Logic gates

Although there are lots of types of digital logic gates, you only need to know about three of them for Standard Grade - AND, OR, and NOT (or Inverter). Logic gates may have one or more inputs. For Standard Grade, you only need to know about gates with a maximum of two inputs. You should be able to draw and recognise the symbols for AND, OR and NOT gates.

Happening hints for recognising logic gate symbols

The symbol for an AND gate is shaped like the letter 'D'. A NOT gate inverts the input, so it only has one input.

In logic circuits, a high voltage is called logic 1 a low voltage is called logic 0.

A truth table for a logic gate or circuit shows the output for all possible combinations of inputs. For a NOT gate, the output is the opposite of the input. An AND gate only gives a high output when both inputs are high. An OR gate gives a high output when at least one input is high.

Question 1

Print out the following table and the boxes with symbols and truth tables. Cut up the boxes and stick them in the correct places to complete the table.

	input output 0 1 1 0
input input output 0 0 0 0 1 0 1 0 0 1 1 1		input input output 0 0 0 0 1 1 1 0 1 1 1 1

The Answer

How did you get on?

Check your answers below. You may want to print this table out as a permanent summary.

Gate	Symbol	Truth table
AND		input input output 0 0 0 0 1 0 1 0 0 1 1 1
OR		input input output 0 0 0 0 1 1 1 0 1 1 1 1
NOT		input output 0 1 1 0

You should be able to explain how to use logic gates together in real-life situations.

Try this

1. Which gates are needed to switch the fan on at night when the temperature becomes too high? Click on the correct box for each gate.

2. Which gate is needed to light the lamp when a person goes through either turnstile a or b at a football ground? Click on the correct box.