With Reference to the Circuit Shown Below

ELECTRONICS

’08 Q8c

With reference to the circuit shown below:

(i) Identify the electronic components A, B, C and D.

(ii) Describe the function of A and the function of B.

Ans.

B – Variable resistor.

C – Transistor.

D – Light bulb.

(ii) The component A is a light dependent resistor (l.d.r.), it will change resistance as

light falls on it and is commonly used as a light sensor.

The component B is a variable resistor which can adjust its resistance and is

commonly used as a dimmer switch.

In the circuit, the variable resistor and l.d.r. form a potential divider which

determines the light intensity needed to turn on the transistor. The l.d.r. senses the

light and the variable resistor can adjust the sensitivity of the circuit.

’07 Q8c

With reference to the circuit shown below:

(i) Identify the electronic components A, B, C and D;

(ii) Describe the function of components B and C in the circuit.

See diagram in past papers

Ans.

possible including rack and pinion, linkages, crank and slider, pulleys,

pneumatic control, etc.

A simple suggested solution is to use a rack and pinion mechanism to

automate a sliding door.

The motor driving the pinion is driven by the low voltage circuit. When

light flow to the light dependent resistor is interrupted, by a person

arriving at the doorway, the resistance is reduced. The transistor is

activated switching on the relay. The motor circuit is then switched on

causing the pinion to move the rack and open the door. The push-to-break

switch will prevent the pinion running off the rack.

(d) (i) A – Light bulb.

B – Transistor.

C – Capacitor (polarised).

D – Resistor.

(ii) The transistor will act as an electronic switch, turning the bulb on when it

is activated. It will also turn the bulb off when current to the base leg of

the transistor is removed.

The capacitor will gather, store and release electrical energy. In this

circuit, the size of the capacitor will determine how long the bulbs stay on

for.

’06 Q8c

With reference to the circuit shown below:

(i) Identify the electronic components A, B, C and D.

(ii) (ii) Explain the operation and suggest an application for the circuit.

See diagram in past papers

Ans.

B - Diode.

C – Bulb

D – Transistor.

(ii) Operation: In daylight the LDR has a very low resistance and therefore has a low

potential difference. This produces a low voltage at the base of transistor D.

Therefore no current flows across the base emitter junction and the output (lamp) is

off. When darkness falls the LDR has high resistance. The darlington pair switches

on and the coil of the relay is energized. This switches the AC circuit to on and the

lamp lights. The diode is used to protect damage to the transistors.

Application: Outside lights for security. Emergency lighting. Street lights.

’05 Q8c

With reference to the circuit shown below:

(i) Identify the electronic components A, B, C and D.

(ii) Explain the operation and suggest an application for the circuit

See diagram in past papers

Ans.

(i) A: Light dependent resistor

B: Diode

C: Motor

D: Transistor

(ii) Operation: The circuit uses a relay to power a large voltage circuit from a

smaller powered circuit. In the daytime the LDR has a very low resistance and

the voltage across it is low. Therefore the voltage across transistor D is very

low. No current can pass across the base / emitter junction and the motor is off.

If the LDR is covered or in darkness its resistance increases, the voltage across

the base / emitter rises and the transistor begins to conduct therefore energising

the relay. The relay in turn operates the motor.

Application:

’02 Q8

Describe the operation of the circuit shown, and outline an application for its use.

Page 11

Ans.

The circuit makes use of a BC108 transistor to sense water or damp

conditions. In dampness a small current flows through the water.

This raises the voltage at the base of the transistor, switching it on

and allowing a large current to pass through the LED. In dry conditions

The transistor is switched off and hence the LED.

Uses might include a simple moisture sensor or to determine if plants

Need

PNEUMATICS

’06 Q1 sect b

(n) Outline three advantages of using pneumatic sequential control in

industry.

(o) With reference to the pneumatic sequential control circuit shown:

(i) Name the components C, D, E and F;

(ii) Describe the function of any two components named.

See diagram in past papers

(p) With reference to pneumatic sequential control describe any two of the

following:

(i) Cascade circuit, (ii) PLC, (iii) FRL.

(q) (i) State two industrial applications where pneumatic sequential control

is used.

(ii) What are the benefits of using compressed air systems in industry?

(r) Utilising the diagram shown below, describe the sequence of operations

which will ensure that the component is drilled safely.

See diagram in past papers

Ans.

Section B – 50 marks

(n) Advantages of using pneumatic sequential control:

• Advanced industrial automation.

• Reduced labour costs.

• Can be extended to control a variety of complex tasks.

• High degrees of repeated accuracy.

• Reliability.

(o) (i) C: Double acting pneumatic actuator/cylinder.

D: 3/2 roller operated spring return valve (3-port valve)

E: Flow control valve or restrictor valve.

F: 5/2 control valve (5-port valve) (Name)

(ii) C – A double acting pneumatic actuator/cylinder provides power and motion to

pneumatic systems. It has a port at each end. Compressed air entering the rear port

moves the piston forward and the piston rod goes positive. Air in front of the piston

exhausts out of the front port. The reverse occurs when the air enters the front port. The

force it exerts is determined by the cylinder’s bore. The stroke of the cylinder

determines the max. linear movement it can produce. It is usually controlled by a 5-port

valve.

D – The 3/2 roller operated spring return valve. It’s basic function is to switch and

control the air flow. It has one port to which the supply from the compressed air main is

connected, a second port connected to the cylinder or an other valve and a third port

through which the cylinder can exhaust when the valve is turned off.

E – The flow control valve or restrictor valve is used to independently control the

outstroke speed and the in-stroke speed of a piston rod. A unidirectional flow regulator

has free flow in one direction and adjustable restricted flow in the other direction.

F- The 5/2 control valve or 5-port valve is usually used to control a double acting

cylinder. It is like two 3-port valves combined into one. It has two different air flow

patterns. (Any two)

(p) (i) Cascade circuit: To overcome the problem of in-operable circuits due to opposing

signals cancelling each other a cascade system can be utilised. The basis or this system

is the division of movement into groups. Each of these groups are provided with air

supplies, where only one air supply is pressurised at any one time. Air supply is changed

from valve to valve by activating a change over valve.

Page 6 of 21

(ii) PLC: Programmable Logic Controller. The use of electronics for the control of

sequencing of pneumatic applications. This further reduces any complications

associated with cascade circuits or time delays where high degrees of accurate

repeatability is essential. The PLC operates by monitoring input signals from sources

such as heat, pressure sensors, limit switches or reed switches. The PLC interacts with a

user-programmed internal logic switching network to produce an appropriate output

signal which in turn operates a pneumatic sequence.

(iii) FRL: This stands for filter, regulator and lubricator. The FRL unit ensures that the air

supply is clean and at the correct pressure. It also carries fine particles of oil in the air

to lubricate all essential parts within the system. (Any two)

(q) (i) Industrial applications include:

• Packaging

• Painting

• Plastic moulding

• Drilling/machining

(ii) Benefits of using compressed air systems in industry:

• Versatile

• Safe in use

• Reliable power source

• Requires simple maintenance.

(r) 1 – Cylinder A positive, this closes the vice and clamps the work-piece.

2 - Cylinder B positive, the drill comes down and drills the work-piece.

3 - Cylinder B negative, the drill is withdrawn and returns to it’s starting position.

4 - Cylinder A negative, The vice opens and releases the work-piece.