place your candidate electronics tasc exam...2. answer all questions. 3. answers must be written in...

© Copyright for part(s) of this examination may be held by individuals and/or organisations other than the Office of Tasmanian Assessment, Standards and Certification.

Pages: 32

Questions: 16

Attachment: Information Sheet

Tasmanian Certificate of Education

External Assessment 2019

ELECTRONICS (ELT315114)

PART 1 Time allocated: 90 minutes

On the basis of your performance in this examination, the examiners will provide results on each of the

following criteria taken from the course document:

Criterion 1 Apply knowledge and skills in designing, testing, building, and experimenting with circuits.

Criterion 4 Apply knowledge and understanding of digital and analogue circuits and their components.

Section Total

Criterion 1 /45

Criterion 4 /45

PLACE YOUR CANDIDATE

LABEL HERE

Candidate Instructions

1. You MUST make sure that your responses to the questions in this examination paper will

show your achievement in the criteria being assessed.

2. Answer ALL questions.

3. Answers must be written in the spaces provided on the examination paper.

4. You should make sure you answer all parts within each question so that the criterion can

be assessed.

5. This examination is 3 hours in length. It is recommended that you spend approximately 90

minutes in total answering the questions in this booklet.

6. The 2019 External Examination Information Sheet for Electronics can be used throughout

the examination.

7. No other written material is allowed into the examination.

8. All written responses must be in English.

ELT315114 of 32

BLANK PAGE

ELT315114 of 32

When candidates are asked to ‘show that’:

A candidate should calculate their own answer to three significant figures and use this

subsequently.

A candidate who is unable to determine the required value should use the value given by

the examiner in subsequent questions.

A spare set of diagrams has been provided in the back of the answer booklet for you to use if

required (pages 28 - 30).

If you use the spare diagrams, you MUST indicate you have done so in your answer(s) to these

questions, and cross out the original diagram answered.

Additional Instructions for Candidates

ELT315114 of 32

This section assesses Criterion 1.

Answer ALL questions in this section.

Question 1

A qualified technician removes the metal casing of a signal generator which is not working.

(a) The red (live) wire from the mains plug is connected to a fuse.

Name an instrument which could be used to test if the fuse is broken. (1 mark)

……………………………………………………………………………………………………..

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(b) The broken fuse has 100 mA written on it.

State the danger of using a 5 A fuse as a replacement. (1 mark)

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(c) Explain what might happen if a 50 mA fuse was used as a replacement. (1 mark)

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(d) State TWO different effects on the body that could occur if the technician were to touch

the mains live wire while the power was on. (2 marks)

Effect 1 - ………………………………………………………………………………………….

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Effect 2 - ………………………………………………………………………………………….

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Question 1 continues.

SECTION A

For

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ELT315114 of 32

Question 1 (continued)

(e) Name the devices described below (Table 1) and draw their circuit symbols. (3 marks)

Table 1

Description Name Symbol

A thin piece of wire that is designed

to melt if too much current flows

through it

A polarised device that can store a

lethal charge even after it has been

disconnected from the electrical

supply

A device which detects a small

current difference between the

Active and Neutral wires, and

almost immediately ‘shuts down’

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No answer required

in this square

ELT315114 of 32

Question 2

An audio amplifier is connected to a signal generator and an oscilloscope for testing. The

signal generator is set at 100 mV at about 500 Hz, so that the output of the amplifier being

tested can be inspected for frequency, amplitude and possible distortion.

(a) List in about 5 steps how you would manually set up the oscilloscope (cathode ray,

CRO, or digital, DSO) to suitably display about 3 cycles of the amplifier output (so the

frequency and amplitude can be measured).

YOU MUST NOT USE ANY AUTO (Setup) CONTROLS. (3 marks)

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(b) Considering the oscilloscope controls available, describe the effect on the waveform

displayed if each of the following changes is made: (3 marks)

(i) the ‘Horizontal’ control is increased, for example from 10 to 50.

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(ii) the ‘Vertical’ control is increased, for example from 10 to 50.

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(iii) the ‘Trigger’ control is changed.

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ELT315114 of 32

Question 3

(a) Two items of test equipment are an oscilloscope (CRO or DSO) and a multimeter.

Suggest a test situation where each would be suitable. (2 marks)

Oscilloscope

Multimeter

(b) A printed circuit board (PCB) is suspected of having a break in one of its tracks. The

section of diagram below (Figure 3) shows the probes of a multimeter used to test the

PCB.

Figure 3

(i) State the most suitable function setting for the multimeter to test the PCB. (1 mark)

……………………………………………………………………………………………………...

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(ii) With the probes as shown and the multimeter set to the most suitable function

and range, the display was observed to read 0.00.

