IGCSE Systems and Control Presentation

What is Technology?

KNOWLEDGE?

Questions?y What is Technology? y Is Technology neutral? y What does it mean when he says technology

is selfish and generous? y Why is there a dilemma with the use of Technology?

Design Process

Course Work RequirementsCriterion 1. Identification of a need or opportunity with a brief analysis leading to a design brief 2. Research into the design brief resulting in a specification Level of Response A statement of what is to be made. Consideration of the design need or the intended user(s) leading to a design brief. Consideration of both the design need and the intended user(s) leading to a clear design brief. Limited examination of the design brief with a specification identifying some basic requirements. Meaningful research of the design brief with some data identified. A specification including key features of the intended product. Thorough research of the design brief with relevant data identified and collected. Analysis of the research leading to a detailed specification for the intended product. A limited range of ideas with a tendency to focus on a single concept. Little or no evaluation of ideas. A range of appropriate solutions proposed. Ideas examined with evaluations leading to the identification of possible ideas for development. A wide range of appropriate solutions with imaginative interpretation. Detailed evaluation of ideas and consideration of the requirements of the specification. Mark Range 1 23 45 5 Maximum Mark 13 47 810 17 813 1420 20 10

3. Generation and exploration of design ideas

Course Work Requirements Contd.Criterion4. Development of proposed solution

Level of ResponseSome decisions made about form, materials and/or construction methods. As a result of investigation, appropriate decisions made about form, materials and construction/production methods. Evidence of some testing and/or trialling. Appropriate testing and trialling resulting in reasoned decisions about form, materials, construction/production methods and other items. Limited evidence of any forethought. A working drawing with little detail. A simple plan showing awareness of the main processes involved. A clear working drawing showing overall layout and major dimensions. Clear and detailed planning showing an effective order for the sequence of operations. Drawings and other information give full details of the final product. The product will exhibit a reasonable standard of outcome, be mainly complete and satisfy some aspects of the specification. The product may have some minor inaccuracies and blemishes but will be complete and function as intended. The product will be completed to a high standard of outcome with precision and accuracy. It will meet fully the requirements of the product specification.

Mark Range 15 6101115 13 46 710 110 1120 2130

Maximu m Mark 15

5. Planning for production

10

6. Product realisation

30

Course Work Requirements Contd.Criterion7. Testing and evaluation

Level of ResponseLittle or no evidence of testing. General overall appraisal with little reference to the specification. Appropriate reporting and/or comment on simple testing. Reference to the specification with some conclusions leading to possible modifications or improvements. Objective testing with reference to the specification and user. Detailed and meaningful conclusions leading to proposals for further development.

Mark Range13 46 710

Maximum Mark10

Review Study Guide

Review study guidey Pages 41

73

y Pages 73 - 99

Why Present DrawingsDrawings giving detailed information about structure, dimensions, materials, and assembly instructions for a project are done using one of a number of formal techniques. The main ones are isometric projection and perspective drawing (both 3D), and, for working drawings - orthographic projection, section drawings and assembly drawings (2D).

3D - ISOMETRICy In isometric projection all vertical lines on an object remain vertical while horizontal lines are drawn at 30 to the horizontal. Isometric drawings can be sketched or produced with drawing equipment to ensure accuracy. y Isometric projection distorts shapes slightly in order to keep all upright lines vertical (and because perspective is ignored). Their advantage is that they show the object's dimensions accurately and in correct proportion to each other, making it easy to draw the projection correctly to scale.

Sketching Isometric Drawingsy Use of isometric templates. y Practice sketching the isometric of the given example. y Learn how to draw circles. y Learn how to use crating to draw other objects

example cylinder.

