Knowledge recall questions - where you are asked to remember the names of components, recognise which is the correct component to use in a given situation, or match component names to pictures or symbols.
	Calculations - where you are asked to choose the correct formula and complete a calculation using the information provided.
	Partially completed questions - where you are expected to complete a circuit diagram, PCB, stripboard or breadboard assembly, by adding the missing components or parts.
	Written or diagrammatic explanations - where you are asked to explain or draw how a component or circuit works, or how you would complete or carry out a certain task.
	Design questions - where you are asked to design either a jig, PCB or case to as given specification.

Part 1	Part 2	Part 3	Part 4	Part 5	Part 6
Part 7	Part 8	Part 9	Part 10	Part 11	Part 12

	the hut lights must come on automatically as dusk falls;
	the system must be run from a renewable energy source. A block diagram for this system is shown below.

(a)     From the circuits shown below select the correct input for the system.

                 

The correct circuit is ................................

(b)     Explain how a change in light level controls the switching of the transistor.

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

(c)     A suitable transistor was identified in a component catalogue. It is shown here.

The three legs of the transistor must be connected the correct way round in the circuit. Give the name for each of the legs in the space below and add a short description to explain how each of the legs is usually connected into a circuit.

The leg labelled E is called the ..................... and is normally connected to  ............................................................................................................. The leg labelled B is called the ..................... and is normally connected to  ............................................................................................................. The leg labelled C is called the ..................... and is normally connected to  .............................................................................................................

(d)    Below is a circuit diagram showing the switching stage of the system.

(i)    Add the important detail which is missing from the transistor symbol and briefly        explain its importance to the symbol.

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

 (ii) Explain the purpose of the 1K resistor 

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

(e)     The transistor chosen has a hfe (gain) of 40. Calculate the base current when the collector          current is 400mA.

Formula   ...................................... Insert values .................................. Answer   .......................................

(f)    A 12V spot light powered by a lead acid battery is to be used to provide the light in the           chicken huts. A relay could be used as an interface. The circuit symbol for the relay and its contacts is shown below.

Using the information you have been given throughout the question draw a complete circuit for the system in the space below.

 

2.    (a)      Give the full name for each symbol shown below.

(i)		.............................................................
(ii)		.............................................................
(iii)		.............................................................
(iv)		.............................................................
(v)		.............................................................
(vi)		.............................................................
(vii)		.............................................................

          (b)       Complete the sentence under each diagram by inserting one of the following words.

series, polarity, parallel, equilibrium
(i) these resistors are connected in ..................................................	(ii) these resistors are connected in ..................................................

 

          (c)       Calculate the total resistance in the diagram below.

Formula   ...................................... Insert values .................................. Answer   .......................................

          (d)       Resistors are often connected to LED's. What does the resistor do ?

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

(e)       The LED shown below loses 2V across it and uses a current of 20mA.          Calculate the value of resistor required for the circuit shown below.

Formula   ...................................... Insert values .................................. Answer   .......................................

(f)         Choose the nearest value from the E12 series shown below.

                10,  12,  15,  18,  22,  27,  33,  39,  47,  56,  68,  82

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

3.        The circuit below has been designed for use as a dryness detector for use in a greenhouse. The dryness of the soil is tested by the sensor which is pushed into the ground.

       (a)      (i)   What is the name of component W ?........................................................

                 (ii)   Explain the purpose of this component in the circuit. ................................

       (b)      (i)   What is the name of component X ?.........................................................

                 (ii)   Explain the purpose of this component in the circuit. ................................

       (c)      (i)   What is the name of component Y ?........................................................

                 (ii)   Explain the purpose of this component in the circuit. ................................

       (d)      (i)   What is the name of component Z ?.........................................................

                 (ii)   Explain the purpose of this component in the circuit. ................................

      (e)       The relay selected for the circuit has an operating voltage of 6V and has a resistance                  of 200 ohms. Calculate the minimum current needed to activate the relay.    

                Formula ............................................................................

                Working ............................................................................

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

                Answer and units ...............................................................

       (f)        What do the letters NO indicate on the relay contacts ?  ................................

