9[1].4 logic gates

SEK MEN SAINS MUZAFFAR SYAH

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Security lamps, alarm systems, and washing machines can make some simple decisions. The switching on and off operations are controlled by electronic switches made up of logic

gates. Logic gates work using tiny transistors as switches. They are manufactured as integrated

circuit (IC), with each chip holding several gates.

For each gate, the input or inputs are on the left of the symbol. The output is on the right Each input and output can be either high (logic 1) or low (logic 0). A binary “0” represents 0 V, and a binary “1” represents a non zero voltage. A truth table lists all input possibilities and the corresponding output for each input.

Gates Truth table Action1. AND gate For the input to be ON,

both inputs must be ON.Output in ON only whenboth inputs A and B areON.

2. OR gate For the output to be ON atleast one of the inputsmust be ON.Output Q is ON when inputA or B or both is ON

3. NOT gateThe output is ON when theinput is OFF, and viceversa

4. NAND gateIt is equivalent to an ANDgate with its outputinverted by a NOT gate.Output Q is OFF wheninputs A and B are bothON

9.4

Logic gates as switching circuits in electronic systems

A logic gate is a circuit that has one or more input signals but only one output signal.

List, draw symbols and state the action of the logic gates


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5. NOR gateIt is equivalent to an ORgate with its outputinverted by a NOT gate.Output Q is ON when bothinput A and input B areOFF

Built truth tables for logic gates in combination for a maximum of 2 inputs


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1. There are smoke sensors and temperaturedetectors in hotels. If there is no smoke orhigh temperature in a room, the alarm will nottrigger. But if one of the inputs is HIGH orboth the smoke and the temperature sensorsare HIGH, then the alarm will trigger.Complete the truth table

2. Diagram 2 shows a simpleelectronic control system that isused as an automatic andmanual switches to switch onthe street lights when it is dark.The control system allows thestreet lights either to switch onmanually by a switch at any time,or automatically by a lightsensor when it is dark.Light sensor:daytime = 1Dark /night = 0

Diagram 2Complete truth table

Describe applications of logic gate control systems.


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3. A student would like to construct alogic gate circuit to switch on an airconditioner in his home automatically.He has chosen a light – sensitivedevice to detect daytime and nightand a heat – sensitive device to detectthe surrounding temperature. He hasconstructed a truth table to show thefunction of the logic gate circuit toswitch on the air conditioner.

Draw the logic gate circuit that will switchon the air conditioner based on thefunction stated in the truth table

Input P Input Q Output RNight = 0 Cold = 0 Off = 0Night = 0 Hot = 1 On = 1Day = 1 Cold = 0 On = 1Day = 1 Hot = 1 On = 1

4. You are given two sensors and aswitch.(a) temperature sensor A is hot =

ON (1) cool = OFF (0)(b) Light sensor B is sunlight =

ON night = OFFDesign a combination gate from thetruth table given below. Using anytwo of the given logic gates (NOT,AND and OR), place the gatecombination in the box provided.


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TUTORIAL 9.4

1. The diagram shows an electric circuit.

Which of the following symbol isequivalent to the circuit above?

2. The diagram shows an electric circuit.

Which of the following logic gate isequivalent to the circuit above?A AND gateB OR gateC NAND gateD NOR gate

3. The diagram shows a logic gate G whosetruth table is a shown in the table below.

M N X0 0 01 0 00 1 01 1 1

A AND gate B OR gateC NAND gate D NOR gate

4. Which of the following is equivalent to aNOT gate?

5. The diagram shows a circuit containingthree logic gates.

Which of the following logic gate isequivalent to the circuit above?A OR gate B AND gateC NOT gate D NAND gate

8 The diagram shows the combinationthree logic gates.

The truth table for the combination ofthree logic gates is as follows.

Input OutputP Q R0011

0101

1110

What is the gate X?A AND B ORC NAND D NOR


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9. The diagram shows a circuit containingfour NAND gates.

Which of the following logic gate isequivalent to the circuit above?

A OR gate B AND gateC NOR gate D NAND gate

10 A NOR gate with input signalsM = 01010101 and N = 011010100 . Whatis the output signal of the logic gate.A 01010101 B 10000010C 01111101 D 01000000

11 The diagram shows a circuit containingthree logic gates.

If the input signals P and Q arerespectively 1001 and 1100 what is theoutput signal at X?A 0001 B 0010C 1010 D 1110

12 The diagram shows a logic gates circuit.

Which of the following truth tablerepresents output R in the circuit?(2003)

A P Q R B P Q R0 0 1 0 0 00 1 0 0 1 11 0 1 1 0 01 1 0 1 1 1

C P Q RD

P Q R

0 0 0 0 0 00 1 1 0 1 11 0 0 1 0 11 1 0 1 1 0

13. The diagram shows a combination ofthree logic gates.

Which truth table is correct for thecombination of three logic gates?

A P Q R B P Q R0 0 1 0 0 01 0 1 1 0 00 1 1 0 1 11 1 1 1 1 0

C P Q RD

P Q R

0 0 0 0 0 11 0 0 1 0 10 1 0 0 1 01 1 0 1 1 1

14. The diagram shows the combination offour logic gates.

Which of the following is the correcttruth table for output X?

A X Y Q B X Y Q0 0 0 0 0 10 1 1 0 1 11 0 1 1 0 11 1 0 1 1 0


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C X Y QD

X Y Q

0 0 1 0 0 00 1 0 0 1 11 0 0 1 0 11 1 1 1 1 1

15. The diagram shows a circuit containingtwo logic gates.

Which of the following is the correcttruth table for output R?