Perspective Drawingsy y

y

In perspective drawing the forward face or edge of the object is drawn first, with the other lines receding away from the viewer and gradually approaching each other just as they appear to do when you look at a real object. If the receding lines are extended they will meet at points that are called vanishing points. Perspective drawing can be done using drawing equipment or freehand. Perspective drawing can use one, two or three vanishing points. One-point perspective is often used for room interiors. Two-point perspective has many applications for developing ideas in 3D. Three-point perspective is often used for drawings of tall buildings. The advantage of perspective drawing is that it makes objects appear more realistic, as objects appear to get smaller as their distance from the observer increases.

Practice Drawing a Perspectivey On an A4 piece of paper, practice drawing the L shape

previously shown with single and double point perspective.

Working Drawingsy Drawings which contain all the information needed to make the

object you have designed, including dimensions and details of components, materials (bill of materials) and assembly instructions, are called working drawings. Although working drawings for simple products may sometimes be done in isometric projection, the normal drawing technique for working drawings is orthographic projection. y Some products may need a section drawing to give extra structural information, or an assembly drawing to show how parts fit together.

Orthographic Projectiony Orthographic projection shows

complex objects by doing a 2D drawing of each side to show the main features. Orthographic drawings usually consist of a front view, a side view and a plan, but more views may be shown for complex objects with lots of detail. A drawing board and T-square is used to project one view from another. y Orthographic drawing may be done using first angle projection or third angle projection. The graphic on the right shows the differences between the two.

Section Drawingsy Section drawings show the various parts of a

product as if it had been sliced in half. (Sometimes they are called cross-sections.) The position of the imaginary cut is called a section plane, sometimes represented by a line consisting of long and short dashes. y The purpose of a section drawing is to make clear how a product is constructed. Parts of the object that are cut through are shaded with lines at 45 and spaced 4mm apart - called cross-hatching. If two parts of a product are touching, then the cross-hatching goes in opposite directions. Parts such as nuts and bolts and axles are not normally sectioned.

Assembly Drawingy An assembly drawing shows the various

parts of a product drawn to show exactly how they fit together. They are often used for products such as construction and model kits or flat-pack furniture, to show the user how to assemble the parts. y They can be drawn in two ways.y A fitted assembly drawing shows the

parts put together, and can be drawn in 2D or 3D. y An exploded drawing shows the parts separated, but in the correct relationship for fitting together. Exploded views are usually drawn in 3D, as illustrated.

Practice Sketching the Drawing Showny Use A4 piece of paper or

Isometric Template Paper. y Show construction lines.

Basic Drawing Techniques:

Thick and thin lines.

How they work: If the spider crawls over the edge and you can still see the spider, the line stays THIN. If it crawls over the edge and you cant see it anymore, the line becomes THICK.



How they work: If the spider crawls over the edge and you can still see the spider, the line stays THIN. If it crawls over the edge and you cant see it anymore, the line becomes THICK.



On your sketch of the camera: Add the correct thick and thin lines to it using a pen or dark pencil. See examples on next slide.


Tones

1st surfaces the light hits: Lightest tone. 2nd surfaces the light hits: Medium tone. Surfaces away from the light: Dark tone.


Tones

1st surfaces the light hits: Lightest tone.


Tones

2nd surfaces the light hits: Medium tone.


Tones

Surfaces away from the light: Dark tone.


Tones

1st surfaces the light hits: Lightest tone. 2nd surfaces the light hits: Medium tone. Surfaces away from the light: Dark tone. On your camera drawings, show where the light is coming from and add the correct tones.

