Logic GatesLearning ObjectiveTo be able to describe and use logic gates.

All

Most

Some

• Describe the functions of logic gates. (C)

• Construct truth tables for logic gates. (C)

• Design a circuit to use logic gates to control circuits. (B+)

13 March 2014

George Boole,(1815-1864)

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

A logic gate is a digital circuit which either allows a signal to pass through it or to stop it. There are seven basic logic gates: AND, OR, XOR, NOT, NAND, NOR, and XNOR.

?Logic !

?Logic !

ORGATE

ANDGATE

NOTGATE

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

ORGATE

FRONT DOORBELLSWITCH

BACK DOORBELLSWITCH

DOORBELL

ORGATE

Input

Input

Output

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

cc

ANDGATE

ANDGATE

ON SWITCH FOR ALARM

PERSON SENSOR

BURGLAR ALARM

Input

Input

Output

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

NOTGATE

Input Output

NOTGATE

HEAT DETECTOR(ABOVE 20 C)

CENTRALHEATING

ORGATE AND

GATE

ANDGATENOT

GATE

FINAL OUTPUTON ? OFF ?

Is the Final Output ON (True) or OFF (false) ?

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

?

?

?

?YES, Turn sprinklers on

High temp = 1

Weather dry = 1

Daytime = 1

Soil in plants wet? Yes = 1

Name the logic gates. Test it by answering True or false (Yes or No) for each Input.

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

YES, Alert police

Write conditions in the logic boxes that would alert you to someone breaking into your house. You can choose the logic gates you have learnt. Name the logic gates. Test the alarm system by answering Yes or No ( True or false ).

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

ON or OFF ?

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

ORGATE AND

GATE

BEDROOM SENSOR

FRONT ROOM SENSOR

BURGLAR ALARM

ON SWITCH FOR ALARM

NO ONE IN EITHER ROOM

1.1.

2.2.

3.3.

6.6.

4.4.

5.5.

?

?

?

?

?

?

The output is ON or OFF ?

The output is ON or OFF ?

?

8.8.

9.9.

10.10.

?

?

?7.7.

The output is ON or OFF ?

The output is ON or OFF ?

Complete a labelled diagram to show how two transistors can be connected to form an “AND” gate.

Explain how this works....

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

If a current is flowing through wire A, it enters the transistor through the Base. This switches the current on between the collector and the emitter. This current can only flow through the lower transistor if there is a current flowing through wire B so that the lower transistor is also switched on.

Can you construct an OR gate using transistors?

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

Can you construct an OR gate using transistors?

The collectors of both transistors are connected to the same positive input – you can think of them being in parallel with each other. If input A OR input B have a current flowing then a current will flow through either or both transistors to the output Q – so that the output is high if either of the inputs A and B are high.

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

Can you construct a NOT gate using transistors?

When the input at A is 0 no current flows through the transistor and all of it flows through the output at Q. When the input at A is 1, a current flows through the base which switches the current through the transistor on. As it is easier for the current to flow through the transistor than to output Q, the output at Q is zero when A is 1.

Logic gates

Logic gates can have only one of two values 0 or 1, low or high.

As the signals can only have one of two values, they are DIGITAL signals.

The outputs for logic gates depend upon the inputs and are shown in a truth table.

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

Logic gate Circuit symbol

NOT

OR

AND

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

Logic gate Circuit symbol

NOR

NAND

All – describe the functions of logic gates. (C)Most – construct truth tables for logic gates. (C+)Some – Design a circuit to use logic gates to control circuits. (B+)

NOT

Input Output

0

1

1

0

NOT gate inverts the input signal

OR

Input A Input B Output

0 0

0 1

1 0

1 1

0

1

1

1

Output is 1 if either of the inputs is 1, or if both inputs are 1.

AND

Input A Input B Output

0 0

0 1

1 0

1 1

0

0

0

1

Output is 1 only if both of the inputs is 1.

NAND

Input A Input B Output

0 0

0 1

1 0

1 1

1

1

1

0

Output is opposite of AND gate.

NOR

Input A Input B Output

0 0

0 1

1 0

1 1

1

0

0

0

Output is opposite of OR gate.