TA2: Electronic and programmable systems, components and devices

What you need to know

• The purpose of system block diagrams • How to interpret system block diagrams.

The systems approach allows electronic systems to be communicated clearly and effectively. It involves the use of block diagrams to show the different inputs, processes and outputs that will be used.

The systems approach A block diagram gives a top-down overview of a system. • The blocks represent groups of components, or subsystems. • The arrows show the signals that flow between them. Input Process Output Block diagrams • An input device takes an environmental signal and turns it into an electronic signal. For example, a light sensor. • A process device changes the electronic signal in some way. For example, by turning it on for a set amount of time. • An output device turns the electronic signal back into an environmental signal. For example, a buzzer produces sound. Input Process Output Light sensor Timer Buzzer Light level Electronic signal

Sound Electronic signal DRAFT

Practise it! Remember it!

1 State what is represented by each of the following in a block diagram: (a) the blocks (1 mark) (b) the arrows. (1 mark) 2 Name the three types of blocks used in a systems diagram. (3 marks) A block diagram shows a top-down overview of a system in terms of its inputs, processes and outputs.

What you need to know

• The difference between open and closed loop systems • How feedback loops are used in electronic and programmable systems.

• Closed loop systems use feedback, whereas open loop systems do not. • Feedback is when the output signal becomes an input signal to the system.

The main two types of systems are open loop and closed loop. Understanding the differences between them is an essential part of system design.

Open and closed loop systems A feedback loop is created when the output signal of a system becomes an input signal to the same system. Feedback allows systems to ‘self-correct’. • Systems without a feedback loop are called open loop systems. For example, temperature control and robotic arm systems. • Systems with at least one feedback loop are called closed loop systems. For example, simple lighting systems, simple timers and counters. A closed loop system A heating system is a good example of a closed loop system. • The heater is turned on by the microcontroller when the sensor detects that the temperature has dropped below the set level. • The new temperature information is then ‘fed back’ to the input of the system. • The microcontroller therefore knows when it is warm enough to turn the heater off again. Temperature sensor

Microcontroller Heater Input Process Output Feedback DRAFT

Practise it!

1 Describe what is meant by a closed loop system. 2 Give two examples of closed loop systems. (2 marks)

(2 marks)

Remember it!

What you need to know

• The purpose of circuit schematics • How to interpret circuit schematics.

Circuit schematics show how all the individual components in a circuit are joined together. This is very useful for allowing engineers to clearly see the technical detail of each circuit that they are designing.

Interpreting circuit schematics This is a circuit schematic for a transistor switching circuit. Circuit schematics are usually drawn from left to right, starting with the power supply Standard symbols are used to represent the components Abbreviations are used to label the components

VR1 42k R2

R1 525 lux The value of a component is shown, if it is known

BL1 B1 9V Q1 1k Wires are drawn with straight lines Why circuit schematics are used • They use standard symbols. Therefore, it is clear which components are being shown in each circuit. • They show how all the individual components relate to each other. • They take up less space than pictorial diagrams. • If drawn in computer aided design (CAD) software, they can often be exported to automatically produce printed circuit board (PCB) layouts. Remember it!Practise it!DRAFT 1 Describe the purpose of circuit schematics. (2 marks) 2 State the abbreviation that should be used for each of the following: (a) the first battery in a circuit (1 mark) (b) the third resistor in a circuit (1 mark) (c) the second transistor in a circuit. (1 mark) • Schematics show how all the individual components in each circuit relate to each other. • Standard symbols and abbreviations are used to represent the components.

What you need to know

• The purpose of PCB layouts • How to interpret PCB layouts.

Once a circuit has been designed, a printed circuit board layout needs to be produced ready for manufacture and assembly.

The purpose of PCB layouts Printed circuit board (PCB) layouts show the physical layout of the circuit board that is to be manufactured, including: • the layout of the copper tracks that join the components together • the pads where components will be soldered. A PCB layout can be used: • to produce a mask or a CAD file, to use for manufacturing the circuit board • to aid with assembling the components on a circuit board after the board has been manufactured. Interpreting PCB layouts PCB layouts are usually produced using CAD software. • The ‘artwork’ view shows the mask that is used during manufacture.

• The ‘real world’ view shows what the PCB will look like with all the components soldered in place. DRAFT

Practise it! Remember it!