       (g)       To begin with the ground is damp and gradually dries out. Explain what happens to:

                          (i) the voltage at the transistor base ............................................................

                          (ii) the transistor ........................................................................................

                          (iii) the relay ..............................................................................................

                          (iv) the relay contacts ................................................................................

                          (v) the buzzer .......................................................................................

         (h)     Complete the circuit diagram so that the dryness detector now operates as a moisture detector.

    

4. You have been asked by a local baby equipment manufacturer to complete a circuit for a night light. The circuit is shown in part (f) below.

The night light is a monostable which is operated by the switch SW2. The light L1 then comes on for a set amount of time.

(a)    What type of switch is SW1 ? ............................................................ 

(b)     What type of switch is SW2 ? ........................................................... 

(c)    Which components in the circuit control the time delay ?

(i)    .................................................................

(ii)   ................................................................. 

(d)    What modification would you make to the circuit to allow the time delay to be adjustable ?

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

(e)    The manufacturer wishes to set the 'on-time' to 5 minutes. Calculate the value of the resistor R2 needed to achieve this.

Formula    ............................................................................

Working    ............................................................................

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

Answer and units     .............................................................. 

(f)    The output from the timer was not powerful enough to operate the lamp.

(i)    What two components would you need to add to solve this problem ?

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

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

(ii) Complete the circuit diagram below to show how these components would be used.

 

(g)   Use the circuit diagram above to complete the track layout for the PCB for the night light shown below.

 

5

(a)   Complete the circuit diagram above by drawing in the red and green LED's                                                                                                                        
                                                                                                      

(b)   The Op-Amp has two inputs.

(i)   What is the name of the '+' input   .....................................

(ii)   What is the name of the ' - ' input  ....................................

(iii)   If the voltage at the ' - ' input is greater than at the '+' input, what

 will the output voltage be ?  .................................................. 

(c)   What voltage would you expect to find at point X in the circuit ?

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

(d)   The LED's need 2V and a current of 25 mA to operate. 

(i)   Calculate the value of the series resistor needed for the LED's.

Formula  .......................................................................................

Working  .......................................................................................

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

Answer and units   .........................................................................  

(ii)  Choose a resistor from the E 12 series shown below.

   10,  12,  15,  18,  22,  27,  33,  39,  47,  56,  68,  82          

(e) (i)  The potential divider used in the circuit is shown below.  Thermistor R1 has a resistance of 20K @ 25⁰C. Calculate the output from the potential divider when the temperature is exactly 25⁰C.

Formula  .......................................................................................

Working  .......................................................................................

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

Answer and units   .........................................................................

(ii)   Will the red LED light up ?  ............................................ 

 

6. A small, local cycle equipment manufacturer has designed a new red 'flashing' cycle light which it intends to produce in small batches. The circuit is shown below.

 

(a)   Describe what is meant by small batch Production. .................... 

(b)   A PCB mask is to be produced using a process known as CAD.

(i) What does CAD stand for ? ..............................................   

(ii) Give three advantages of using CAD to produce PCB masks.

Advantage 1 .......................................................................  

Advantage 2 .......................................................................  

Advantage 3 ....................................................................... 

(c)   Give two reasons why DIL (Dual in Line) sockets are used with IC's

Reason 1 ..........................................................................

Reason 2 ..........................................................................

7.  A supermarket owner has asked you to make a model of an automatic lighting system for their car park.

The lighting needs to come on when it is dark and the supermarket is open. As an added safety feature the lights must be able to be switched on at any time.

The main specifications for the system are:

	A time clock is used to set the on and off periods
	A dark sensor is used to detect when it becomes dark
	A manual override switch is used to switch the lights on at any time.

(a)  (i)  Name the logic gate shown. .................................................  

      (ii)  Complete the truth table for the logic gate shown in (i)

Inputs		Output
0	0
0	1
1	0
1	1

                                                                                                     

(b)   Specification No. 2 asks for a dark sensor. complete the circuit below by           drawing in the necessary components to give a logic '1' when it is dark.

   

(c)   Specification No. 3 asks for an override switch. Complete the circuit below by drawing the necessary components to give a 'logic 1' when the switch is pressed.