Explain what this reading indicates about the PCB tracks. (1 mark)

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ELT315114 of 32

Question 4

A burglar alarm system for a garden shed makes use of a sensor fitted to the door and a sensor fitted to the window. The system has the following specifications:

The alarm system only operates when the outside light level drops below a pre-set value.

When the door is opened, the sensor produces a logic 1.

When the window is opened, the sensor produces a logic 1.

The alarm is triggered when either the door or the window is opened.

The battery operated system turns on a mains powered alarm when activated.

(a) Draw a system diagram (using only the subsystems given below) to show how this alarm should operate. (5 marks)

alarm AND gate comparator door sensor

driver light level

detector OR gate relay

light level

reference

window

sensor


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ELT315114 of 32


(b) (i) Name the subsystem in which an analogue signal is being converted to a digital

signal. (1 mark)

…..………………………………………………………………………………………………...

(ii) Name the subsystem that could contain an op-amp. (1 mark)

……………………………………………………………………………………………………..

(iii) Name a component that could be used to detect the light in the light level

detector subsystem. (1 mark)

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ELT315114 of 32

Question 5

(a) This table (Table 5) shows the characteristics of two operational amplifiers X and Y.

One is a general-purpose device whereas the other is for more critical applications.

Characteristic X Y

Input impedance (MΩ) 2 4

Output impedance (Ω) 100 10

Open-loop Gain (Volts/V) 15k 4M

Unity Gain Bandwidth (MHz) 1 14

Max Supply Voltage (V) ± 15 ± 18

Output Voltage Swing (V) ± 12 ± 14

Slew Rate (VІμV) 0.5 4.5

Which op amp, X or Y, is nearer to being ideal?

Give two reasons for your answer. (2 marks)

Reason 1: ………………………………………………………………………………………...

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Reason 2: ………………………………………………………………………………………..

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Consider the op amp X in a circuit without feedback (Figure 5). The inverting input is at 0 V

while the non-inverting input is at 1.0 mV.

Figure 5

(b) With reference to the data given in (a) above, explain carefully what the output voltage

is likely to be. (2 marks)

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Table 5

ELT315114 of 32

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ELT315114 of 32

Question 6

When designing electronic circuits, the correct operation of the circuit must be tested before

manufacture. The two main ways would be using a solderless breadboard and using CAD

software.

Figure 6

(a) State two reasons for using CAD software to test a circuit. (2 marks)

1. …………………………………………………………………………………………………

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2. …………………………………………………………………………………………………

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(b) Describe two advantages, other than cost, of using a breadboard instead of using

CAD software. (2 marks)

1. …………………………………………………………………………………………………

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2. …………………………………………………………………………………………………

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Figure 6

ELT315114 of 32


(c) You are required to answer Question 6 (c) (i) OR (c) (ii)

EITHER:

(i) PCB design software will often allow information to be saved in a way that is

suitable for directly operating a CNC drill or milling machine (router).

Explain why this method is mainly used for batch production of circuit boards

rather than for job (one off) production. (2 marks)

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OR:

(ii) Chemicals used in the manufacture of PCBs can be hazardous.

State two considerations when working with and properly disposing of

the chemicals.

(2 marks)

1. …………..…………………………………………………………………………………….

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2. ……………..………………………………………………………………………………….

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ELT315114 of 32

Question 7

A warning device indicates that a car's headlights have been left on.

Figure 7.1 shows the circuit diagram and breadboard layout.

Figure 7.1

(a) Describe the changes to the breadboard layout that will be needed to make the circuit

work. (2 marks)

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(b) Figure 7.2 below shows details of suitable buzzers for the headlight warning device from a supplier's

catalogue.

Figure 7.2

(i) Explain which buzzer with leads (mini piezo buzzer or surface mount buzzer) you

would recommend be used for connection to a circuit board.

The fixing should not be permanent, but it should be secure against vibration.

(2 marks)

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(ii) Use sketches to show how that buzzer with leads can be fixed to a circuit board.

(1 mark)

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ELT315114 of 32

Question 8

The car headlight warning device referred to in Question 7 above has 3 inputs to its logic

system:

Input 1 – Any lights left on give a logic 1 (High) signal.

Input 2 – The engine switched off gives a logic 0 signal.

Input 3 – the driver’s door open gives a logic 0 signal.

Figure 8.1 shows the PCB layout for the logic system, which is built from four NOR gates.

Figure 8.1

Use the above information to complete Figure 8.2 below to show the circuit diagram of the

logic gates. (4 marks)

Figure 8.2

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ELT315114 of 32



Question 9

Two resistor networks each contain three resistors all with equal resistance. Network A has

resistors connected in a series whilst Network B has its resistors connected parallel to each

other. Each network is connected to a large 12 V battery.

