WELSH JOINT EDUCATION COMMITTEE CYD-BWYLLGOR ADDYSG CYMRU

General Certificate of Secondary Education Tystysgrif Gyffredinol Addysg Uwchradd

ELECTRONICS

TERMINAL EXAMINATION

HIGHER TIER

A.M. TUESDAY, 24 June 2003

(75 minutes)

VP*(0298/2)

298/02

Centre Number

Candidate’s Name (in full)

Candidate’s Examination Number

For Examiner’suse only.

1

2

3

4

5

6

7

8

9

10

Total

INSTRUCTIONS TO CANDIDATES

Write your centre number, name and candidate number in the spaces provided above.

Answer all questions.

Write all the answers in the spaces provided in this booklet.

INFORMATION FOR CANDIDATES

The number of marks is given in brackets at the end of each question or part-question.

No certificate will be awarded to a candidate detected in any unfair practice during theexamination.

2

INFORMATION SHEET

This information may be of use in answering the questions.

1. Resistor ColourCodes

BLACK 0

BROWN 1

RED 2

ORANGE 3

YELLOW 4

GREEN 5

BLUE 6

VIOLET 7

GREY 8

WHITE 9

The fourth bandcolour gives thetolerance as follows:GOLD ± 5%SILVER ± 10%

2. Preferred Values for Resistors

E 12 SERIES OF PREFERRED VALUES10; 12; 15; 18; 22; 27; 33; 39; 47; 56; 68; 82 and multiples thereafter

3. Resistance =

4. Effective resistance, R, of two resistors R1

and R2

in series is given by R = R1

+ R2.

5. Effective resistance, R, of two resistors R1

and R2

in parallel is given by

6. Voltage Divider

voltagecurrent

; R = VI

7. Power = voltage ¥ current;

8. LED The forward voltage drop across a LED is 2V.

9. Transistors

(i) Current gain =

(ii) The forward voltage drop across the base emitter junction is 0·7V.

R R RR R

1

1= +

2

2

P VI I R VR

2

= = =2 .

Collector currentBase current

h IIFE

C

B; =

+VS

0V

R1

R2 V R

R R VOUT 2

1 2S= ¥

+

.

.

.

(0298/2)

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Answer all questions in the spaces provided.

1. (a) The diagram shows a resistor with its four coloured bands.The resistor colour code is given in the information sheet on page 2.

(0298/2)

Brown Orange

Red Silver

Write down its resistance:

(i) in ohms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [2]

(ii) in kilohms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(b) The coloured bands on three resistors, 1, 2 and 3, show that their resistance should be 220Ω.The tolerance bands are gold.Their resistance is measured.

Complete the table to show whether each resistor is good or faulty. [2]

Resistor

1

2

3

Measured value of resistance

222Ω

208Ω

244Ω

Good or Faulty?

(c) Work out the total resistance for each of the combinations shown below.

(i) 3kW 6kW

Total resistance = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(ii) 3kΩ

6kΩ

Total resistance = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [2]

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Turn over.

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2. The block diagram for a burglar alarm is shown below.

(0298/2)

Lightsensing

unit

Pressuresensing

unit

X Latch

Resetswitch

BuzzerTransducerdriver

The burglar alarm has two sensors:1. a light-sensing unit which sets the alarm automatically only at night,2. a pressure switch, under the door mat.

(a) The light sensing unit outputs logic 0 in darkness, and logic 1 in daylight.The pressure sensing unit outputs logic 0 when pressed, and logic 1 when released.

X is a logic gate.It must output a logic 1 only when someone stands on the doormat when it is night-time.

(i) Complete the truth table. [1]

Light sensing unit

0

0

1

1

Pressure sensing unit Output of X

0

1

0

1

(ii) What logic gate must be used in sub-system X? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(b) Part of the latch circuit is shown below.Draw the four connections needed to make the latch set and reset. [4]

5V

Signal fromsub-system

X

D Q

R

0V

Resetswitch

Output oflatch

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3. (a) Here are the symbols for five types of logic gates, labelled A, B, C, D and E:

(0298/2)

A B C D E

(i) Which is the symbol for an OR gate? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(ii) Which of these logic gates gives the opposite behaviour to an OR gate?

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(iii) Which of these logic gates gives an output of logic 1 only when both inputs are at

logic 1? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(b) A

BP

Q

Complete the truth table for the logic system shown above. [2]

A

0

0

1

1

B

0

1

0

1

P Q

(c) (i) Redraw the system replacing the logic gates with their NAND gate equivalents. [2]

(ii) Draw a line through any redundant gates. [1]

Turn over.

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4. The following diagram shows a resistor-capacitor network.

(0298/2)

9V

0V

Before the switch is closed, the capacitor is not charged.

(a) Which of the following statements is true?After the switch is closed

A. the voltage VC decreases.B. the voltage VC stays the same.C. the voltage VC increases.

Answer . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(b) This circuit can be used to produce a time delay in a monostable circuitWhich of the following combinations of resistor and capacitor will produce the biggest timedelay?

