a row. Electricity flows ONLY if Switch A AND Switch B are both closed.
Parallel Circuit (OR Logic)
Two side-by-side tracks. Electricity flows if Switch A OR Switch B is closed.
Core Gate 1
The AND Gate
Output is 1 (True) ONLY if BOTH Input A and Input B are 1 (True).
| Input A | Input B | Output (A AND B) |
|---|
| 0 (False) | 0 (False) | 0 (False) |
| 0 (False) | 1 (True) | 0 (False) |
| 1 (True) | 0 (False) | 0 (False) |
| 1 (True) | 1 (True) | 1 (True) |
Core Gate 2
The OR Gate
Output is 1 (True) if AT LEAST ONE input is 1 (True).
| Input A | Input B | Output (A OR B) |
|---|
| 0 (False) | 0 (False) | 0 (False) |
| 0 (False) | 1 (True) | 1 (True) |
| 1 (True) | 0 (False) | 1 (True) |
| 1 (True) | 1 (True) | 1 (True) |
Core Gate 3
The NOT Gate (The Inverter)
Takes a single input and flips it to the opposite value.
| Input A | Output (NOT A) |
|---|
| 0 (False) | 1 (True) |
| 1 (True) | 0 (False) |
Real life example: Security alarm light is OFF when door is closed (1), turns ON when door breaks connection (0).
Smartboard Voting
Evaluate This Logic Output!
Input A = True (1)
Input B = False (0)
They pass into an OR gate, and then the result goes into a NOT gate.
Question: What is the final output? (Is it True or False?)
Advanced Gates
Combining Gates: NAND, NOR & XOR
NAND
AND gate followed by NOT. Opposite of AND.
NOR
OR gate followed by NOT. True only if both 0.
XOR
Exclusive OR. True if inputs are different.
Scratch to Python
Remember Scratch Green Boolean Blocks?
In Scratch, you snapped green hexagonal blocks inside `if` blocks:
< (score > 50) and (lives > 0) >
< (key_pressed) or (mouse_down) >
< not (touching color) >
Language Comparison
Visual Blocks vs Python Keywords
Scratch Visual Block
< <age > 12> and <has_ticket = true> >
Python Text Syntax
if age > 12 and has_ticket == True:
Core Concept 4
Python's Three Logical Operators
# 1. and (both must be True)
result1 = (5 > 2) and (10 < 20) # Evaluates to True
# 2. or (at least one is True)
result2 = (5 > 100) or (3 == 3) # Evaluates to True
# 3. not (flips True to False)
result3 = not (5 == 5) # Evaluates to False
Rulebook
Logical Operator Precedence
Just like math has PEMDAS, Python evaluates boolean logic in this strict order:
Spot the Mistake
Parentheses Change Everything!
expr1 = not True and False
# Step 1: not True -> False
# Step 2: False and False -> False
expr2 = not (True and False)
# Step 1: (True and False) -> False
# Step 2: not False -> True!
Real-World Code
Video Game Inventory Check
has_key = True
player_health = 75
is_poisoned = False
can_open_door = has_key and (player_health > 0) and not is_poisoned
print(can_open_door) # What gets printed?
Quick Check · Quiz 1
What is the output of an AND gate when Input A = 1 and Input B = 0?
A1 (True)
B0 (False)
C2
DUndefined
Click to reveal answer
Quick Check · Quiz 2
What does Python evaluate for: (10 > 5) or (2 > 100)?
ATrue
BFalse
CNone
DError
Click to reveal answer
Quick Check · Quiz 3
Which Scratch block corresponds to Python's not (x == y)?
A< (x = y) and (not y) >
B< (x = y) or (x = y) >
C< not < x = y > >
D< join x y >
Click to reveal answer
Quick Check · Quiz 4
Which logical operator flips a Boolean value to its opposite?
Click to reveal answer
Guided Practice
Translate English Rules to Python Expressions
Rule: "Allow roller coaster entry if height is over 120cm AND (age is 12 or older OR parent is present)."
allowed = (height > 120) and (age >= 12 or parent_present == True)
Common Trap
Misconception: if x == 1 or 2: in Python
In English we say "If x equals 1 or 2". But Python interprets or 2 as "is 2 truthy?".
✗ Wrong Python Syntax
if x == 1 or 2:
✓ Correct Python Syntax
if x == 1 or x == 2:
Summary & Takeaways
What We Learned in Chapter 2
- Bits operate on 1 (True / High Voltage) and 0 (False / Low Voltage).
- AND gates require all 1s; OR gates require at least one 1; NOT gates invert.
- Scratch green blocks map directly to Python keywords `and`, `or`, `not`.
- Always use parentheses to enforce correct logical precedence!
Exit Ticket
Truth Table Challenge
Evaluate on your whiteboard:
a = True, b = False, c = True
What is: not a or (b and c)?
Next Chapter: Python Setup & Syntax!