(a) Which network drains the battery faster? (1 mark)

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(b) How does the current through the second resistor of each network compare to the total

current flowing through the each network? (2 marks)

Network A: …………………………………………………………………………………………

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Network B: …………………………………………………………………………………………

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(c) If Network A is connected in series with Network B across the same battery, which

network’s resistors will heat up more, and why? (1 mark)

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SECTION B

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ELT315114 of 32

Question 10

The diagram below (Figure 10.1) represents a transformer to be used as part of a power

supply that steps-down the input mains voltage (240 VRMS) to an output of 16 VRMS.

The diagram is not to scale and the coil loops are shown simplified.

Figure 10.1

240 VRMS 16 VRMS

(a) Calculate the peak input voltage and the peak output voltage. (1 mark)

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(b) The voltage of the primary (input) coil changes over time and is shown in the upper

graph of Figure 10.2 below.

Sketch the output voltage of the secondary coil on the lower graph, ensuring to include a

vertical scale and label for both graphs. (2 marks)

Voltage

time

Figure 10.2


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ELT315114 of 32


(c) Show by way of a circuit diagram how the 16 VRMS output can be rectified to a smooth

DC output using additional components. (3 marks)

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ELT315114 of 32

Question 11

Below is a circuit diagram for a binary ripple up-counter (Figure 11.1).

Figure 11.1

(a) Are the R inputs on the flip-flops represented above active high or active low? (1 mark)

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(b) What is the number of possible output states? (1 mark)

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(c) Which feature indicates that the circuit is a ripple counter rather than a synchronous

counter? (1 mark)

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(d) Which one of the three outputs is the MSB? (1 mark)

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(e) On which edge of the clock signal are these flip-flops triggered? (1 mark)

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(f) Complete this timing diagram for the counter through to and including Q2 output.

(3 marks)

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Q0

Q1

ELT315114 of 32

Question 12

(a) Complete this table of capacitor values: (2 marks)

pF code nF µF

221

0.150

3n3

(b) Consider this circuit (Figure 12.1):

Figure 12.1

(i) Which components are directly grounded? (1 mark)

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(ii) Which component is/components are not directly connected to the output? (1 mark)

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(iii) What is connected to the base of the transistor? (1 mark)

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(c) Three capacitors, 1.0µ, 220n and 330n are available. Show diagrammatically how any

two of these capacitors are combined to give a: (calculations are not required)

(i) maximal total capacitance (1 mark)

(ii) minimal total capacitance (1 mark)

/7

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ELT315114 of 32

Question 13 The following circuit (Figure 13.1) was designed to pick up the pulse of a clock motor as it

energises to drive the second hand of a clock, then amplify the pulse and create a 1 Hz

square wave at the output of IC1f.

Figure 13.1

(a) What is the name of the component L1 which senses the pulse? (1 mark)

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(b) Initially which components are used to amplify the pulse from L1? (1 mark)

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(c) What is the function of C3? (1 mark)

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Although the output was found to be exactly 1 Hz, the pulse width was too narrow (below left)

and needs to be widened as shown below right (see Figure 13.2):

T= 1 second

Figure 13.2

(d) Suggest additional circuits or components that will widen the pulses without altering the

frequency. (2 marks)

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OUTPUT

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ELT315114 of 32

Question 14

Consider this 7 segment common cathode display pin diagram

(Figure 14.1):

Figure 14.1

(a) Complete this table for the omitted cells. (Note: dp = ∙ ) (2 marks)

Digit to Display ∙ g f e d c b a Hex code

0 0011 1111 3F

1 0000 0110

2 5B

9

(b) Which segments need to be lit to display the letter ‘H’? Give its hex code.

(1 mark)

…………………………………………………………………………………………

…………………………………………………………………………………………

Circuit A has a BCD to 7 Segment Decoder (4511) directly wired to the display.

Circuit B has a BCD to 7 Segment Decoder (4511) with separate 220 ohm resistors wired to

the display (Figure 14.2).

Circuit A Circuit B

Figure 14.2

(c) State one advantage for Circuit A over Circuit B. (1 mark)

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(d) State one advantage for Circuit B over Circuit A. (1 mark)

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+V

V110V

4511D3D2D1D0ELBILT g

fedcba

U1

ab

cd

ef

g.

Gnd

DISP1

R5

R2

R1

220

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ELT315114 of 32

Question 15

Consider this circuit (Figure 15.1):

https://www.epemag3.com/lib/free_projects/general/1002-%20EPE%20Bounty%20Treasure%20Hunter.pdf

Figure 15.1

(a) What are the components IC1a-f (six small triangles)? (1 mark)

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(b) Describe the output at pin 4 of the component IC1b (encircled). (1 mark)

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(c) Which components control the gain of IC2a (centre)? (1 mark)

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https://www.epemag3.com/lib/free_projects/general/1002-%20EPE%20Bounty%20Treasure%20Hunter.pdf

ELT315114 of 32

Question 16

The 7 LED die-face circuit below (Figure 16.1) can simulate the rolling of a 6-sided die, and

can be controlled with a PICAXE 08 microcontroller.