1. When identifying possible markets for a product, you will seek to establish: Who would buy the product? Why would they buy it? Where would it be used? Who would use the product? All of the above

2. Designing products to meet the demand from consumers is called consumer pull or push? Pull Push

3. When designing a product, a designer has to make sure that it meets the design specification. Which one of the following would you NOT expect to see in a specification? Dimensions Ergonomics Materials Employee wages Environmental considerations

4. What are the two key aspects to quality in Design and Technology? y the quality of design and the quality of the packaging y the quality of design and the quality of the sales staff y the quality of the design and the quality of the manufacture y the quality of design and the quality of the marketing y the quality of design and the quality of the website

5. These two aspects of quality can be independent of each other: true or false? True False

1. Social and cultural influences study whether the product reflects or offends the conventions of society: true or false? True False

2. Consumer choice is the technique used by manufacturers to choose their customers: true or false? True False

3. Product maintenance considers what maintenance is required, who will maintain it and other aspects of the product [...] The missing word is: Lifecoach Lifestyle Lifetime Lifecycle Lifespan

4. Which of the following would NOT normally be considered under moral and environmental issues? Sustainability Productivity Recycling Conservation Health and safety

5. What sort of technology protects the environment from the pollution caused by industry and transport?High technology Information technology Design and technology Organic technology Green technology

6. Which of the following energy sources is NOT renewable? Solar Wind Coal Hydroelectric Tidal

What is a System?A system is a set of devices or things which are connected and work in conjunction with each other in order to perform a specific function.

All systems haveINPUT PROCESS OUTPUT

Example:

INPUT

PROCESS

OUTPUT

INPUT Door sensor

A system is sometimes made of subsystems joined together.OUTPUT

INPUT Window sensor INPUT Panic button

PROCESS

Siren Flashing light

Burglar Alarm

INPUTInput sensors usually convert a change in light, movement, temperature or humidity into changes in electrical signals. The inputs are often tiny changes of voltage, current or resistance.

PROCESSThe process or processor is the part of a control system which reacts to the inputs. The changes to the inputs are usually too small to operate output devices like buzzers or motors, so they have to be amplified to give a large enough change to operate the output devices.

Processing DeviceAmplifiers Electronic switches Timers Counters Computers Micro-processors

ExampleAmplifies small inputs Switches at different levels Switches after time delay Counts input pulses Detects signals from inputs Detects signals from inputs

OUTPUTOutput devices are those which convert electrical signals into sound, light or movement.Output DeviceLamps Buzzers/Bells Speakers Motors Indicators Light Sound Sound Movement Information

Example

1. A closed loop system consists of four separate sections. The first section is called the:Ingredients Start Feedback Input User

2. Raw materials and information are changed in which part of a closed loop system?Product Process Ingredients Backchat Users

3. The result or what happens at the end is called the: Output Process Product Feedback Ingredients

4. A result of the system that influences the input is called: Output Backchat Product Input Feedback

5. What type of system takes information from the output to control the system? Circle loop Open loop Closed loop Single loop Whole loop

6. The feedback in a control system is the information from the output, which loops back to influence and control the processes. True False

7. Which of the following is an advantage of using control systems in manufacturing? They provide a high level of accuracy. They monitor safety and performance. They automate tedious repetitive tasks. They are quick and can operate continuously. All of the above

8. A system is a set of components working together to carry out a particular function. Systems can consist of: Mechanical components Electrical or electronic components Pneumatic components Hydraulic components Systems can consist of any or all of the above.

Binary Numbers

Binary Numbers

Logic Gates

A & B = Input, Q = Output

A Q 0 1 1 0

A 0 0 1 1

B 0 1 0 1

Q 0 1 1 1

A 0 0 1 1

B 0 1 0 1

Q 0 0 0 1

Logic Tables or Truth Tables

Logic Gates

A 0 0 1 1

B 0 1 0 1

Q 0 1 1 1

A 0 0 1 1

B 0 1 0 1

Q 1 0 0 0

A 0 0 1 1

B 0 1 0 1

Q 0 0 0 1

A 0 0 1 1

B 0 1 0 1

Q 1 1 1 0

NAND = Opposite of AND (Not AND)

