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A range of logic gates exist and they are represented as symbols, each with its own truth table (sometimes called a logic table). Gates have inputs and produce outputs and these are in the form of 1s and 0s. Remember, a 1 represents an input or output of electrical current. Each truth table clearly shows the ‘state’ of inputs and outputs at any one time.
Study the symbols and tables below. You will soon find that they can be combined to design interesting logic circuits.

The AND gate will only output current (produce a 1 at Q) if both logic states at inputs A and B change to 1.

The NAND gate has the opposite outputs to the AND gate. How does the NAND gate symbol differ to the AND gate?

The OR gate will output current at Q if either of the logic states of inputs A and B change to1.

The NOR gate has the opposite outputs to the OR gate. How does the NOR gate symbol differ to the OR gate?

The INVERTER gate reverses input. For example, if the input is 1 then the output is 0.
This is a very useful gate especially when designing logic circuits.

QUESTIONS:
1. Draw each of the logic gates shown above and explain how each gate works.
2. Learn and remember each of the logic tables.

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