     

(d) the logic part of the process sub-system is designed using two logic gates. Complete the diagram below using the necessary circuit symbols.

  

(e)  In the space below using your previous answers complete the entire circuit diagram for the lighting system.

 

 

Q8.    The circuit diagram for a simple fuse tester is shown below. A fuse is placed across the       contacts and if it is in good condition the LED lights up.

(a) Complete the system diagram below for the fuse tester.

(b) Why is the resistor R2 needed in the circuit? ...................................................

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

(c) The diagram above shows that the LED uses 2V and the power supply gives 9V. what voltage (labelled X) in the circuit is used by the resistor.

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

(d) The theoretical value of R2 has been calculated at 350 ohms. Choose the nearest appropriate resistor to that value from the E12 series shown below.

10,  12,  15,  18,  22,  27,  33,  39,  47,  56,  68,  82

(e) If the case for the fuse tester was to be made from a thermoplastic. What method of production would be used to make

(i) 100 cases ? ...................................................

(ii) 20 000 cases ? ..............................................

(f) When soldering the circuit board it is necessary to protect the transistor against excessive heat. How would you do this ?

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

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

Q9.  Study the timing circuit below and the information given for the IC. It is to be used as a kitchen timer.

(a)  Describe simply the operation of the above circuit, commenting on the function of SW1, SW2, R1, C1 and the state of pin 3 when the buzzer sounds.

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

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

(b) Calculate the time constant using the values given in the circuit.

Formula ....................................................................................

Working ....................................................................................

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

Answer and units .......................................................................

(c) How would you change the circuit to make the time constant variable ?

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

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

(d) The timer is to be assembled on a prototype board to test it. 

Complete the diagram below by drawing in the wire connections needed to make a working circuit. The connecting wires for pins 4, 6 and 7 and SW1 have already been drawn.

(e) Using the circuit diagram, complete the track layout of the PCB for the kitchen timer. For economy you should only use the size of board indicated below.

 

Q10.  A student has been investigating designs for a simple, cheap and compact throw away torch.

(a) The first circuit investigated by the student is shown below.

Explain how the circuit operates (in your answer refer to SW1, SW2 and the correct names for the other components)

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

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

(b) An alternative solution used a relay. Complete the circuit diagram below so that it performs the same function as the circuit in part (a).

(c) if it was decided to start a business manufacturing torches the student would have to ensure that the quality of the product was suitable for the intended user.

With this in mind, describe in detail how the product could be tested to ensure:

(i) That the switches are durable in use:

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

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

(ii) That the product is easy and comfortable to use.

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

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

Q11. The function of a gas central heating system is described in the system diagram below.

(a) The circuit shown below is the input used as the room thermostat. Explain how the circuit operates by referring to Vs, R1 and R2.

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

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

(b) The boiler should only come on if the three sensors are at 'logic 1' or when the override switch is pressed.

Draw a logic system below which can be used to solve this problem. Use only 2-input logic gates.

 

 

(c) The transducer driver part of the system is an NPN transistor. The transistor selected has an hFE of 100. Calculate the base current when the collector current is 500mA.

Formula ....................................................................................

Working ....................................................................................

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

Answer and units ......................................................................

(d) Draw the circuit symbol for an NPN transistor and label each of the legs with its correct name.

 

(e) A solenoid valve, controls the flow of gas to the boiler.

(i) Complete the circuit below by adding the transducer driver.

(ii) As the solenoid valve works in a similar way to a relay there is the possibility that the transistor could be damaged by 'back emf'. Name a component that could be added to the circuit to prevent this damage.

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

(iii) Add a component to the diagram above which would protect the transistor from 'back emf'.

Q12.  A baby equipment company has decided to produce a bath temperature alarm using an operational amplifier. A CAD program with the ability to simulate the function of electronic components was used to test the initial ideas for the new circuit.

(a) Give three advantages of using CAD programs for testing ideas.

1.   ......................................................................................................

2.   ......................................................................................................

3.   .....................................................................................................

(b) A possible input circuit is shown below.