Logic Gates

A 0 0 1 1

B 0 1 0 1

Q 0 1 1 0

A 0 0 1 1

B 0 1 0 1

Q 1 0 0 1

Exclusive OR and Exclusive NOR

Logic Gates

A Q 0 1 1 0

A 0 0 1 1

B 0 1 0 1

Q 0 1 1 1

A 0 0 1 1

B 0 1 0 1

Q 0 0 0 1

A 0 0 1 1

B 0 1 0 1

Q 1 0 0 0

A 0 0 1 1

B 0 1 0 1

Q 1 1 1 0

A 0 0 1 1

B 0 1 0 1

Q 0 1 1 0

A 0 0 1 1

B 0 1 0 1

Q 1 0 0 1

Switches as logic Gatesy What happens when I

press the switch ?y Not pushed = logic 0 y Pushed

= logic 1

What do I need to do to get the LED to light ?Button A Button BButton A Button B LED

What do I need to do to get the LED to light ?Button AButton A Button B LED

Button B

What do I need to do to get the LED to light ?Button A Button C

1 Button B

Conclusiony Logic gates create conditions for when things become true (or false!) y True: something turns on y False: something turns off y Conditions could be y The state of inputs (switches, sensors etc) y The state of outputs (are motors turned on properly) y The state of process control pins y Allows the automatic control of your circuit

Using Logic GatesInputs Switch in Gatehouse A Switch under barrier (on when barrier closed) B Switch (pressure pad) before barrier on road C Switch (pressure pad) under barrier D Switch (pressure pad) after barrier on road E Outputs Motor (on barrier raised, off barrier lowered) Red Light Green Light

Design using Logic Gates systems to; 1) show a green light when the barrier opening and red when closing 2) automatically open the gate when the car approaches, then close it when it has passed 3) allow the gatehouse switch to close the gate unless a car is under it.

Draw the Logic Diagrams of this example.

Approach to takey Identify relevant Inputs y Identify relevant Outputs y Others are irrelevant y Draw truth table and populate y Work out gate or combination of gates.




Conditionsy Switch open = 0, closed = 1 y Lights on = 1, lights off = 0 y Barrier lowered = 1, raised = 0

Answer to part 1Input B 0 1 B Green Light 1 0 R G Red Light 0 1




Answer to part 2C 0 0 0 0 1 1 1 1 D 0 0 1 1 0 0 1 1 E 0 1 0 1 0 1 0 1 GATE 1 0 0 0 0 0 0 0 C D E

Plan view of road.




Answer to part 3A 0 1 0 1 D 0 0 1 1 Gate Open Closed (1) Open Open A D

A couple more questionsy How would you make a NOT gate from a NAND gate? y How would you make an AND gate with 3 or 4 inputs? y What would the Truth Table for a 3 input AND gate

look like?

Prefix Multipliers1,000,000,000,000 1,000,000,000 1,000,000 1,000 1 Tera (T) 1 Giga (G) 1 Mega (M) 1 Kilo(K) 1012 109 106 103 10-3 10-6 10-9 10-12

10.001

1 milli (m) 1 micro () 0.000001 1 nano (n) 0.000000001 0.000000000001 1 pico (p)

Review your metric conversion table at the back of your study guides

SI Unitsy Voltage y Current y Resistance y Capacitance y Time y Frequency y Power y Charge y Energy

Volts Amps Ohms Farads Seconds Hertz Watts Colombs Joules

Ohms LawThe most important equation you will ever learn in electronics

V = voltage (Volts) I = current (Amps) R = resistance (Ohms)

V = IRV I R

Worked ExamplesWhat current flows through a 1M voltage of 9V is applied across it? I = V/R = 9V/1M = 9/(1x106) Amps = 9 x 10-6 Amps =9 A Resistor when a

am x an = am+n am / an = am-n

Worked ExamplesWhat is the time constant for an RC (resistor/capacitor) filter of a 220 f capacitor and 3k3 resistor? T = CxR = 220 f x 3k3 = 220x3.3 x (10-6x103) = 726 x (10-3) = 0.73sam x an = am+n am / an = am-n

Some Exercisesy What voltage must be applied to a 2k

I Rresistor in order for a

V

10mA current to flow ?

y A coil has a current of 50 mA flowing through it when the

voltage is 12 V. What is the resistance of the coil?

y A 100 V battery is connected across a resistor and causes a

current of 5mA to flow. Determine the resistance of the resistor.

