Q.Identify the logic gates marked P and Q. Rewrite the truth table with output Y for input conditions shown in the truth table for the combination of logic gates and identify the logic operation carried out by the combination of gates. OR A semiconductor X is made by doping a germanium crystal with indium. A second semiconductor Y is made by doping germanium with arsenic. The two are joined end to end and connected to a battery as shown (a diagram shows blocks X and Y joined in series and connected to a battery) :

A truth table is the simplest way to show exactly what a logic gate does. Before you memorise any table, picture a light switch. The switch has two states: ON or OFF. In digital electronics, we call those states 1 (ON, high voltage) and 0 (OFF, low voltage). A logic gate is a tiny circuit that takes one or more of these 1/0 inputs and produces a single 1/0 output.

The truth table is just a complete list: for every possible combination of inputs, what output does the gate give? That's all. No hidden rules, no guesswork — it's the gate's entire behaviour written in a table.

The precise statement

A truth table for a logic gate is a tabular listing of all possible input combinations (in binary order) and the corresponding output for each combination. For a gate with $n$ inputs, there are $2^n$ rows.

So a 2-input gate has $2^2 = 4$ rows; a 3-input gate has $2^3 = 8$ rows, and so on.

The simplest example: the NOT gate (inverter)

The NOT gate has only one input. It flips the signal: 1 becomes 0, 0 becomes 1.

That's the truth table. Two rows, because $2^1 = 2$. The output is always the opposite of the input.

A 2-input gate: the AND gate

The AND gate gives output 1 only when both inputs are 1. Otherwise, output is 0.

Notice the pattern: the inputs are listed in binary counting order (00, 01, 10, 11). This is the standard way to write truth tables so you never miss a combination.

Why this matters

A truth table is the definition of a logic gate. When you see a gate symbol (like the AND gate's D-shape), the truth table tells you exactly what it does. You don't need to guess or remember a vague description — the table is the complete, unambiguous behaviour.

For exam problems, you'll often be asked to:

• Write the truth table for a given gate
• Identify a gate from its truth table
• Combine gates and produce a truth table for the whole circuit

In every case, start with the inputs, list all $2^n$ combinations in binary order, then work out the output for each row. That's the entire method.

One more example: the OR gate

The OR gate gives output 1 if at least one input is 1.