The resistance of the thermistor is 82K at 10⁰C. Calculate the value of Vs at this temperature.

Formula ....................................................................................

Working ....................................................................................

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

Answer and units ......................................................................

(c) Complete the Op-amp circuit shown below to make it work as a comparator, which will indicate a high temperature by lighting the red LED and a low temperature by lighting the green LED.

Show clearly :

	the components used to sense the temperature connected to the inverting input
	the components used to set a reference voltage at the non-inverting input.

(d) Explain in detail how the components you have used as the temperature input to pin 2 control the red LED.

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

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

(e) It was suggested that the red LED should latch on if the temperature is too high and the circuit was modified as shown below.

(i) Explain the function of the 10K resistor

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

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

(ii) The red LED failed to latch when the circuit is tested. Explain in detail, why the circuit did not work correctly.

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

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

(iii) Draw a modified circuit which would latch the LED correctly, by adding a single component to the diagram above.

 

 

 

 

 

Answers

 

1. An egg producer wants to fit an automatic lighting system to a chicken hut.
   
   the two main specifications for the system are:

	the hut lights must come on automatically as dusk falls;
	the system must be run from a renewable energy source. A block diagram for this system is shown below.

(a)     From the circuits shown below select the correct input for the system.

                 

The correct circuit is  Circuit A

(b)     Explain how a change in light level controls the switching of the transistor.

As the light falling on the LDR falls its resistance increases. This causes a similar increase in the voltage dropped across it. As a result the output voltage increases to a value which will switch the transistor on (0.7V)

(c)     A suitable transistor was identified in a component catalogue. It is shown here

The three legs of the transistor must be connected the correct way round in the circuit. Give the name for each of the legs in the space below and add a short description to explain how each of the legs is usually connected into a circuit.

The leg labelled E is called the Emitter and is normally connected to The zero volt rail The leg labelled B is called the Base and is normally connected to a protective resistor The leg labelled C is called the Collector and is normally connected to an output device

(d)    Below is a circuit diagram showing the switching stage of the system.

(i)    Add the important detail which is missing from the transistor symbol and briefly explain its importance to the symbol.

The arrow shows that it is an NPN transistor. It indicates the direction of current flow through the transistor.

(ii) Explain the purpose of the 1K resistor 

To protect the transistor from damage caused by excessive currents

(e)     The transistor chosen has a hfe (gain) of 40. Calculate the base current when the collector current is 400mA.

Formula  Insert values   Answer

(f)    A 12V spot light powered by a lead acid battery is to be used to provide the light in the chicken huts. A relay could be used as an interface. The circuit symbol for the relay and its contacts is shown below.

Using the information you have been given throughout the question draw a complete circuit for the system in the space below.

2.    (a)      Give the full name for each symbol shown below.

(i)		Fixed resistor
(ii)		Polarised Capacitor (Electroytic Capacitor)
(iii)		Diode
(iv)		Thyristor
(v)		Thermistor
(vi)		Light Dependent Resistor
(vii)		NPN Transistor

          (b)       Complete the sentence under each diagram by inserting one of the following words.

series, polarity, parallel, equilibrium
(i) these resistors are connected in Series	(ii) these resistors are connected in Parallel

 

          (c)       Calculate the total resistance in the diagram below.

Formula   Insert values Answer

          (d)       Resistors are often connected to LED's. What does the resistor do ?

                     Protects the LED by limiting the current flowing through it    

(e)       The LED shown below loses 2V across it and uses a current of 20mA.          Calculate the value of resistor required for the circuit shown below.

Formula Insert values Answer

(f)         Choose the nearest value from the E12 series shown below.

                10,  12,  15,  18,  22,  27,  33,  39,  47,  56,  68,  82

The nearest higher value resistor is 390 ohm

3.        The circuit below has been designed for use as a dryness detector for use in a greenhouse. The dryness of the soil is tested by the sensor which is pushed into the ground.