1 Describe the purpose of

PCB layouts. 2 State the purpose of each of the following PCB layout views: (a) artwork

(b) real world. (2 marks)

(1 mark)

(1 mark) • PCB layouts show how the copper pads and tracks are to be arranged on the physical circuit board. • They can be used to produce a mask or exported to computer aided manufacturing (CAM) equipment for manufacture.

What you need to know

• The purpose and function of different types of switches • The typical uses and applications of different types of switches.

• Switches allow or prevent current flow, depending on certain conditions. • Different types include SPST, push, tilt and reed switches.

Input devices respond to changes in the environment around them. They change environmental information into an electronic signal. Switches are a common type of input device.

SPST switch Push switch

A single pole single throw (SPST) switch allows current to flow when closed and stops current flow when open. SPST switches can be used for turning power supplies on and off, and for latching output devices on and off. For example, light switches. Tilt switch A tilt switch allows current to flow when tilted to one side. Tilt switches can be used for sensing movement and tip-over warnings. For example, for silos or items on conveyor belts.

There are two types of push switch: • Push-to-make switches allow current to flow when pressed. • Push-to-break switches prevent current flow when pressed. Push switches can be used for games and score counting systems, and electronic doorbells. Quantum tunnelling composite (QTC) switches vary their resistance with applied pressure. They can be used as more complex push switches, or touch sensors. Reed switch A reed switch allows current to flow when close to an applied magnetic field. Reed switches can be used for door or window sensors in home security systems, and automatic door locks. DRAFT

Revise it!

• Create a table showing different types of input devices (switches and sensors). • Add columns for their function, purpose and typical applications. • Draw the circuit symbols for each input device.

Remember it!

What you need to know

• The purpose and function of different types of sensors • The typical uses and applications of different types of sensors.

• Sensors detect changes in the environment around them. • Different types include light, temperature and infrared sensors.

Input devices respond to changes in the environment around them. They change environmental information into an electronic signal. Sensors are another common type of input device.

Light sensors There are two types of light sensors: • Light-dependent resistors (LDRs) have a resistance that decreases as the light level increases, and vice versa. • Photodiodes use light energy to produce a flow of current. LDRs and photodiodes can be used for night and street lighting, light meters and infrared receivers. Pressure sensor A pressure sensor allows current to flow according to the pressure level. Pressure sensors can be used for monitoring pressure (for example, within a gas boiler), and fluid flow measurement. Touch screen A touch screen reacts when touched by another conductive medium, such as a finger. This changes the capacitive charge at the point of contact. Touch screens can be used for car infotainment systems, tablet computers and mobile phones.

NTC thermistor A negative temperature coefficient (NTC) thermistor has a resistance that decreases as the temperature level increases, and vice versa. NTC thermistors can be used for sensing temperature and climate control systems. Infrared sensor An infrared sensor measures the presence of infrared light within its field of view. Infrared sensors can be used for motion detectors in alarm systems, and proximity sensors for robots. Smart sensor A smart sensor is any type of sensor that can send and receive wireless signals to or from a main processing device. Smart sensors can be used for home automation systems, such as automatic curtain openers and lighting systems. Remember it!Practise it!DRAFT Suggest a suitable input device for each of the following applications. (a) Movement sensor for a home alarm. (1 mark)

(b) Climate control system for a food growing facility.

(c) User interface for a smart home control system. (1 mark)

(1 mark)

What you need to know

• The purpose and function of amplifiers • The typical uses and applications of amplifier circuits.

• Amplifiers are used to increase the size of signals, such as voltage or current. • Operational amplifiers (op-amps) are often used in comparator circuits.

Amplifiers are process devices. Amplifiers are used in a huge range of electronic products, from audio systems to complex communications devices.

Purpose and function of amplifiers Amplifiers increase the size of the signal that is flowing into them. The signal is usually either a voltage or a current. The amount of amplification is called the amplifier gain. It is calculated by dividing the output signal by the input signal: gain (A v) = V out Vin or gain (AI) = I out Iin Op-amp • An operational amplifier (op-amp) is a type of amplifier integrated circuit (IC) that produces an extremely high gain. • Op-amps have two inputs (inverting and non-inverting) and one output. • Op-amps are often used when comparing an input signal to a reference. Applications Uses

Audio amplifiers and speaker systems Temperature control Communications systems Comparator circuits –+ DRAFT

Practise it!

