Applied Electronics, systems and Computing
Answer ALL the questions from Section A and B
Section A – Applied Electronics
1. (a) Sketch the circuits that you might use for;
(i) Half – wave rectification of an a.c. signal.
(ii) Full – wave rectification of an a.c. signal.
Explain how these circuits rectify the a.c. signal and how these rectified signals can be converted to a d.c. signal. What is ripple voltage and how does it differ for Half- wave and Full- wave rectification?
(10 marks)
(b) A Zenner diode with a Zenner voltage Vz = 5.1 V and a maximum power rating of Pmax = 15 mW is used to deliver a steady d.c. voltage level. The d.c. power supply used in this circuit is 10 V and a resistor is used to limit the current through the diode. Draw a suitable diagram for the circuit and calculate the minimum value for the current limiting resistor in the circuit.
(9 marks)
(c) A resistor is used to limit the current flowing through an LED to 12 mA which is sufficient to turn on the LED. Draw a suitable circuit diagram for this arrangement. Assuming the voltage across the LED is 1.8V and the power supply voltage is 16 V, calculate the minimum value of the limiting resistor required to operate this LED circuit.
(6 marks)
(Total 25 marks)
2. A combinational logic system has FOUR inputs and a single output. The output signal from the system is high (logical ‘1’) only when the signals on two or more of the inputs lines are low (logical ‘0’), otherwise the output signal is low.
(a) Draw a truth table for the system.
(7 marks)
(b) Use a K-map to obtain a minimal Boolean expression for the output.
(7 marks)
(c) Implement the design using AND, OR and NOT gates as necessary. Draw a clearly labelled circuit diagram of your completed system.
(7 marks)
(d) How many IC packages would be needed to build your system? Assume six NOT gates or four 2-input gates or three 3-input gates are on each individual IC package.
(4 marks)
(Total 25 marks)
3. (a) What type of circuit is shown in Figure 1? What is the phase angle relationship between the input and output signals? Explain why there is this phase relationship.
(6 marks)
Figure 1
(b) What is the purpose of the two capacitors C1 and C2 in this circuit? What is the purpose of the capacitor C3 in this circuit?
(4 marks)
(c) For the transistor circuit shown above the following values are given;
R1 = 41.6 k, R2 = 6.4 k, R3 = 1 k, RL = 5.6 k
Vcc = 12 V and hfe = 150.
Determine the following;
(i) Voltage across R2 (Vb).
(ii) Voltage across R3 (Ve).
(iii) Emitter current (Ie) in mA.
(iv) Collector current (Ic) in mA.
(v) Voltage on collector relative to ground (Vc).
(vi) Base current (Ib) in A.
(15 marks)
(Total 25 marks)
Continued…
Section B – Systems
4. (a) The diagram below shows a standard definition (SD) TV picture with three grey and one black vertical bars. The lightest shade of grey is on the left, and the second lightest is on the right.
Draw the signal waveform you would expect to see if this TV picture, below in Figure 2, was displayed on an oscilloscope. Name six of the parts of the wave form and annotate your sketch with two standard numerical voltage, or time values for an SD TV picture.
(12 marks)
Figure 2
(b) If the right-most grey bar has a luminance value of 50%, what
voltage value would it have? State what assumptions you have made.
(4 marks)
(c) What two pieces of information are required in a TV signal, before the active line signal commences? In each case state what they are used for?
(5 marks)
(d) In what proportions do the colour components combine, to form the luminance signal Y?
(4 marks)
(Total 25 marks)
Continued…
5. (a) A mobile telephone charger connects to a 240 V ac mains socket and provides a 5V d.c. supply.
(i) Briefly describe how the electrical mains voltage is converted to the d.c. supply.
(6 marks)
(ii) Calculate the current delivered to the battery if the 5V d.c. supply delivers 10 watts of power to the battery.
(4 marks)
(iii) How long will this charger take to charge a 3000mAh smartphone battery if it is initially fully discharged?
(4 marks)
(b) (i) Describe the internal construction of a TV Aerial cable, and state where in the cable the TV signal energy is transmitted.
(6 marks)
(ii) Explain how you would investigate the fault in a TV aerial cable which does not pass a transmission signal from the TV aerial to the television.
(5 marks)
(Total 25 marks)
6. (a) Describe briefly the importance of international standards. Name two organisations which create these standards for the broadcast or communications industries.
(6 marks)
(b) Briefly describe what the CE letters mean when seen stamped on an item of equipment.
(4 marks)
(c) Describe one example of a standard which has been adopted throughout Europe related to the field of media and communications and explain why this standardisation was useful.
(3 marks)
(d) State TWO media broadcast or communications systems which have changed in the last 5 years, and describe for each, in detail, how these have changed.
(2 x 6 marks, 12 marks)
(Total 25 marks)