When the button is pushed, a pattern for each number ‘rolled’ can be seen.

Figure 16.1

The circuit diagram (Figure 16.2) shows how the die-face LEDs are connected to the outputs

of the PICAXE IC.

Figure 16.2

The code on the next page has a loop which counts through all the possible dice patterns

quickly so they cannot be seen.

Line numbers (‘1-‘21) have been added as comments for ease of reference to your answers.


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ELT315114 of 32

Question 16 (continued) main:

low 0 ‘1

low 1 ‘2

low 4 ‘3

high 2 ‘4

if input3 is on then display ‘5

low 2 ‘6

high 4 ‘7


high 2 ‘9


low 2 ‘11

high 1 ‘12


high 2 ‘14


low 2 ‘16

high 0 ‘17


goto main ‘19

display:

wait 2 ‘20

………………………. ‘21

(a) In the main program how is the display number ‘2’ achieved? (2 marks)

………………………………………………………………………………………………………

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(b) Write the code missing from line 21 to ensure that after displaying the number the

program continues. (1 mark)

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(c) How long (in seconds) is a number displayed when the button is pushed? (1 mark)

………………………………………………………………………………………………………

(d) How many LEDs are on when the button is NOT pressed? (1 mark)

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(e) There is a second way to display the digit ‘3’. How? (1 mark)

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(f) Compare the current drawn from the supply voltage when displaying the value for ‘1’

with when displaying ‘6’. (1 mark)

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ELT315114 of 32

Question 4 (a) Draw a system diagram (using only the subsystems given below) to show how this

alarm should operate.

alarm AND gate comparator door sensor

driver light level

detector OR gate relay

light level

reference

window

sensor

Question 8

Figure 8.2

SPARE DIAGRAMS

ELT315114 of 32

Question 10

(b) The voltage of the primary (input) coil changes over time and is shown in the upper

graph of Figure 10.2 below.

Sketch the output voltage of the secondary coil on the lower graph, ensuring to include

a vertical scale and label for both graphs. (3 marks)

Figure 10.2

(c) Show by way of a circuit diagram how the 16 VRMS output can be rectified to a smooth

DC output using additional components. (3 marks)

ELT315114 of 32

Question 11

(f) Complete this timing diagram for the counter through to and including Q2 output.

(2 marks)

Question 12

(c) Three capacitors, 1.0µ, 220n and 330n, are available. Show diagrammatically how any

two of these capacitors are combined to give a:

(i) maximal total capacitance (1 mark)

(ii) minimal total capacitance (1 mark)

Question 14

(a) Complete this table for the omitted cells. (Note: dp = ∙ ) (2 marks)

Digit to Display ∙ g f e d c b a Hex code

0 0011 1111 3F

1 0000 0110

2 5B

9

Q0

Q1

ELT315114 of 32

BLANK PAGE

ELT315114 of 32

This

examination paper and any materials associated with this examination (including answer booklets, cover sheets, rough note paper,

or information sheets) remain the property of the Office of Tasmanian Assessment, Standards and Certification (TASC).

© Copyright for part(s) of this examination may be held by individuals and/or organisations other than the Office of Tasmanian Assessment, Standards and Certification.

Pages: 36

Questions: 14

Attachment: Information Sheet

Tasmanian Certificate of Education

External Assessment 2019

ELECTRONICS (ELT315114)

PART 2 Time allocated: 90 minutes

On the basis of your performance in this examination, the examiners will provide results on each of the

following criteria taken from the course document:

Criterion 5 Apply knowledge of digital and analogue systems in describing the function and operation of components and circuits.

Criterion 7 Apply knowledge and understanding of mathematical concepts in electronics

Section Total

Criterion 5 /45

Criterion 7 /45

Candidate Instructions

1. You MUST make sure that your responses to the questions in this examination paper will

show your achievement in the criteria being assessed.

2. Answer ALL questions.

3. Answers must be written in the spaces provided on the examination paper.

4. You should make sure you answer all parts within each question so that the criterion can

be assessed.

5. This examination is 3 hours in length. It is recommended that you spend approximately 90

minutes in total answering the questions in this booklet.

6. The 2019 External Examination Information Sheet for Electronics can be used throughout

the examination.

