Logic Gates AIM: To know the different types of logic gate To understand how truth tables describe the action of logic gates PRIOR KNOWLEDGE: That digital signals can be represented by 1 (on) and 0 (off) www.pfnicholls.com

Introduction Logic gates are the basic building block of combinational logic circuits. Combinational logic circuits are circuits where the output only depends on the inputs The output of a combinational logic circuit is usually labelled Q The inputs are usually labelled A, B, C etc Logic gates can have two or more inputs The output of the logic gate, for each combination of inputs, is shown in a truth table A very common series of logic Integrated Circuits is called the 4000 series. These chips are CMOS chips and work at any voltage between about 3V and 18V but they do not provide very much output current

The NOT gate (4069) A Q 1 The NOT gate is the most basic logic gate When A = 1 then Q = 0 When A = 0 then Q = 1 The output (Q) is NOT the same as the input (A) A Q 1

The AND gate (4081) The AND gate needs both inputs to be ON to make the output come ON A = 1 AND B = 1 then Q = 1 Otherwise, for all other combinations, Q = 0 A B Q 1

The OR gate (4071) The OR gate gives the output ON when any input is ON When A = 1 OR B = 1 OR both A & B = 1 together then Q = 1 When A & B are both 0, Q = 0 A B Q 1

The NAND gate (4011) The NAND gate is the opposite of an AND gate. It is NOT AND When A = 1 AND B = 1 then Q is NOT ON, Q = 0 Otherwise, Q = 1 A B Q 1

The NOR gate (4001) The NOR gate is the opposite of the OR gate. It is NOT OR When A =1 OR B = 1 OR both A & B are 1 then the output is NOT ON, Q = 0 Otherwise, when A = 0 as well as B = 0 then Q = 1 A B Q 1

Input Circuits for Logic Gates Logic circuits need an input that is definitely either ON (Logic 1) or OFF (Logic 0) An input that is left disconnected or floating will take any logic value and cause unexpected results. This is avoided by using a resistor to make sure the input is always connected to something Pressed = Logic 1 Not pressed = Logic 0 The pull up resistor keeps the input to the logic circuit at Logic 1 when the button is not pressed The pull down resistor keeps the input to the logic circuit at Logic 0 when the button is not pressed Pressed = Logic 0 Not pressed = Logic 1

Summary You just have to learn these symbols and truth tables

Questions Why is an AND gate called and AND gate? Why is an OR gate called an OR gate? Why is a NOT gate called a NOT gate? How do you make a NAND gate from AND and NOT gates? How do you make a NOR gate from OR and NOT gates?

Answers The output is ON when input A is ON AND input B is ON The output is ON when either input A is ON OR input B is ON OR inputs A and B are both ON The output is NOT ON (i.e. OFF) when the input is ON and vice versa so the output is NOT what the input is An AND gate followed by a NOT gate An OR gate followed by a NOT gate