Wednesday 7 June 2017 Afternoon Time allowed: 1 hour 30 minutes

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1 Please write clearly in block capitals. Centre number Candidate number Surname Forename(s) Candidate signature A-level ELECTRONICS Unit 4 Programmable Control Systems Wednesday 7 June 2017 Afternoon Time allowed: 1 hour 30 minutes Materials For this paper you must have: a pencil and a ruler a calculator Data Sheet (enclosed). Instructions Use black ink or black ball-point pen. Fill in the es at top of this page. Answer all questions. You must answer the questions in the spaces provided. outside the around each page or on blank pages. Do all rough work in this book. Cross through any work you do not want to be marked. Information The marks for each question are shown in brackets. The maximum mark for this paper is 80. For Examiner s Use Question Mark TOTAL *JUN17ELEC401* IB/M/Jun17/E1 ELEC4

2 2 Answer all questions in the spaces provided. 1 Figure 1 represents a system diagram for a generalised microprocessor system. Figure 1 The names of 16 subsystems are given below. ADC address bus clock data bus data latch DAC microprocessor PIC port power supply program counter RAM ROM stack tri-state buffer watchdog timer *02*

3 3 Various subsystems in Figure 1 have been identified by the letters A J and these are given in Table 1. Complete Table 1 with the most appropriate subsystem names from the list. Each name may be used once, more than once or not at all. [10 marks] Table 1 Subsystem Name A B C D E F G H I J 10 Turn over for the next question Turn over *03*

4 4 2 A spectrophotometer is a machine which measures how much a sample of material absorbs different wavelengths of light. Figure 2 shows a simplified diagram. Figure 2 The white light is split into separate colours by the diffraction grating. By rotating the diffraction grating, different colours can be selected. Some light from the diffraction grating then passes through the sample and is detected by photodiode A. Some light passes beneath the sample and is detected by photodiode B. 2 (a) The diffraction grating is rotated by a stepper motor. State one reason why a stepper motor is more suitable in this application than a conventional motor. [1 mark] *04*

5 5 2 (b) The stepper motor has three centre-tapped coils. Figure 3 shows the partially-completed circuit diagram of the driver circuit for coil 1. The manufacturer can use either npn junction transistors or n-channel MOSFETs in the driver circuit. Figure 3 Complete Figure 3 by adding two npn junction transistors and any other components and wire links that you need. [4 marks] 2 (c) (i) The manufacturer decides instead to use MOSFETs as the drivers for the coils. The stepper motor is controlled by a microcontroller with a 1 MHz clock. The gates of the six MOSFET drivers for the coils are connected to outputs D 0 to D 5 of PORT A of the microcontroller. State the assembler instructions needed to set PORT A bits D 0 to D 5 as outputs and D 6 and D 7 as inputs. [3 marks] 2 (c) (ii) The microcontroller has to wait for approximately 10 ms between sending each new instruction to PORT A when altering the position of the stepper motor. Explain why this time delay is necessary. 10 Turn over *05*

6 6 3 You do not need to have attempted question 2 before attempting this question. A spectrophotometer is a machine which measures how much a sample of material absorbs different wavelengths of light. Figure 4 shows a simplified diagram. Figure 4 The white light is split into separate colours by the diffraction grating. By rotating the diffraction grating, different colours can be selected. Some light from the diffraction grating then passes through the sample and is detected by photodiode A. Some light passes beneath the sample and is detected by photodiode B. *06*

7 7 3 (a) (i) Figure 5 shows the circuit diagram of a photodiode amplifier. Figure 5 Increasing the light level on the reverse biased photodiode increases the current through it. Explain why a change in light level affects the output voltage V out. 3 (a) (ii) When light of wavelength 540 nm and with an intensity of 10 lux is received by one of the photodiodes, a current of 5 µa passes through the diode. Calculate the output voltage from the photodiode amplifier. Question 3 continues on the next page Turn over *07*

8 8 3 (b) Photodiode A receives light that has passed through the sample. Photodiode B receives light directly from the diffraction grating. The absorption of light by the sample is then determined from the output of the difference amplifier. Figure 6 shows the partially-completed circuit diagram of the difference amplifier. Complete the circuit diagram and give suitable component values so that the difference amplifier has a voltage gain of 1. [4 marks] Figure 6 3 (c) The current through the photodiodes is proportional to the incident light intensity. A particular sample of material has a transmission of 25% at a wavelength of 540 nm. Photodiode B receives 10 lux at this wavelength. Calculate the output voltage from the difference amplifier. 10 *08*

9 9 4 An incubator for hatching hens eggs needs to maintain a constant temperature of 37.5 C. Figure 7 shows a system diagram for controlling the temperature of the incubator. Figure 7 4 (a) Tick ( ) the next to the correct description of this system. [1 mark] open loop with negative feedback open loop with positive feedback closed loop with negative feedback closed loop with positive feedback 4 (b) The temperature in the incubator must be maintained within as small a range as possible. A comparator or a Schmitt trigger can be used as a processor. Explain which would be better to use in this situation. Refer to the properties of both a comparator and a Schmitt trigger. [3 marks] *09* Question 4 continues on the next page Turn over

10 10 4 (c) (i) The heater for the incubator is rated at 100 W at 12 V. For the driver subsystem, there is a choice of a relay, a junction transistor or a MOSFET. Explain why a relay would be unsuitable in this application if a comparator is used as the processor. 4 (c) (ii) All of the available junction transistors have a saturated voltage of 0.2 V from collector to emitter and the best MOSFET available has a saturated drain to source resistance of 0.01 Ω. Explain whether a junction transistor or a MOSFET would be the best device to use in this situation and support your answer with relevant calculations. [4 marks] 10 *10*