7. No other written material is allowed into the examination.

8. All written responses must be in English.

PLACE YOUR CANDIDATE

LABEL HERE

ELT315114 Page 2 of 36

BLANK PAGE


When candidates are asked to ‘show that’:

A candidate should calculate their own answer to three significant figures and use this subsequently.

A candidate who is unable to determine the required value should use the value given by the examiner in subsequent questions.

A spare set of diagrams has been provided in the back of the answer booklet for you to use if required (pages 32 - 35).

If you use the spare diagrams, you MUST indicate you have done so in your answer(s) to these questions, and cross out the original diagram answered.

Additional Instructions for Candidates




Question 17

A student wishes to design a three-way (tweeter, midrange, woofer) speaker system to filter

audio frequencies from 20 – 20 000 Hz with frequency-crossover points at 100 Hz and 1400

Hz.

The lowest frequencies are to go to an amplified woofer (W) with a rated safe-range 2 Hz - 100 Hz. The highest frequencies are to go to an amplified tweeter (T) with a rated safe-range 1400 Hz- 60 kHz. All frequencies between the crossover points are to be amplified to drive a midrange speaker (M). The student’s design has five (5) filters. The first is a 2 Hz high-pass which blocks all sub-sonic frequencies and the four (4) others are as shown with their cut-off frequencies in figure 17.1.

Figure 17.1

(a) Use the four (4) function blocks in Figure 17.1 to illustrate the connections between the

five (5) filters and the three (3) amplified speakers in the student’s design. (3 marks)

Figure 17.2


SECTION A

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One of the filters in the system contains the following component configuration (Figure 17.3):

Figure 17.3

(b) What is this configuration called? (1mark)

……………………………………………………………………………………………………..

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(c) What is the gain of this filter? (1 mark)

……………………………………………………………………………………………………..

……………………………………………………………………………………………………..

(d) Explain the advantage(s) of using this configuration over a simple passive filter.

(2 marks)

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(e) Which of the filters (see Figure 17.1) is shown in Figure 17.3? (1 mark)

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Question 18

Consider the circuit represented below (Figure 18.1), supplying voltage to a load.

Figure 18.1

The waveform that could be observed across the input of the 7812 linear regulator with an

oscilloscope is shown as a graph below (Figure 18.2).

Figure 18.2

(a) On the graph above (Figure 18.2) draw the output of the regulator.

Add a vertical scale to the graph. (2 marks)

In this photo (Figure 18.3), four additional components have

been added to a breadboarded version of the regulator

circuit.

(b) Show the four new circled components as symbols on

the partial schematic below (Figure 18.4). (2 marks)

Figure 18.3

Figure 18.4


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(c) Often it is advised to use a transformer with a higher voltage output to drive the

regulator circuit. Explain reasons for this advice. (2 marks)

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Another method to regulate voltages includes a zener diode-based design (Figure 18.5).

Figure 18.5

(d) Contrast the zener diode design to a linear regulator design. (2 marks)

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Question 19

The following schematic diagram (Figure 19.1) shows an incomplete logic system.

Figure 19.1

(a) Using Boolean algebra, write the expression for point D and point E in terms of the

inputs A, B and C. (2 marks)

D = ……………………………………………………………………………………………….

E = ……………………………………………………………………………………………….

(b) When complete, the logic system is designed to detect the prime numbers 2, 3, 5 and 7

presented at the inputs as a binary number CBA.

Output Q is to become logic 1 when a prime number is detected.

(i) Complete the truth table below (Table 19.1) for the completed intended logic system.

(2 marks)

Decimal number C B A D E F Q

0 0 0 0

1 0 0 1

2 0 1 0

3 0 1 1

4 1 0 0

5 1 0 1

6 1 1 0

7 1 1 1

Table 19.1

(ii) Complete the logic system schematic diagram Figure 19.1 so that it works as

described above, by including a logic gate (or gates) after points E and F. (1 mark)


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(c) A second output, R, is to be added so that it will be equal to logic 1 when an odd

number is at the inputs CBA.

On the previous schematic diagram Figure 19.1 show this new output. (1 mark)

(d) A third output, S, is to be added, which is to be equal to logic 1 when the following

condition is met:

S = �̅� ∙ (B + C)

State which decimal numbers in the truth table above (Table 19.1), when presented as

CBA at the inputs would cause output S to be equal to logic 1. (1 mark)

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ELT315114 Page 10 of 36

Question 20

An LDR is being used as a light sensor in a system that will switch a carport light ON when it

gets dark. The characteristic for the LDR is graphed as shown in Figure 20.1.

Figure 20.1

When the daylight intensity is below 20 000 Lux the carport is considered dark and unsafe.

(a) What is the resistance of the LDR when the light intensity falls to 20 000 Lux? (1 mark)

……………………………………………………………………………………………………..

This circuit (Figure 20.2) represents a partial solution to controlling the carport light (LED).