1 Describe what is meant by amplifier gain. 2 State two applications of amplifier circuits. (2 marks)

(2 marks)

Remember it!

What you need to know

• The purpose and function of counters • The typical uses and applications of counter circuits.

• Counters add up the number of input signals received. • Decade counters are decimal counters that can count from 0 to 9.

Counters are process devices. Counters are used in a huge range of electronic products and applications, from digital clocks to score counters for games.

Purpose and function of counters Counters add up the number of input signals or pulses received. This is usually in relation to a ‘clock’ input. Once the counted signals have exceeded the maximum value for the particular IC, the counter will reset back to zero. Decade counter • A decade counter is a type of counter IC that can count ten decimal digits (0–9). • Results of counting can be outputted using a series of light emitting diodes (LEDs) or a digital display. • A push-to-make switch or a pulse generator can provide the input signals for each ‘count’. Applications

L1 Q1 L2 Q2 L4 Q4 L8 Q8 UP DN LD C BCL Uses

Measurement devices Waveform generators Score counters Digital clocks DRAFT

Practise it!

1 Describe the function of a decade counter IC. 2 State two applications of counter circuits. (2 marks)

(2 marks)

Remember it!

What you need to know

• The purpose and function of timers • The typical uses and applications of timer circuits.

Timers are process devices. Timers are used in a huge range of electronic products, from security systems to electronic kitchen equipment.

Purpose and function of timers Timers produce an output signal, pulse or time delay that lasts for a certain period of time. For example, turning on a buzzer for five seconds when a doorbell is pressed. A simple timer can be created using a resistor–capacitor (RC) network, but this can lack accuracy. 555 timer IC • The 555 timer IC is a type of timer IC that can be used in either astable (pulse generator) or monostable (timer) mode. • In monostable mode, the length of each timed pulse is set using resistor and capacitor values. Applications Kitchen timers Automated lighting systems

Alarms and security systems Sports timers Electronic doorbells Uses

8 5 4 6 3 2 1 7 DRAFT

Practise it!

1 State two methods of achieving an electronic timer. (2 marks)

2 State two applications of timer circuits. (2 marks)

Remember it!

• Timers produce an output signal that remains high or low for a set period of time. • A 555 timer IC can be used in monostable mode to achieve this.

What you need to know

• The purpose and function of latches • The typical uses and applications of latching circuits.

• Latches, once triggered, ‘lock’ the output signal into either a high or a low state. • Thyristors can be used to achieve a simple latching function.

Latches are process devices. Latches are used in a huge range of electronic products, from security systems to electronic kitchen equipment.

Purpose and function of latches Latches produce an output signal that stays the same until it is reset. They effectively ‘lock’ it in place. Latches are triggered by a momentary high (on) or low (off) input signal, depending on the type used. The output signal can also be high or low. A bistable multivibrator is a type of latch that can be used to store logic state information. Thyristor • A thyristor is a component that can be used to provide a simple latching function. • A push-to-make switch is typically used as the reset. Applications Uses

Computer memory Alarms and security systems Data storage Power switching R (reset) S (set) set–reset latch DRAFT Q

Q’

Practise it!

1 Describe the function of a latching circuit. 2 State two applications of latching circuits. (2 marks)

(2 marks)

Remember it!

What you need to know

• The purpose and function of pulse generators • The typical uses and applications of pulse generator circuits.

• Pulse generators produce a sequence of

‘high’ and ‘low’ pulses. • A 555 timer IC can be used in astable mode to achieve this.

Pulse generators are process devices. Pulse generators are used in a huge range of electronic products, from flashing lighting systems to electronic circuit test equipment.

Purpose and function of pulse generators Pulse generators produce a continuous series of digital pulses. The output signal cycles between high and low states until stopped. For example, when producing a series of flashing LEDs for a cycle safety light system. 555 astable • The 555 timer IC acts as a pulse generator when in astable mode. • The time that the output signal is high (time high) or low (time low) depends on the resistor and capacitor values used. Applications Uses

Flashing lights Signal waveform generators Providing input signals to counters/digital clocks Pulsing alarm sounders

8 5 7 6 3 2 1 4 DRAFT

Practise it!

1 State two components that control the ‘time high’ and ‘time low’ of an astable circuit. (2 marks) 2 State two applications of pulse generator circuits. (2 marks)

Remember it!

