Logic Gates: Make a Digital Decision
Understand that logic gates combine 0s and 1s by fixed rules, that AND, OR and NOT can be wired together to build any decision, and that a truth table lists every possible input combination.
Before you try
What if a lamp should light only when exactly one of two switches is on, never both?
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The gates switch instantly and perfectly. Real gates take a tiny amount of time to switch, and need power to work.
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Try these ideas
- Compare AND and OR with the same two switches set to 1 and 0.
- Trace the XOR circuit and find the two rows that give 1.
- Press Predict the output, work out the answer, then reveal it.
Why it works
Digital circuits deal in just two values, 0 and 1, and gates are the tiny rules that combine them. AND outputs 1 only when both its inputs are 1. OR outputs 1 when at least one input is 1. NOT simply flips its input. Wire them together and you can build any decision at all: NOT after AND gives NAND, and an OR combined with a NOT AND gives XOR, which is 1 when exactly one input is 1.
A truth table is the complete description of a circuit, listing every possible input combination and the output for each. Two inputs give four rows, three inputs give eight. In this simulator every wire carries a visible 0 or 1 tag as well as a colour, so you can trace the signal through gate by gate, and a prediction mode hides the lamp and asks you to work out the answer first.
Check your understanding
Question 1 of 4
An AND gate has inputs 1 and 0. What is its output?
An OR gate has inputs 1 and 0. What is its output?
How many rows does a truth table for three inputs have?
What does a NOT gate do?
Explain it in your own words
What changed? Why did it happen? Where might you notice this in everyday life?
For parents & teachers
- Ask when a real staircase light should switch on, and write the rule as a table.
- Discuss how a seatbelt warning uses AND: engine on and belt not fastened.
- Look up how many gates are inside a modern phone chip.