Figure 20.2

(b) What is the voltage at point X at 20 000 Lux? (2 marks)

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(c) What value for R1 would allow the voltage at point Y be the same as the voltage in

your answer to part (b)? (1 mark)

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(d) Select a suitable E12 value for resistor R1. (1 mark)

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(e) Show on Figure 20.2 above the connections from points X and Y to the op-amp inputs

so that the carport LED switches on when the light level falls below the described

value in part (a). Justify your answer. (2 marks)

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(f) (i) If the op-amp used is not ideal, describe what problem might happen to the LED

when the light level is around 20 000 Lux. (1 mark)

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(ii) Suggest how the problem could be solved. (1 mark)

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ELT315114 Page 12 of 36

Question 21

Consider this circuit diagram for a lap counter for two-track slot car racing (Figure 21.1).

IC5 is an LM1458 op amp IC.

Output pin 1 will drive the track 1 counter for a

car’s total laps and output pin 7 will drive the

track 2 counter for the other car’s total laps.

The Hall sensor will provide a voltage of about

3V when a car carrying a small magnet passes

over it and about 2V without a magnetic field.

(C)Silicon Chip 2002

Figure 21.1

(a) What is the supply voltage for the circuit? (1 mark)

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(b) Why are pins 2 and 6 tied together then joined to 10k and 12k resistors? (2 marks)

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(c) Why are there no feedback resistors on the two op amps IC5a and IC5b? (1 mark)

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(d) Pin 6 is supplied with about 2.5 volts. Describe the changes that occur at pin 5 and

pin 7 when a car passes the sensor on track 2. (2 marks)

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ELT315114 Page 14 of 36

Question 22

This circuit (Figure 22.1) allows a normal clock to lose 50 minutes

a day, and be useful as a high (sea) tide indicator. Part of this

circuit shuts off an electric clock for 5 seconds out of every 144 s,

making the clock lose exactly 50 minutes every 24 h, so high tides

match the clock reading.

The design uses LS90 Decade and LS92 Divide-By-Twelve

counters. Part of this is shown in Figure 22.1. Figures 22.2 and

22.3 indicate pins and functions for each of these ICs.

Points labelled A, B, C and D indicate input and output testing

positions.

Figure 22.1

Divide-By-Ten Counter (LS90) — The Q3 output must be externally connected to the CP0

input ie pin 11 to pin 14. The input count is then applied to the CP1 input, pin 1 and a divide-

by ten square wave is obtained at output Q0, pin 12.

Figure 22.2


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ELT315114 Page 15 of 36


Divide-By-Twelve Counter (LS92) — Join pin 1 to pin 12.

The CP0 input receives the incoming count and Q3 produces a divide-by-twelve square wave

output.

Figure 22.3

(a) What is the supply voltage for the counter’s ICs? (1 mark)

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(b) What types of flip-flop are used in the LS90 counter IC? (1 mark)

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(c) At point A, a 60 Hz square wave is applied to pin 14 of IC1 (LS92).

What is the effect of this wave on IC1? (1 mark)

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(d) Why are pins 6, 7 and 10 grounded on each of the ICs? (2 marks)

(i) Pin 10: ……………………………………………………………………………………

……………………………………………………………………………………………

(ii) Pin 6 & 7: ………………………………………………………………………………

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The following block (system) diagram Figure 22.4 shows how the 60 Hertz signal is divided

to give a square wave with a period of 24 seconds at point D.

Figure 22.4

(e) Show on the diagram below (Figure 22.5) how the circuit can be redesigned for a

50 Hz signal, so that point D still has a 24 second period. (2 marks)

Figure 22.5

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ELT315114 Page 17 of 36



Question 23

Two resistors are connected in series as shown

in Figure 23.1:

Figure 23.1

(a) Show that the voltage V1 is approximately 4.7 V. (1 mark)

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A 1k resistor is then added to the circuit, in two different configurations as shown.

Figure 23.2 (A) Figure 23.3 (B)

(b) Calculate the output voltage V2. (2 marks)

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(c) If the 1k resistor is moved as shown in circuit B, describe the effect on the output

voltage. (1 mark)

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SECTION B

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A B

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ELT315114 Page 18 of 36

Question 24

Shown below is a CR combination and a charging curve for this circuit (Figures 24.1, 24.2).

The capacitor was initially uncharged; at t = 0, the switch was closed.

Figure 24.1 Figure 24.2

(a) What was the supply voltage? (1 mark)

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(b) The time constant is about 0.2 s. Show that the capacitor has a value of

approximately 17 μF. (1 mark)

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(c) Suppose C were replaced by a capacitor of twice the value. On the graph above

(Figure 24.2), show the expected charging curve for this new combination. (1 mark)

An LR combination is now used (Figure 24.3), with the same supply voltage.

