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Logic Gates Learning Objective To be able to describe and use logic gates. All Most Some Describe the functions of logic gates. (C) Construct truth tables.

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Presentation on theme: "Logic Gates Learning Objective To be able to describe and use logic gates. All Most Some Describe the functions of logic gates. (C) Construct truth tables."— Presentation transcript:

1 Logic Gates Learning Objective To 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

2 George 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. ?Logic ! OR GATE AND GATE NOT GATE 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 OR GATE FRONT DOORBELL SWITCH BACK DOORBELL SWITCH DOORBELL OR GATE 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+)

5 c c AND GATE AND GATE ON SWITCH FOR ALARM PERSON SENSOR BURGLAR ALARM 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+)

6 NOT GATE Input Output NOT GATE HEAT DETECTOR (ABOVE 20 C) CENTRAL HEATING

7 OR GATE AND GATE AND GATE NOT GAT E FINAL OUTPUT ON ? 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+)

8 ? ? ? ? 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+)

9 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+)

10 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+) OR GATE AND GATE BEDROOM SENSOR FRONT ROOM SENSOR BURGLAR ALARM ON SWITCH FOR ALARM NO ONE IN EITHER ROOM

11 ? ? ? ? ? ? The output is ON or OFF ?

12 ? ? ? ? 7. The output is ON or OFF ?

13 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?

14 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.

15 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.

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 gateCircuit 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 gateCircuit 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 InputOutput NOT gate inverts the input signal

20 OR Input AInput BOutput Output is 1 if either of the inputs is 1, or if both inputs are 1.

21 AND Input AInput BOutput Output is 1 only if both of the inputs is 1.

22 NAND Input AInput BOutput Output is opposite of AND gate.

23 NOR Input AInput BOutput Output is opposite of OR gate.


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