A P Q RB

P Q R

0 0 0 0 0 00 1 0 0 1 11 0 0 1 0 01 1 1 1 1 0

C P Q R D X Y Q0 0 0 0 0 10 1 0 0 1 01 0 1 1 0 01 1 0 1 1 0

16 The diagram shows a circuit containingthree logic gates.

Which of the following is the correcttruth table for output R?

A P Q R B P Q R0 0 0 0 0 10 1 1 0 1 01 0 1 1 0 11 1 1 1 1 1

C P Q R D X Y Q0 0 1 0 0 10 1 0 0 1 01 0 1 1 0 01 1 0 1 1 1

17 The diagram shows the combination ofAND gate , OR gate and NOT gate.

Which of the following is the correcttruth table for output C?

A A B C B A B C0 0 0 0 0 10 1 0 0 1 11 0 0 1 0 11 1 0 1 1 1

C A B C D A B C0 0 0 0 0 10 1 1 0 1 01 0 0 1 0 11 1 1 1 1 0

18 The table shows a truth table for acombination of logic gates.

P Q R0 0 10 1 01 0 01 1 0

Which of the following is true to show acircuit according to the truth tableabove?


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19 The figure shows a OR gate with inputsignals P and Q.

Which of the following is the outputsignal of the OR gate?

20 The figure shows a OR gate with inputsignals P and Q.

Which of the following is the outputsignal of the OR gate?

21 The diagram shows a logic gate circuitwith input signals, K and L.

Which of the following output signals isproduced by the logic gate circuit?


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22 (a) The diagram shows a logic gate ,Lwith input signals A and B.

(i) Name the logic gate L.

_________________________

(ii) Draw the output signal C in thegraph below.

(c) Later , logic gate L is connected tologic gates M and N as shown inthe diagram below.

(i) Complete the truth table below.

X Y Z0 00 11 11 1

23 The diagram shows a circuit is used as asafety system in a house.

(a) Name logic gate R.

_________________________

(b) Using two switches , two dry cellsand a bulb draw a circuit thatequivalent to logic gate S.

(c) The following truth table representsthe logic gate in the figure above.

A B L M0 00 11 01 1

(i) Complete the truth table above.

(ii) What happen to door and thegate when the main switch isclosed?

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(iii) Give two methods to open thedoor and the gatesimultaneously.

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24(a) Figure shows a circuit made up froma combination of a few basic logicgates.

Figure (a)

(i) Based on the arrangement of thelogic gates in the Figure (a)complete the truth table below.

Input OutputK L M0 00 11 01 1

(ii) State a single logic gate whichproduced the same output asthe output in (a)(i).

………………………………………

(b) Figure(b) shows a circuit diagram ofa vending machine . The vendingmachine accepts 10 sen coins, 20sen coins and 50 sen coins. Threelight dependent resistors (LDR) , X ,Y and Z are used to check the sizeof coins slotted in the machine. Thelight sources of the slot machinewhere the coins are slotted are asshown in Figure (c).

Figure (b)

Figure (c)

(i) Complete the truth table in the tablebelow.

State of LDR Input OutputX Y Z K L M S

Nocoin 010

sen 020

sen Bright Dark Dark 0 1 1 050

sen 1

(ii) Name the gate that is probably inthe black box.

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SPM 2004 No. 725. Ali wishes to design a security system in

his house as shown in Figure 1.1. Whena person crosses the infrared beam atnight, the infrared sensor will send asignal to a logic gate which then lightsup a spotlight. The spotlight has a 240 Valternating current (a.c) supply and willonly light up at night.

Figure 1.1

The circuit that could help Ali todevelop the security system is shownin Figure 1.2.

Figure 1.2

(a) If the potential difference acrossresistor W is 2 V, what is thepotential difference across theinfrared sensor?

_____________________________[1]

(b) Component X has a high resistancewhen the surroundings are dark and alow resistance in bright surroundings.Name the component X.

_________________________________

(c) Component S is a switch thatconnects the 240 V a.c. spotlightcircuit.(i) What type of switch is component

S?

______________________________[1]

(ii) Why is component S necessary inthe circuit?

______________________________[1]

(d) The following truth table shows theoperation of a logic gate in the circuitin Figure 26.2

Key:Dark surroundings : high input voltageat P, logic ‘1’Bright surroundings : low input voltageat P, logic ‘0’Person crossing the path : high inputvoltage at Q, logic ‘1’Nobody crossing the path : low inputvoltage at Q, logic ‘0’Switch S is on : high output voltage atR, logic ‘1’Switch S is off : low output voltage atR, logic ‘0’

Input P Input Q

SurroundingsInput

voltageat P

Personcrossing

Inputvoltage

at Q

OutputR

Bright 0 No 0Bright YesDark NoDark Yes

(i) Using the given key, complete thetruth table above. [4]

(ii) Name the logic gate in the circuit inFigure 26.1.

______________________________[1]


213

SPM 2006 No. 726. A student wants to build s simple lift

motor control system which operatesusing two buttons, A and B, for a two-storey building.

A: Up buttonB: Down button

The lift motor only activates whensomeone presses any one of thebuttons.Figure (a) shows the circuit that can beused to activate the motor.

Figure (a)

Keys:

Buttons A and B:When pressed,logic “1”Not pressed, logic “0”X Output: Motor is activated,logic “1”

(a) The table is a truth table whichshows the operations of the logicgates in a lift motor.Using the keys given, complete thetable.

A B X0 00 11 01 1

(b) Why is a relay switch needed in thecircuit?

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9[1].4 logic gates

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