Figure 24.3

(d) Show that the time constant for this LR circuit is approximately the same as for

the original CR circuit. (1 mark)

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(e) A copy of the original capacitor charging curve graph is shown below.

On this graph (Figure 24.4) sketch the expected voltage ~ time curve for the inductive

circuit shown above. (2 marks)

Figure 24.4

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ELT315114 Page 20 of 36

Question 25

A band-pass filter has an input signal of constant 5.0 V with variable frequency.

The bandwidth is 500 Hz and the lower cut-off frequency is 3.0 kHz. At centre

frequencies, the maximum power output delivered from the filter is 200 mW.

(a) Determine, at cut-off frequency:

(i) the output power (1 mark)

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(ii) the output voltage (1 mark)

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(b) Sketch a labelled Vout ~ freq graph for this filter, indicating all relevant features.

Include: (3 marks)

the pass region

bandwidth

cut-off frequencies

half-power points

voltage at cut-off points

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ELT315114 Page 21 of 36

Question 26

You must show your working for this question.

(a) Convert the binary number 10 11002 to:

(i) hexadecimal (1 mark)

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(ii) decimal (1 mark)

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(b) This number, 10 11002, is to be added to the number 11 10102.

Use binary addition to add the numbers, and show that the answer (in decimal value) is

correct. (3 marks)

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ELT315114 Page 22 of 36

Question 27

Figure 27.1

(a) Calculate the output voltage of the above circuit (Figure 27.1) if the input voltage is

5.0 V. (2 marks)

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The following circuit (Figure 27.2) represents a simple Digital-to-Analogue Converter (DAC).

Each of the four inputs at A, B, C and D can be either 0 V or 5.0 V, represented by

0 and 1 respectively.

D is the most significant bit.

Figure 27.2


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(b) Find Vout if the input is:

(i) 00102 (1 mark)

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(ii) A16 (2 marks)

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(c) What is the greatest magnitude (value) of the output voltage? (1 mark)

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ELT315114 Page 24 of 36

Question 28

A simple power supply has an unregulated DC output. Unfortunately there is ripple in the

output voltage; the ripple increases as the load current increases (Figure 28.1).

Figure 28.1

A simple regulator can be constructed with a zener diode and 100R resistor which

will regulate the output voltage, i.e. keep it at a constant voltage of 12 V (Figure 28.2).

Figure 28.2

To maintain a constant 12 V across the zener diode, a minimum of 2.0 mA must flow

through it.

(a) Show that the maximum current which can flow though this 400 mW zener

without it overheating is approximately 30 mA. (1 mark)

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(b) Determine the minimum voltage across the resistor when the circuit is properly stabilised

(i.e. when a minimum of 2.0 mA is flowing). (1 mark)

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(c) Hence, what is the minimum voltage which must be supplied to the regulator to keep

the output properly regulated? (1 mark)

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(d) When a load is connected and begins to draw current, what happens to the voltage

across the resistor R1? Explain. (2 marks)

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ELT315114 Page 26 of 36

Question 29

Figure 29 shows an RC circuit:

Figure 29

(a) Calculate the value of C (in nF) which makes the rms voltage across C

equal to the rms voltage across the resistor. (2 marks)

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(b) What is the value of this voltage (VR, VC)? (2 marks)

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(c) Why does VR + VC not equal Vin? (2 marks)

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ELT315114 Page 27 of 36

BLANK PAGE EXAM CONTINUTES OVER THE PAGE

ELT315114 Page 28 of 36

Question 30

Below is a design for a cat feeder based on the PICAXE 08M (Figure 30.1). When the cat

steps on a pressure mat, a motor is activated to release some food.

Figure 30.1

When the Push button is pressed the motor activates to release food subject to

the following:

The motor will run for 3 seconds

The motor will not run if it has been activated in the previous 10 minutes.

An LED will light for 30 seconds after the motor activates.

(a) On the circuit diagram above, label the PICAXE with appropriate leg/pin numbers.

(1 mark)

The flowchart outline on the next page shows an approach to programming the feeder

(Figure 30.2).

(b) Use the flowchart template and appropriate symbols to write the subroutine (or section

of program) called motor_run for this part of the description above: (2 marks)

motor_run Runs the motor for 3 seconds, lights LED

for 30 seconds

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ELT315114 Page 29 of 36


Figure 30.2

[NOTE: There is a summary of some PICAXE BASIC commands on page 31]

(c) Use the flowchart template and appropriate symbols to write the subroutine (or section

of program) called motor_delay for this part of the description above (or equivalent to):

(2 marks)

motor_delay The motor will not run if it has been activated

in the previous 10 minutes. Include an

occasional LED ‘flash’ so this delay period

can be seen.