(a)      (i)   What is the name of component W ? this is a Diode, it is often called a flywheel diode when used in parallel with a relay coil.                  (ii)   Explain the purpose of this component in the circuit. It allows the high voltage produced when an electromagnetic coil is switched off (back e.m.f.) to be safely dissipated. If it is not present the transistor would be damaged by the high current produced.
(b)      (i)   What is the name of component X ? this is an NPN bipolar transistor           (ii)   Explain the purpose of this component in the circuit. It amplifies the signal from the signal from the sensor and controls the current flowing through the relay coil.
(c)      (i)   What is the name of component Y ? this is a buzzer            (ii)   Explain the purpose of this component in the circuit. it produces an audible output.
(d)      (i)   What is the name of component Z ? this is a potentiometer used as a variable resistor           (ii)   Explain the purpose of this component in the circuit. it allows you to adjust the sensitivity of the sensor.

   

  (e)       The relay selected for the circuit has an operating voltage of 6V and has a resistance  of 200 ohms. Calculate the minimum current needed to activate the relay.    

                Formula            

                Working            

                                          

                Answer and units                                                                                  

    (f)        What do the letters NO indicate on the relay contacts ?  Normally Open              

    (g)       To begin with the ground is damp and gradually dries out. Explain what happens to:

                          (i) the voltage at the transistor base  will increase

                          (ii) the transistor will switch on when the base voltage reaches 0.7V

                          (iii) the relay coil will generate a magnetic field

                          (iv) the relay contacts will change over

                          (v) the buzzer will sound                                                                        

     (h)     Complete the circuit diagram so that the dryness detector now operates as a moisture detector.

          

       

4. You have been asked by a local baby equipment manufacturer to complete a circuit for a night light. The circuit is shown in part (f) below.

The night light is a monostable which is operated by the switch SW2. The light L1 then comes on for a set amount of time.

(a)    What type of switch is SW1 ? S.P.S.T. (Single pole single throw switch) 

(b)     What type of switch is SW2 ? P.T.M.. (Push to make switch) 

(c)    Which components in the circuit control the time delay ?

(i)                        Resistor R2  

(ii)                        Capacitor C1                                         

(d)    What modification would you make to the circuit to allow the time delay to be adjustable ?

                    Change resistor R1 to a variable resistor                  

(e)    The manufacturer wishes to set the 'on-time' to 5 minutes. Calculate the value of the resistor R2 needed to achieve this.

Formula                      T (secs) = C1 x R2

Working               5 mins = 5 x 60 secs = 300 secs

               300 = .0001 x R2   therefore R2 = 300 / .0001

Answer and units     3 000 000 ohms or 3 M ohms                         

(f)    The output from the timer was not powerful enough to operate the lamp.

(i)    What two components would you need to add to solve this problem ?

                                     A transistor

                                   A base resistor  

(ii) Complete the circuit diagram below to show how these components would be used.

 

(g)   Use the circuit diagram above to complete the track layout for the PCB for the night light shown below.

 

5.      The partially completed circuit below is for a temperature alarm which uses an Op-Amp (operational amplifier). A red LED will light up when the temperature exceeds a set value and a green LED will light when it is below that value.

(a)   Complete the circuit diagram above by drawing in the red and green LED's.
                                                                                            

(b)   The Op-Amp has two inputs.

(i)   What is the name of the '+' input  The Non-inverting input   

(ii)   What is the name of the ' - ' input  The Inverting input 

(iii)   If the voltage at the ' - ' input is greater than at the '+' input, what

 will the output voltage be ?         It will be - 9V.       

(c)   What voltage would you expect to find at point X in the circuit ?

                                  4.5 V                                 

(d)   The LED's need 2V and a current of 25 mA to operate. 

(i)   Calculate the value of the series resistor needed for the LED's.

Formula   R = V / I    V = 9 - 2 = 7V      I = .025 A

Working                                R = V / I

                                 R = 7 / .025

Answer and units                R = 280 Ohms 

(ii)  Choose a resistor from the E 12 series shown below.

   10,  12,  15,  18,  22,  27,  33,  39,  47,  56,  68,  82            

                  Nearest Larger value =  330 ohms

(e) (i)  The potential divider used in the circuit is shown below.  Thermistor R1 has a resistance of 20K @ 25⁰C. Calculate the output from the potential divider when the temperature is exactly 25⁰C.