What you need to know

• The purpose and function of analogue to digital converters (ADCs) • The typical uses and applications of analogue to digital converter circuits.

• ADCs take analogue signals and change them into digital signals. • This is useful in microcontroller systems that need to interact with analogue sensors.

ADCs are process devices. Digital integrated circuits cannot understand analogue signals. For example, from an analogue sensor. This means they need to be converted before they can be processed.

Purpose and function of ADCs Analogue to digital converters (ADCs) change analogue signals into digital signals. For example, converting the continuous signal from a temperature or light sensor into a discrete signal that a microcontroller can process. Many modern microcontrollers have ADCs built into them.

ADC

analogue signal digital signal Applications Uses

Signal processing Light sensing systems Temperature control systems Computer and microcontroller systems DRAFT

Practise it!

1 Describe the purpose of analogue to digital converters (ADCs). (2 marks) 2 State two applications of

ADC circuits. (2 marks)

Remember it!

What you need to know

• The purpose and function of different logic gates • The truth tables for AND, OR, NOT and NAND logic gates.

• Logic gates are digital devices that respond to, and output, signals of 1 or 0. • The truth table for each gate shows what the output state is depending on the inputs.

Logic gates are process devices. Logic gates respond to, and output digital signals. They produce an output based on the input signals that they receive. The output signal for each combination of input signals is shown using a truth table.

AND gate An AND gate produces a high (1) output when both inputs are high. The output is low (0) when either or both inputs are low.

Input A Input B Output 0 0 0 0 1 0 1 0 0 1 1 1 Truth table NOT gate A NOT gate produces a high (1) output when the input is low, and a low (0) output when the input is high.

Input Output 0 1 1 0 Truth table

OR gate An OR gate produces a high (1) output when either or both inputs are high. The output is low (0) when both inputs are low. Input A Input B Output 0 0 0 0 1 1 1 0 1 1 1 1 Truth table NAND gate A NAND gate produces a low (0) output when both inputs are high. The output is high (1) when either or both inputs are low. Input A Input B Output 0 0 1 0 1 1 1 0 1 1 1 0 Truth table DRAFT

Revise it!

• Produce a mind map of everything that you know about logic gates. • Draw the logic gate symbols and truth tables for AND, OR, NOT and NAND gates. • Add typical uses and applications of logic gates to your mind map.

Remember it!

What you need to know

• How logic gates are used both singly and in combination.

• Logic gates can be used on their own or in combination with other gates. • Combining logic gates allows more complex functions to be achieved.

Logic gates can be used either on their own or in combination with other gates.

Combining logic gates

Sometimes single logic gates cannot perform the functions required by the system designer. Combining them allows the system to perform more complex functions. These can be logic gates of the same or different types. For example, this diagram shows a lift system with two AND gates. The lift only moves upwards when a person has entered, the floor button has been pressed and the doors have fully closed. Applications Latching circuits Traffic control systems

Uses Lift systems Simple computer memory Alarm systems AND AND DRAFT

Practise it!

1 Identify four types of logic gates. (4 marks) 2 State two applications of logic circuits. (2 marks)

Remember it!

What you need to know

• The purpose and function of different light output components and devices • The typical uses and applications of different light outputs.

• Light outputs change an electronic signal into visible light. • This can then be adapted in the form of electronic displays to show information.

Output devices change an electronic signal into an environmental signal that we can understand. For example, light outputs produce visible light, which can be used to light rooms or show information.

Lamp A lamp produces light when current flows through and heats up a filament. Lamps can be used for decorative lighting, task lighting and indicator bulbs. Liquid crystal display A liquid crystal display (LCD) uses liquid crystals and a fluorescent backlight to produce visible text and images. LCDs can be used for instrumentation and information panels, TV/monitor screens and digital watches/clocks.

Light emitting diode A light emitting diode (LED) produces light when current flows from the anode (+) to the cathode (−). LEDs can be used for on/off indicators, garden solar lighting, car indicators, room lighting and coloured/ambient lighting. LED display An LED display works like an LCD, but the backlight is provided by LEDs, rather than fluorescent lighting. LED displays can be used for instrumentation panels, TV/monitor screens, digital watches/clocks DRAFT and information screens.

Revise it!