A. R = 10kΩ C = 1 mFB. R = 10kΩ C = 10 mFC. R = 1kΩ C = 1 mFD. R = 1kΩ C = 10 mF

Answer . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(c) Here is the circuit diagram for a monostable, used in an egg timer.

0V

9V

330mF

4 8

3

7

6

555

1

2Input Output

R

C VC

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(0298/2)

The variable resistor has a maximum resistance of 1MΩ.Use the formula:

T = 1·1RC

to calculate the maximum cooking time for the egg, in seconds. [2]

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Turn over.

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5. A transistor is used to switch on a 100mA buzzer when the light level gets too bright.The circuit diagram for this system is shown below.

(0298/2)

6V

6V 100mABuzzer

0V

V1

V3

V2

(a) The table gives information about four transistors.

Which transistor A,B,C, or D will drive the buzzer correctly and draw the smallest possiblecurrent from the light sensing unit?

(i) Answer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [2]

(ii) For your chosen transistor in (i), calculate the base current when the collectorcurrent is 100mA. [1]

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Transistor

A

B

C

TO18

E-line

TO39

TO5

100

200

50

40

300mA

50mA

800mA

1·2AD

PackageCurrent gain

hFE

MaximumCurrent

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(0298/2)

(b) The transistor is switched on (saturated). Find the value of

(i) the voltage V1, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(ii) the voltage V2, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(iii) the voltage V3, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(iv) the power dissipated in the buzzer, [2]

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(v) the power dissipated in the transistor. [1]

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Turn over.

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(0298/2)

6. The block diagram for PA (public address) amplifier system is shown below.

Microphone . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Microphone

Loudspeaker

(a) (i) Identify the unnamed blocks, using sub-systems from the following list.Write your answers on the dotted lines provided. [3]

preamplifier voltage divider power amplifier

comparator Schmitt trigger mixer

Hint: One of these sub-systems is used twice.

(ii) How does the job of the preamplifier differ from that of the power amplifier? [2]

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(b) Here are four possible bandwidth choices for this PA amplifier system.

A. 100Hz to 12kHz

B. 12Hz to 50kHz

C. 50Hz to 100kHz

D. 100Hz to 1MHz

(i) Which is the best choice for this amplifier system? . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(ii) Give a reason for your answer to part (i) [1]

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7. (a) Draw the circuit diagram for a non-inverting amplifier, which makes use of an op-amp andtwo resistors. [3]

+

_

(b) Calculate suitable values for the resistors so that the amplifier has a voltage gain of 40.Label the resistors, on your circuit diagram, with the values you have chosen. [2]

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Turn over.

12 Examineronly

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(0298/2)

(c) The output of this amplifier saturates at ± 10V.A signal is applied to the input of the amplifier. This is shown in the diagram below.Draw the output signal produced by the amplifier, using the axes provided. [2]

0

10

20

–10

–20

Voltage/mV

Input Time

0

200

400

–200

–400

600

–600

Voltage/mV

Output Time

13 Examineronly

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(0298/2)

8. The block diagram for a simple radio receiver is shown below.

A

B C D

(a) Identify the sub-systems labelled A, B, C and D. [3]Choose your answers from the following list:

demodulator aerial headphones tuned circuit

Sub-system A = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Sub-system B = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Sub-system C = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Sub-system D = . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

(b) Which sub-system:

(i) picks up the radio waves from free space: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(ii) selects the desired radio station; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(iii) is the output device; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(iv) contains a diode; . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(v) contains a variable capacitor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . [1]

(c) What is the name of the component in this radio receiver that allows current to flow throughit in one direction only? [1]

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Turn over.

14 Examineronly

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(0298/2)

9. Two processes are needed to digitise an analogue signal.The signal must first be sampled to produce a PAM (pulse amplitude modulated) signal.The PAM signal is then converted to a binary number by an ADC.

(a) What does the abbreviation ADC stand for?

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(b) The graph shows an analogue signal, which is being sampled at the times shown.

Voltage / V

Firstsamplingpulse

Secondsamplingpulse

Thirdsamplingpulse

Complete the table to show the voltages of the PAM signal. [2]

Sampling pulse

First

Second

Third

PAM voltage

(c) The PAM signal is then processed by the ADC.The next table shows the binary output for various values of PAM voltage.Some rows have already been completed. Use this information to complete the rest. [2]

PAM voltage

0·1V

0·2V

0·3V

0·4V

1·0V

ADC binary output

0001

0010

0011

Time0

0.4

0.8

1.2

10. Passengers travelling on an underground railway in a big city use a lift to travel between streetlevel and the railway platform, below ground.

Part of the lift control system counts the number of people entering the lift. When 10 people haveentered the lift, it switches on a ‘Lift Full’ sign. The remainder of the control program then takesover.

Draw a flowchart for the first part of the control system, using the following instructions: [7]

Instructions: Does the counter = 10?Has someone entered?

Reset the counterSwitch on the ‘Lift Full’ sign

Add 1 to the counter

15 Examineronly

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(0298/2)

Start

No

Yes

Rest of programme

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