Formula                       

Working                             

Answer and units                 

                                           = 4.4 V

(ii)   Will the red LED light up ?             No.               

 

6. A small, local cycle equipment manufacturer has designed a new red 'flashing' cycle light which it intends to produce in small batches. The circuit is shown below.

 

(a)   Describe what is meant by small batch Production. 

Small batch production means making only a small number of products at any one time . Each group of products is known as a batch.                                                                                   

(b)   A PCB mask is to be produced using a process known as CAD.

(i) What does CAD stand for ?      Computer Aided Design 

(ii) Give three advantages of using CAD to produce PCB masks.

Advantage 1             It is usually quicker. 

Advantage 2         It is usually more accurate 

Advantage 3       Changes can be made easily                         

(c)   Give two reasons why DIL (Dual in Line) sockets are used with IC's

Reason 1          To avoid heat damage during soldering

Reason 2          So faulty 'chips' can be easily replaced            

7.  A supermarket owner has asked you to make a model of an automatic lighting system for their car park.

The lighting needs to come on when it is dark and the supermarket is open. As an added safety feature the lights must be able to be switched on at any time.

The main specifications for the system are:

	A time clock is used to set the on and off periods
	A dark sensor is used to detect when it becomes dark
	A manual override switch is used to switch the lights on at any time.

(a)  (i)  Name the logic gate shown. An AND gate.  

      (ii)  Complete the truth table for the logic gate shown in (i)

Inputs		Output
0	0	0
0	1	0
1	0	0
1	1	1

                                                                                                     

(b)   Specification No. 2 asks for a dark sensor. complete the circuit below by           drawing in the necessary components to give a logic '1' when it is dark.

 

(c)   Specification No. 3 asks for an override switch. Complete the circuit below by drawing the necessary components to give a 'logic 1' when the switch is pressed.

  

(d) the logic part of the process sub-system is designed using two logic gates. Complete the diagram below using the necessary circuit symbols.

 

(e)  In the space below using your previous answers complete the entire circuit diagram for the lighting system.

 

 

Q8.    The circuit diagram for a simple fuse tester is shown below. A fuse is placed across the       contacts and if it is in good condition the LED lights up.

(a) Complete the system diagram below for the fuse tester.

(b) Why is the resistor R2 needed in the circuit? To limit the current flowing in the LED

(c) The diagram above shows that the LED uses 2V and the power supply gives 9V. what voltage (labelled X) in the circuit is used by the resistor.          9 - 2V =7V

(d) The theoretical value of R2 has been calculated at 350 ohms. Choose the nearest appropriate resistor to that value from the E12 series shown below.

10,  12,  15,  18,  22,  27,  33,  39,  47,  56,  68,  82

Nearest higher value - 390 ohms

(e) If the case for the fuse tester was to be made from a thermoplastic. What method of production would be used to make

(i) 100 cases ?            Vacuum Forming

(ii) 20 000 cases ?       Injection Moulding

 

(f) When soldering the circuit board it is necessary to protect the transistor against excessive heat. How would you do this ?

Hold each leg as it is soldered with a pair of pliers to conduct heat away from transistor

or 

solder a transistor socket onto the PCB and fit the transistor into the socket

Q9.  Study the timing circuit below and the information given for the IC. It is to be used as a kitchen timer.

(a)  Describe simply the operation of the above circuit, commenting on the function of SW1, SW2, R1, C1 and the state of pin 3 when the buzzer sounds.  Pressing SW2 takes pin 2 low. This causes pin 3 to go high for a period of time determined by R1 and C1. After this period pin 3 goes low and the buzzer sounds.

(b) Calculate the time constant using the values given in the circuit.

Formula                             T (secs) = C x R

Working                     T (secs) = .000 1 x 100 000

                                       T (secs) = 100

Answer and units     Time constant = 100 (secs)

(c) How would you change the circuit to make the time constant variable ?

                                Make R1 a variable resistor

(d) The timer is to be assembled on a prototype board to test it. 

Complete the diagram below by drawing in the wire connections needed to make a working circuit. The connecting wires for pins 4, 6 and 7 and SW1 have already been drawn.

