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A computer is basically a collection of transistors and circuits. These components have two states:

• on - a current is flowing through the component
• off - a current is not flowing through the component

These two states can easily be represented by using binary notation:

• 1 = on (TRUE)
• 0 = off (FALSE)

State refers to the output of a circuit. If a circuit has one input and one output, and the input and output each have two states - on and off - this gives two combinations of state:

• input off, output off
• input on, output on

More complex circuits have many inputs, and the output state depends on the various combinations of the inputs and the logic gates they pass into.

Logic gates

A logic gate is a series of transistors connected together to give one or more outputs. Each output is based on the input or combination of inputs supplied to it. There are three types of gate to consider at GCSE:

• AND gate
• OR gate
• NOT gate

Each type of gate can be represented either as a diagram, in algebraic form, or as a truth table. In a diagram, each gate is a different shape.

Boolean algebra

Boolean algebra is a notation used to represent logic. For example:

• Q = A AND B
• Q = A OR B
• Q = NOT A