• Make a list of all the different light output devices you can see around your home or in school/college. • State what each one is used for.

Remember it!

What you need to know

• The purpose and function of different sound and movement output devices • The typical uses and applications of different sound and movement outputs.

• Sound outputs change an electronic signal into audible sound. • Motors change an electronic signal into rotary movement.

Output devices change an electronic signal into an environmental signal that we can understand. For example, sound outputs produce audible sounds, whereas motors produce rotary movement.

Buzzer Piezo sounder

A buzzer uses an internal oscillator to produce a buzzing or bleeping sound when current flows through it. Buzzers can be used for door buzzers, kitchen timers, quiz buzzers and collision warnings. Motor A motor produces rotary movement when current flows through it. Reversing the direction of the current also reverses the direction of movement. Motors can be used for portable fans, robot arms and electric vehicles.

A piezo sounder uses the piezoelectric effect to produce sound when current flows through it. Piezo sounders can be used for door and window alarms, musical birthday/gift cards and smoke alarms. Generator A generator is a motor in reverse. Generators can be used for producing electrical energy when they are rotated. For example, through wind turbines and hydroelectric dams. DRAFT

Practise it!

1 Identify two output devices that could be used to produce indicator lighting for a car. (2 marks) 2 Identify two output devices that could be used in a door alarm system to produce a warning sound. (2 marks)

Remember it!

What you need to know

• The purpose and function of transistors, Darlington drivers and relays • The typical uses and applications of drivers and interface devices.

• Drivers increase the signal from the process device to the output of a system so that it can function correctly. • Transistors can operate as an amplifier or as an electronic switch.

Sometimes the process device in a system cannot supply enough current for the output to work effectively. In these instances, a driver (interface device) is needed.

NPN transistor

An NPN transistor allows current to flow from the collector to the emitter when the base voltage is 0.6 V or higher. An NPN transistor can be used as an electronic switch or as a current amplifier. Collector Base Emitter

Darlington driver A Darlington driver is a pair of transistors connected together to create a much higher current gain. A Darlington driver can be used when an output device requires a high amount of current to function. For example, when using a microcontroller to control a motor. Relay A relay is an electrically operated switch. It uses a magnetic field to operate internal switch contacts. A relay can be used as an interface between two separate circuits. For example, switching on a high voltage circuit from a low voltage supply.DRAFT

Practise it!

1 Explain the function of a transistor. 2 Describe one application of a relay. (3 marks)

(2 marks)

Remember it!

What you need to know

• The purpose and function of resistors • The typical uses and applications of resistors in circuits.

• Resistors reduce the flow of current. • They can be fixed or variable. • They are passive components.

Sometimes it is necessary to control the flow of current around a circuit. This is where resistors are useful.

Purpose and function Resistors reduce the flow of current through a circuit. There are two types of resistors: • Fixed resistors have a set resistance value. • In variable resistors, the resistance value can be adjusted. Resistors are passive components. This means they do not introduce any new energy into a circuit or require their own power supply. Fixed resistors are non-polarised. Therefore, they can be connected any way round in a circuit. Uses in circuits • To limit the current to protect other components from too much current. • To set the length of time periods or time delays. For example, in an RC network or monostable circuit. This diagram shows an RC timing network. • To create potential divider circuits. For example, to allow sensors to interact with process devices. B1 Fixed resistor symbol R1 C1

Variable resistor symbol + DRAFT

Practise it!

1 Explain the difference between fixed and variable resistors. (2 marks) 2 State two applications of resistors in circuits. (2 marks)

Remember it!

What you need to know

• The purpose and function of diodes • The typical uses and applications of diodes in circuits.

• Diodes allow current to flow in one direction only. • They are polarised components. • LEDs are special types of diodes that emit light.

Sometimes it is necessary to control the direction in which current can flow in a circuit. This is where diodes are useful.

Purpose and function Diodes allow current to flow in one direction only, from the anode to the cathode. Most diodes are passive components. However, some diodes are active components, because they can add energy into a circuit. For example, an LED is an active component. An LED is a special type of diode that emits light. Diodes are polarised. Therefore, they must be connected the right way round in a circuit. Anode + Cathode – Applications in circuits • To convert AC to DC. For example, as shown in the diagram, in bridge rectifier circuits. • For signal processing. • For protecting components (for example motors) from back emf, which opposes the driving voltage and can cause overheating. • Clipping diodes are used in audio applications. DC output AC input –

+ DRAFT

Practise it!

