7 - Logic Gates - 2025 - New

• Computing
• Upper Secondary (Key Stage 4)
• GCSE

Guess the topic?

LEARNING LOGIC GATES

What are they 

Why we need them

and how they work

Name any electronic device you know

All electronics need logic gates to function

Logic Gate

• Are building blocks in a circuit
•  Make decisions 
• Based upon inputs
• Give an output of either ON or OFF

There are three you need to know well.

AND, OR and NOT gates .

AND, OR and NOT gates

Go to this website

Build three simple circuits for each main  AND, OR and NOT gates. Use toggles and lamps for input and outputs

1. How does the AND gate turn the light on?
2. How does the OR gate turn the light on?
3. How does the NOT gate turn the light on?

AND

Output is only ON when all inputs are ON

Can you think of any device/system that needed you to press two buttons at once to make something happen?

Everyday Examples:

Microwave Oven

The microwave starts only if the door is closed AND the start button is pressed.

Lift Safety System

The lift moves only if doors are shut AND floor button is pressed.

Car Starter System

The engine starts only if brake pedal is pressed AND key is turned (or start button is pressed).

Computer Keyboard (Key Combination)

A shortcut like Ctrl + Alt + Delete sends a signal only if all keys are pressed together.

Technical / Digital Examples:

Digital Locks

Opens only if correct code entered AND confirm button pressed.

Alarm Systems

Alarm activates only if motion detected AND system armed.

CPU Control Logic

A processor performs certain operations only when multiple internal control signals are all true.

Traffic Light Controller

Changes state only if timer reached AND no pedestrian button active.

Logic Circuits in Robotics

Robot moves forward only if path is clear AND start signal received.

OR

Output is ON if ANY input is ON (including multiples)

Can you think or any system where if one of the sensors was triggered, you would want the system do something about it?

Everyday Examples:

Room Lighting with Two Switches

The light turns on if switch A OR switch B is on (common in staircases or hallways).

Car Door Alarm System

The alarm sounds if driver’s door OR passenger’s door OR boot is open.

Smoke and Heat Alarms (Combined System)

Alarm triggers if smoke is detected OR heat exceeds threshold.

TV Remote Control

The TV responds if you press the power button on the remote OR on the TV itself.

Technical / Digital Examples:

Security System Alerts

Sends alert if motion sensor triggers OR window sensor triggers.

Computer Interrupts

CPU responds if keyboard input OR mouse click OR hardware request is received.

Robotics Sensors

A robot stops if obstacle detected on left OR right sensor.

Voting Systems in Digital Circuits

A result is considered positive if any one of the inputs signals a "yes".

Heating Systems

Turns on the heater if room temp is too low OR outdoor temp is below freezing.

NOT

Output will be the opposite to the input (inverts)

Quick Recall - write down the three gates you just learnt

Everyday Examples:

Automatic Night Light

The light turns on when it’s dark (i.e., when the light sensor detects no light).

The NOT gate inverts the sensor signal:

Light detected = OFF, so NOT(light) = ON → light turns on in the dark.

Refrigerator Door Alarm

Alarm sounds when door is NOT closed.

A NOT gate inverts the signal from the door switch.

Toilet Cistern Warning Light

Light comes on if the water level is not full (i.e., NOT full = light on).

Technical / Digital Examples:

Digital Logic Control

Used to invert signals in CPUs, microcontrollers, and logic circuits.

Logic Locking

Access granted only if a condition is NOT true (e.g., NOT wrong password).

Robotics – Avoidance Logic

Robot moves forward if an obstacle is NOT detected.

Fan Control Systems

Fan stays on if temperature is NOT cool enough (temp low = 0 → NOT(0) = 1 → fan on).

Computer Memory Control (Inverted Enable)

Memory chips often use NOT gates to respond only when the chip enable signal is low (active-low logic).

AND gate with two inputs.
Input A = 1, Input B = 0
What is the output?

1. 1
2. 0

AND gate with 3 inputs.
Input A = 1, Input B = 1, Input C = 0
What is the output?

1. 1
2. 0

OR gate with 2 inputs.
Input A = 0, Input B = 0
What is the output?

1. 1
2. 0

OR gate with 3 inputs.
Input A = 0, Input B = 0, Input C = 1
What is the output?

1. 1
2. 0

OR gate with 2 inputs.
Input A = 1, Input B = 1
What is the output?

1. 1
2. 0

Logic Gate Expressions

Output = a AND NOT b

a	b	Output
0	0
1	0
0	1
1	1

Lets complete a truth table.

What would the output be in the red highlighted line?

1. 0
2. 1

What would the output be in the red highlighted line?

1. 0
2. 1

What would the output be in the red highlighted line?

1. 0
2. 1

What would the output be in the red highlighted line?

1. 0
2. 1

Logic Gate Expressions

Output = a AND NOT b

a	b	Output
0	0	0
1	0	1
0	1	0
1	1	0

Lets try a harder one :)

(a OR b) AND NOT c

A	B	C	Output
0	0	0
0	0	1
0	1	0
1	0	0
0	1	1
1	0	1
1	1	0
1	1	1

What would the output be in the yellow highlighted line?

1. 0
2. 1

What would the output be in the yellow highlighted line?

1. 0
2. 1

What would the output be in the yellow highlighted line?

1. 0
2. 1

What would the output be in the yellow highlighted line?

1. 0
2. 1

What would the output be in the yellow highlighted line?

1. 0
2. 1

What would the output be in the yellow highlighted line?

1. 0
2. 1

What would the output be in the yellow highlighted line?

1. 0
2. 1

What would the output be in the yellow highlighted line?

1. 0
2. 1

(a OR b) AND NOT c

A	B	C	Output
0	0	0	0
0	0	1	0
0	1	0	1
1	0	0	1
0	1	1	0
1	0	1	0
1	1	0	1
1	1	1	0

Which Logic gate is this?

1. AND
2. OR
3. NOT

Which Logic gate is this?

1. AND
2. OR
3. NOT

Which Logic gate is this?

1. AND
2. OR
3. NOT

1. 1
2. 0

Research task

Research with google:

NAND gates

NOR gates

XOR gates

Create with logic.ly a drawing using all three of these, and write the truth table for it.