How Do Logic Gates Combine to Process Signals?

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Updated October 31, 2026

A logic gate takes input signals and produces one output signal according to a defined rule. Connecting one gate’s output to another gate’s input builds a circuit with a more complex rule. A truth table records the result for each input combination, so you can use it to check a whole circuit.

Contents

A gate output can become the next gate input.

What you will learn

Read the rules of the AND, OR and NOT gates.
Follow the intermediate signals when several gates are connected.
Check a circuit with a truth table and compare it with the intended requirement.

Before you start

No specialist background is needed; technical terms are explained as they appear.

How does each gate determine its output?

Computers are built from logical switches, and in the modern era those switches are transistors. Each logic gate is built from such switches and carries out one rule. AND and OR gates have two inputs and one output. A NOT gate has one input and one output.

AND returns true when all inputs are true. OR returns true when at least one input is true, including when every input is true. NOT reverses its input: true becomes false and false becomes true. The NOT circuit is also called an inverter.

A truth table records this behaviour in terms of bits. It has one column for each input or output and one row for every combination of input values.

First input	Second input	AND	OR
False	False	False	False
False	True	False	True
True	False	False	True
True	True	True	True

Read across each row. For the same row of inputs, AND and OR can give different results because they apply two different rules.

AND combines conditions that must all be true.

How do connected gates build a larger rule?

One gate’s output becomes the input of the next gate. This is how larger components are built from smaller ones, after which the internals can be set aside and the next level built on top.

Consider an illustrative light that turns on only when someone is present and it is dark. The “daylight” signal passes through NOT. NOT’s result and the “presence” signal then enter AND.

Evaluate in order: take the daylight value, invert it, and only then combine it with presence. Writing down the intermediate result avoids a common mistake, which is applying the later gate’s rule to the original input. The later gate must receive the earlier gate’s output.

NOT reverses the input value.

How can you check the circuit against its intention?

A truth table covers every combination of inputs, so it can check a circuit. For the light, there are four situations: presence with darkness, presence with daylight, absence with darkness, and absence with daylight. Only presence with darkness makes both AND inputs true.

If AND is replaced by OR, the light turns on when there is only presence or only darkness. The circuit’s requirement has changed. State the intended behaviour first, then compare it with the table. A wiring diagram alone does not prove the circuit is right.

Other gates exist, and each is one rule. NAND is the inversion of AND. XOR is true when its two inputs differ. AND, OR and NOT are enough to build any other function, because you can start from a truth table and combine these gates to match it.

OR applies when any input condition may be true.

Real-world example

Work through it on paper with a person entering a dark room. “Presence” is true and “daylight” is false. NOT turns false into true. AND receives two true inputs and gives true, so the light comes on.

When the person leaves, “presence” becomes false. AND gives false even though NOT still gives true, so the light goes off.

For one more check, keep the person in the room and change dark to daylight. NOT now gives false, so the output is false too. You have followed both switching on and switching off without building a circuit.

Common misunderstandings

OR includes the case where every input is true. This differs from everyday speech, where “or” can sound like choosing just one.

A multi-gate circuit has to be read along the signal path. Each gate applies its rule to its own input, and that input may be the output of an earlier gate.

One test situation checks only one row of the table. To be sure, examine every row.

Each gate has its own rule that turns inputs into an output.
Intermediate signals connect logic operations into a circuit.
A truth table checks every input combination against the requirement.

Terms in this article

English and Vietnamese terms, with the meanings used here. Hover, tap or Tab to a term with a dotted underline for an explanation as you read.

logic gatescổng logic: Circuit elements producing outputs according to input rules.
truth tablebảng trạng thái: A table listing outputs for every input combination.
inverterbộ đảo: Another name for the NOT circuit, which reverses its input.
NAND: A gate that gives the opposite of AND for the same inputs.

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