1. Logic Gates Some Most All Learning Objective
13 March 2014
Logic Gates
Learning Objective
To be able to describe and use logic gates.
All
Most
Some
Design a circuit to use logic gates to control circuits. (B+)
Construct truth tables for logic gates. (C)
Describe the functions of logic gates. (C)

2. LOGIc gates George Boole, (1815-1864) Did you know?
George Boole Inventor of the idea of logic gates. He was born in Lincoln, England and he was the son of a shoemaker in a low class family. We describe the functions that logic gates use as parts of Boolean Algebra.
George Boole,
( )
About G. Boole:
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+)

3. 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.
NOT
GATE OR
GATE
AND
GATE
Note that logic gates are not always required because simple logic functions can be performed with switches or diodes:Switches in series (AND function)Switches in parallel (OR function)Combining IC outputs with diodes (OR function)
Did you know?
Logic gates allow the computer to do things such as add, divide, multiply, do simple yes and no reasoning in certain situations along with other things.
?Logic !
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+)

4. ON/OFF ON/OFF ON /OFF Input Y = A OR B OR Output Input OR
An OR gate can have two or more inputs.
The output will be positive (True) if at least one input is true.
Y = A OR B OR
GATE
Output
Input
ON/OFF
ON/OFF
FRONT DOORBELL
SWITCH OR
GATE
DOORBELL
BACK DOORBELL
SWITCH
ON /OFF
The output is "true" if either or both of the inputs are "true." If both inputs are "false," then the output is "false.”
It is doesn’t matter if one input will be off- the output will be on
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+)

5. ON/OFF ON/OFF ON /OFF Input AND Output Input AND BURGLAR ALARM GATE
An AND gate can have two or more inputs.
The output will be positive (true) when both inputs (the input one AND the input two) are positive (true).
Y = A AND B
Input
AND
GATE
Output
Input
ON/OFF c
ON/OFF
PERSON SENSOR
AND
GATE
BURGLAR ALARM
ON SWITCH
FOR ALARM
The output is "true" when both inputs are "true.” Otherwise, the output is "false."
If one input will be off – the output will be off as well.
ON /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+)

6. ON OFF NOT Output Input CENTRAL NOT HEATING
A NOT gate (inverter) has only one input.
It reverses the logic state.
Y = NOT A
NOT
GATE
Input
Output
NOT
GATE
HEAT DETECTOR
(ABOVE 20 C)
CENTRAL
HEATING ON
OFF
NOT gate has only one input. It reverses the logic state.
If the input will be on – the output will be off
If the input will be off –the output will be on.

7. ON OFF Is the Final Output ON (True) or OFF (false) ? OR AND
GATE
AND
NOT ON
OFF
FINAL
OUTPUT
ON ? 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+)

8. GreeNhouse controller:
Name the logic gates. Test it by answering True or false (Yes or No) for each Input. ? ON
YES,
Turn sprinklers on
High temp = 1
Weather dry = 1
Daytime = 1
Soil in plants wet? Yes = 1
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+)

9. ON Design A burglar alarm
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 ). ON
YES,
Alert police
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+)

10. To add more sensors to alarm
GATE
AND
BEDROOM
SENSOR
FRONT ROOM
BURGLAR
ALARM
ON SWITCH
FOR ALARM
ON or OFF ?
NO ONE IN EITHER ROOM
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+)

11. Test 1: ? ? ? ? ? ? The output is ON or OFF ? 1. 4. ON OFF ON OFF 5.2. ? ON ? ON
OFF 6. ? ON
OFF
1- off, 2- on, 3-off, 4- on, 5-off, 6-on 3. ? ON
OFF
The output is ON or OFF ?

12. Test 2: ? ? ? ? The output is ON or OFF ? 7. 10. OFF ON ON OFF OFF 8.9. ON ? ON
7- on, 8-off, 9-off, 10- on ON ON ON ON
The output is ON or OFF ?

13. Can you construct an OR gate using transistors?
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.
Complete a labelled diagram to show how two transistors can be connected to form an “AND” gate.
Explain how this works....
Can you construct an OR gate using transistors?
7- on, 8-off, 9-off, 10- on
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+)

14. 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.
7- on, 8-off, 9-off, 10- on
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+)

15. 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.
7- on, 8-off, 9-off, 10- on
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+)

16. 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+)

17. 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+)

18. 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+)

19. NOT
Input
Output 1 1
NOT gate inverts the input signal

20. OR Input A Input B Output 1 1 1 11 1 1 1
Output is 1 if either of the inputs is 1, or if both inputs are 1.

21. AND Input A Input B Output 1 11 1
Output is 1 only if both of the inputs is 1.

22. NAND
Input A
Input B
Output 1 1 1 1
Output is opposite of AND gate.

23. NOR
Input A
Input B
Output 1 1
Output is opposite of OR gate.