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ELT315114 Page 30 of 36


(d) Now write the complete annotated (commented) program to execute this scenario,

using the symbols, values and subroutines already shown.

You do not need to rewrite the subroutine(s) or program code for motor_run

or motor_delay unless you are modifying it/them. (2 marks)

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(e) If the variable ‘counter’ is used to keep track of the number of times the cat has been

fed, how would you display it on this type of circuit?

Explain what (and how) you would add to the circuit to display this feed count.

(1 mark)

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ELT315114 Page 31 of 36

Summary of some PICAXE BASIC commands:

Command Example

Comments Comment/explain the program ‘comment

Symbol Assign value to variable symbol red = 4

Let Perform mathematical operation (on variable)

let counter = counter + 1

Label Identify section/place in program main:

High (pin#) Output High signal to (pin#) high 4

High (symbol) Output High signal to pin indicated by (symbol)

high red

Low (pin#) / (symbol)

As above, for Low output low 4 or low red

Pause n Halt program for n milliseconds pause 500

Toggle (pin# or symbol)

Change (pin#) or (symbol) tp reverse logic level

toggle led

If (decision) then (action)

If (decision) is true then follow (action), otherwise continue to next line

If pbswitch = 0 then …

Endif End of an ‘if’ command (esp. multiple lines)

endif

Goto (label) Return to part of program starting with (label)

goto main

Gosub (name) Go to subroutine called (name) gosub subroutine1

Return Go back to program line after subroutine call

return

End End of program end

And/or AND / OR keywords (Boolean logic) If pin2=1 and pin3 = 1 then …

For - next loop Repeat contents of loop specified number of times

for counter = 1 to 15 … next counter

Do – until loop Repeat loop until a certain condition is met (‘loop’ indicates end of loop)

do high 4 until counter > 10

Exit Exit from a ‘do’ or ‘for’ command loop

Nap Sleep for short period of time

Sleep Sleep for a period of time

Wait Delay for a number of seconds (less than 65 secs)

wait 3

Stop Stop program execution

Readadc Read value from analogue sensor/input (varying voltage)

readadc 1,b0

Sound Sound output to piezo buzzer eg. sound pin#, (freq, length)

sound 2,(50,100)

Play Play pre-defined sequence of 1 of 4 preset tones

play 1,2

Tune Play tune from Tune Wizard tune 0,8,(1,2,3)

ELT315114 Page 32 of 36

Question 17(a)

Question 18

Figure 18.2

(a) On the graph above (Figure 18.2) draw the output of the regulator.

Add a vertical scale to the graph. (2 marks)

(b) Show the four new components as symbols on the partial schematic below (Figure

18.4). (2 marks)

Figure 18.3

Figure 18.4

SPARE DIAGRAMS

ELT315114 Page 33 of 36

Question 19

(b) (i) Complete the truth table below (Table 19.1) for the completed intended logic system. (2 marks)

Decimal number C B A D E F Q

0 0 0 0

1 0 0 1

2 0 1 0

3 0 1 1

4 1 0 0

5 1 0 1

6 1 1 0

7 1 1 1

Table 19.1

(ii) Complete the logic system schematic diagram Figure 19.1 so that it works as described above by

including a logic gate (or gates) after points E and F. (1 mark)

Figure 19.1

Question 20

This circuit (Figure 20.2) represents a partial solution to controlling the carport light (LED).

Figure 20.2

(e) Show on Figure 20.2 above the connections from points X and Y to the op-amp

inputs so that the carport LED switches on when the light level falls below the

described value in part (a). (2 marks)

ELT315114 Page 34 of 36

Question 22

(e) Show on the diagram below (Figure 22.5) how the circuit can be redesigned for a

50 Hz signal, so that point D still has a 24 second period. (2 marks)

Figure 22.5

Question 24 (c) Suppose C were replaced by a capacitor of twice the value. On the graph below

(Figure 24.2), show the expected charging curve for this new combination. (1 mark)

Figure 24.2

(e) On this graph (Figure 24.4) sketch the expected voltage ~ time curve for the inductive

circuit shown above. (2 marks)

Figure 24.4

ELT315114 Page 35 of 36

Question 25

(b) Sketch a labelled Vout ~ freq graph for this filter, indicating all relevant features.

Include: (3 marks)

the pass region

bandwidth

cut-off frequencies

half-power points

voltage at cut-off points

ELT315114 Page 36 of 36

This examination paper and any materials associated with this examination (including answer booklets, cover sheets, rough note

paper, or information sheets) remain the property of the Office of Tasmanian Assessment, Standards and Certification (TASC).

place your candidate electronics tasc exam...2. answer all questions. 3. answers must be written in...

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