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**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)

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**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+)

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**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+)

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**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+)

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**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+)

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**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.

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**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+)

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**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+)

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**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+)

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**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+)

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**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 ?

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**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 ?

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**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+)

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**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+)

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**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+)

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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+)

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**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+)

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**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+)

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NOT Input Output 1 1 NOT gate inverts the input signal

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**OR Input A Input B Output 1 1 1 1**

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

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**AND Input A Input B Output 1 1**

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

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

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

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