11 11 5 A project needs an 8-bit Digital to Analogue Converter (DAC). Figure 8 shows the partially-completed diagram of the circuit. Figure 8 5 (a) (i) State the value of R 1. [1 mark] 5 (a) (ii) Label the two virtual earth points with the letter P. 5 (a) (iii) State the function of the op-amp circuit in A. [1 mark] [1 mark] 5 (a) (iv) State the purpose of the subsystem in B. [1 mark] Question 5 continues on the next page Turn over *11*

12 12 5 (b) Calculate the output voltage (V out ) from the DAC when the input is Assume that a logic 1 input is +5 V and that a logic 0 input is 0 V. The power supply to the op-amps is ±12 V. 5 (c) The project needs the DAC to be able to supply a large current and it is suggested that this could be achieved using a MOSFET source follower. This is shown in Figure 9. Figure 9 *12*

13 13 Figure 10 shows the characteristic for the MOSFET used. Figure 10 When the circuit was tested, a problem was detected. It was found that V x did not correspond to the output voltage of op-amp 2. 5 (c) (i) Estimate, with a reason, the output voltage, V x, when there is a digital input of 0 to the DAC. 5 (c) (ii) Suggest how the problem could be solved. You might consider modifying the input circuit to op-amp Turn over *13*

14 14 6 Figure 11 shows the system diagram for an experimental weighing machine based on the displacement of a loudspeaker cone. Figure 11 In operation, a weight is placed onto the loudspeaker cone and displaces it. The start switch is then pressed. The microprocessor increases the output of PORT A to the DAC by counting up from 0. The output from the DAC passes through the loudspeaker coil and pushes the loudspeaker cone back up. 6 (a) State the maximum value, in hexadecimal, that can be sent from PORT A to the DAC. [1 mark] *14*

15 15 6 (b) The reflective opto switch gives a logic 1 output when the loudspeaker cone is driven back to its undisplaced position. This alerts the microprocessor to stop counting. The microprocessor then sends the count to the display subsystem. Draw a flow chart to represent the operation of the system from when the start switch is pressed until the count is sent to the display subsystem. [3 marks] Question 6 continues on the next page Turn over *15*

16 16 6 (c) The reflective opto switch is connected to D 0 of PORT B. Write the assembler instructions for the microcontroller to read PORT B, isolate bit D 0 and jump to a label called reflect if the output of the reflective opto switch is not logic 1. [3 marks] 6 (d) When the reflective opto switch gives a logic 1, the value stored in PORT A needs to be transferred to PORT C to be displayed. Write the assembler instructions to transfer the value in PORT A to PORT C and then set PORT A to 0. [3 marks] 10 *16*

17 17 7 A display subsystem in a project receives an 8-bit binary input and shows the decimal value on three 7-segment LED displays. Figure 12 shows part of the circuit diagram for the display. Figure 12 7 (a) The 7-segment displays are multiplexed. Explain what is meant by the term multiplexed in this context and give two advantages and one disadvantage of multiplexing the 7-segment displays. [5 marks] Question 7 continues on the next page Turn over *17*

18 18 7 (b) The 8-bit binary input is received on PORT C, but before it can be displayed it must be converted into three binary coded decimal (bcd) numbers by the microcontroller. Calculate the three bcd numbers that result from an input of 0xBE and give your answer as three hexadecimal numbers. 7 (c) Each bcd number has to be further converted into a byte which represents the segment pattern of the 7-segment displays, and which can be sent from PORT A of the microcontroller. Table 2 shows how the data bits of PORT A are mapped to the display segments. Table 2 PORT A bits D 0 D 1 D 2 D 3 D 4 D 5 D 6 D 7 Display segments a b c d e f g dp For example, to show the number 1 on a display, the byte 0x06 must be sent from PORT A. Calculate the byte of data needed to display the number 5. Give your answer as a hexadecimal number. [3 marks] 10 *18*

19 19 8 Figure 13 shows the top view and front view of a small three-wheeled robotic vehicle. Figure 13 8 (a) The robotic vehicle is powered by two lithium polymer (LiPo) cells. Discuss the advantages and disadvantages of LiPo cells compared with other types of rechargeable batteries. [3 marks] Question 8 continues on the next page Turn over *19*

20 20 Each of the two large front wheels has its own motor, gear and controller. Figure 14 shows part of the circuit diagram of each motor controller. Figure 14 8 (b) Complete Figure 14 by adding a correctly connected p-channel MOSFET. 8 (c) (i) Complete the truth table (Table 3) for the logic gate circuit in Figure 14. [3 marks] Table 3 X Y A B C (c) (ii) Explain why the logic circuit has been designed so that the state where B is logic 0 and C is logic 1 can never occur. 10 END OF QUESTIONS Copyright 2017 AQA and its licensors. All rights reserved. *20*

ELEC2 (JUN15ELEC201) General Certificate of Education Advanced Subsidiary Examination June Further Electronics TOTAL. Time allowed 1 hour

ELEC2 (JUN15ELEC201) General Certificate of Education Advanced Subsidiary Examination June Further Electronics TOTAL. Time allowed 1 hour Centre Number Surname Candidate Number For Examiner s Use Other Names Candidate Signature Examiner s Initials Question Mark Electronics General Certificate of Education Advanced Subsidiary Examination