(e) Using the circuit diagram, complete the track layout of the PCB for the kitchen timer. For economy you should only use the size of board indicated below.

Q10.  A student has been investigating designs for a simple, cheap and compact throw away torch.

(a) The first circuit investigated by the student is shown below.

Explain how the circuit operates (in your answer refer to SW1, SW2 and the correct names for the other components) Pressing switch SW1 allows a small gate current to flow through the 1k resistor. This latches the thyristor and a larger current flows from the anode to the cathode causing the bulb to glow. Pressing SW2 shorts out the anode/cathode current and effectively 'resets' the thyristor.

(b) An alternative solution used a relay. Complete the circuit diagram below so that it performs the same function as the circuit in part (a).

(c) if it was decided to start a business manufacturing torches the student would have to ensure that the quality of the product was suitable for the intended user.

With this in mind, describe in detail how the product could be tested to ensure:

(i) That the switches are durable in use: Use a solenoid to operate the switch to destruction in order to find out how many operations are possible before failure.

(ii) That the product is easy and comfortable to use. Conduct a user survey test to find out peoples views about how easy to use and how comfortable the product was.

Q11. The function of a gas central heating system is described in the system diagram below.

(a) The circuit shown below is the input used as the room thermostat. Explain how the circuit operates by referring to Vs, R1 and R2.

When the temperature is low R2 has a higher resistance than R1. Most of the voltage is lost across R2 and Vs is therefore high. When it is warm the reverse happens R2 has a low resistance compared with R1 and Vs is low.

(b) The boiler should only come on if the three sensors are at 'logic 1' or when the override switch is pressed.

Draw a logic system below which can be used to solve this problem. Use only 2-input logic gates.

(c) The transducer driver part of the system is an NPN transistor. The transistor selected has an hFE of 100. Calculate the base current when the collector current is 500mA.

Formula                      hFE = Collector Current / Base Current

Working                     hFE = Collector Current / Base Current

                                 Base Current = Collector Current / hFE

                                       Base Current = 500 / 100

Answer and units                 Base Current = 5mA

(d) Draw the circuit symbol for an NPN transistor and label each of the legs with its correct name.

(e) A solenoid valve, controls the flow of gas to the boiler.

(i) Complete the circuit below by adding the transducer driver.

(ii) As the solenoid valve works in a similar way to a relay there is the possibility that the transistor could be damaged by 'back emf'. Name a component that could be added to the circuit to prevent this damage.

Diode

(iii) Add a component to the diagram above which would protect the transistor from 'back emf'.

Q12.  A baby equipment company has decided to produce a bath temperature alarm using an operational amplifier. A CAD program with the ability to simulate the function of electronic components was used to test the initial ideas for the new circuit.

(a) Give three advantages of using CAD programs for testing ideas.

1.   It is cheaper because components are not wasted

2.   It is easier to make changes

3.   It is much quicker to build and test

(b) A possible input circuit is shown below.

The resistance of the thermistor is 82K at 10⁰C. Calculate the value of Vs at this temperature.

Formula                                

Working                                 

                                             

Answer and units                    

(c) Complete the Op-amp circuit shown below to make it work as a comparator, which will indicate a high temperature by lighting the red LED and a low temperature by lighting the green LED.

Show clearly :

	the components used to sense the temperature connected to the inverting input
	the components used to set a reference voltage at the non-inverting input.

(d) Explain in detail how the components you have used as the temperature input to pin 2 control the red LED.

When the temperature is high the voltage at pin 2 will be lower than that at pin 3. Because pin 3 has the higher voltage the output will be high and this will cause current to flow through the red LED to 0V

(e) It was suggested that the red LED should latch on if the temperature is too high and the circuit was modified as shown below.

(i) Explain the function of the 10K resistor It provides feedback so that the output of the second gate is always the same as the input of the first.

(ii) The red LED failed to latch when the circuit is tested. Explain in detail, why the circuit did not work correctly. Because if the output of the op-amp goes low the circuit causes the latch to go low.

(iii) Draw a modified circuit which would latch the LED correctly, by adding a single component to the diagram above.