1 Describe the purpose of diodes. (2 marks) 2 State two applications of capacitors in circuits. (2 marks)

Remember it!

What you need to know

• The purpose and function of capacitors • The typical uses and applications of capacitors in circuits.

• Capacitors store charge. • They can be polarised or non-polarised. • They are passive components.

Energy storage plays an important role in all our lives. Capacitors are becoming increasingly useful in this area of electronic engineering.

Purpose and function Capacitors store electrical charge. They are passive components. There are two types of capacitors: • Polarised capacitors have a positive and a negative terminal. For example, electrolytic capacitors. • Non-polarised capacitors can be connected any way round in a circuit. For example, when used as a feedback capacitor in an op-amp circuit. • Most capacitors have a capacitance measured in microfarads. However, newer

‘supercapacitors’ can have capacitances measured in farads or tens of farads. Applications in circuits • To set the length of time periods or time delays. For example, in an RC network or monostable circuit. The diagram shows an

RC timing network. • To filter signals. For example, in an AC–DC converter circuit. • To store electrical energy. For example, capacitors can be used in place of batteries in electric vehicles.

B1

Non-polarised capacitor symbol R1 C1

Polarised capacitor symbol + + DRAFT

Practise it!

1 Explain the difference between polarised and non-polarised capacitors. (2 marks) 2 State two applications of capacitors in circuits. (2 marks)

Remember it!

What you need to know

• How different power supplies work • The suitability of different power supplies for different uses and applications.

• Power supplies ensure electronic and programmable devices have the correct voltage and/or current to work correctly. • Most electronic and programmable systems require a low voltage DC supply.

Electronic and programmable systems need a source of power to be able to function. Engineers must be able to select the correct type for the application needed.

Batteries Photovoltaic cells

Batteries store chemical energy and convert it into electrical energy. They come in a range of different voltages and packages, including 9 V PP3, AA, AAA and C types. They are used when a portable, low voltage power supply is needed. For example, to power mobile phones, tablet computers, digital watches and clocks. They are not suitable when an AC supply is required. Supercapacitors Supercapacitors are a special type of capacitor. They can store much more charge than normal + capacitors. They can be used instead of batteries for some low voltage applications, but need recharging regularly. They are used in some countries to power electric transport systems.

Photovoltaic cells convert energy from the Sun into electrical current. They are a clean and renewable source of energy. They cannot create electricity when there is no sunlight. However, the energy they do create can be stored. Mains adaptors In the UK, mains power is set at 230 V AC. Mains adaptors are used to reduce the voltage and/or convert the AC power supply to a DC supply. This is necessary because most programmable and electronic devices use a low voltage DC supply. Some adaptors can also be used as battery chargers. Mains electricity is extremely dangerous if dealt with incorrectly. + DRAFT

Practise it!

1 Identify two power supplies that would be suitable for powering an electric vehicle. (2 marks) 2 Explain one problem with using mains adaptors to power products. (2 marks)

Remember it!

What you need to know

• The characteristics of different types of wires • The typical uses and applications of different wiring types in circuits.

• Single-strand wire has a single core surrounded by insulation. • Multi-strand wire consists of several wired strands surrounded by insulation.

Wires create electrical connections between different components, thus allowing current to flow. It is important to choose the right wiring type for the application needed.

Single-strand wire Single-strand wire is made up of a single solid core of conducting material surrounded by insulation. Single-strand wire is more suitable for some applications than others: • It is easier to solder than multi-strand wire as it does not fray. • It is better suited to use with breadboards than multi-strand wire. • If the single core within it breaks when the wire is flexed, then conductivity is lost. Multi-strand wire Multi-strand wire is made up of several flexible strands of wire surrounded by insulation. Multi-strand wire is more suitable for some applications than others: • It is well-suited to applications where it might be moved, flexed or placed under mechanical stress. For example, when connecting a battery or a switch to a PCB. • If one strand breaks, the others will still conduct. • It is difficult to place through holes due to fraying. For example, in a breadboard, DRAFT stripboard or a PCB.

Practise it!

1 Explain one application of single-strand wire. 2 Explain one application of multi-strand wire. (2 marks)

(2 marks)

Remember it!