Practice Questionsa)R=180k b)R=3M3 c)R=100 d)R=1M e)R=56k f) R=390k V=9V V=6V V=3V C=220 f C=330pf C=1000 f I=? I=? I=? T=? T=? T=? 0.05mA 1.82 A 30mA 220s 18.5ms 390s

I = V/R

T = CxR

Practice QuestionsI=V/R = 9 / 180k = 9 / 180 x 10-3 = 0.05 x 10-3 = 0.05mA I=V/R = 6 / 3M3 = 6 / 3.3 x 10-6 = 1.82 x 10-6 = 1.82 A I=V/R = 3 / 100 = 0.03A = 30mA

T = CxR = 220 f x 1M = 220 x 1 x (10-6x106) = 220 x (100) = 220s

T = CxR = 330pf x 56k = 330 x 56 x (10-12x103) = 18480 x (10-9) = 18.5 x(10-6) = 18.5 s

T = CxR = 1000 f x 390k = 1000 x 390 x (10-6x103) = 390000 x (10-3) = 390 x (103x10-3) = 390s

Resistors in Series

R total = R1 + R2 + etc

Resistors in ParallelTwo Resistors R total = R1 x R2 = R1 + R2 Product Sum

Three or more Resistors 1 = 1 + 1 + 1 etc R total R1 R2 R3

Resistors

Work out the total resistance.

Resistorsa) Rtotal = 100+100=200 ohm

c) b) Rtotal = 100x100 = 10,000 = 50 ohm 100+100 200

1 = 1 + 1 + 1 Rtotal 100 100 100 1 = 3 Rtotal 100 Rtotal = 100 = 33.3 ohm 3

What Else Does Ohm s Law Tell us?y Rule 1: A resistor with a current flowing through it will always have a voltage drop across it y Example+9VIf I insert a 220 resistor in series with the light bulb : 1. What current flows through the resistor ? 2. What current flows through the light bulb 3. what voltage does the light bulb see now The light bulb demands 50mA In the left hand circuit it has 9V across it & its demand is met

+9V R = 220 50mA 0V

50mA 0V

X

X

Rule 1: Example 2

I R

V

The LED will blow up if it sees more than 15mA The LED has a voltage drop of 1.5V The power supply is +5v So what happens to the other 3.5V ? What is the value of the resistor ?

Rule 2:y I = V/R

I R

V

y A current can t exist without a voltage

y If V = 0, then the value of R is irrelevant y I (current) will always = 0 (NOTHING DIVIDED BY SOMETHING IS ALWAYS NOTHING) y So if any part of your circuit cant see a voltage it won t work !

Rule 3:y Resistors limit current.

I R

V

y The higher the resistance, the lower the current

Rule 4:drop.

I R

V

y The lower the resistance the lower the voltage

Potential Dividersy The fact that resistors drop voltage across them can

be used to create fractions of the supply voltageVs R1

R2 0V

Vo

Why Might I Want One ? (What use are they ?)y If I have 9v supply (for example): y I might want 5v to drive part of my chip y I might want just 1v to drive a transistor y I could use a second supply at a different voltage but

deriving a lower voltage from the primary supply is more efficient

Example 1Resistors R1 & R2 are equal Each will drop the same amount of voltage What would be the voltage seen at the output?

Example 2R2 is 2x bigger than R1 R2 will drop 2x more voltage than R1 What would be the voltage seen at the output?

Its all about ratios!Vo = Vs R2 / (R1 + R2)y If supply voltage = 9V and R1 =2k and R2 = 1k, R2 will drop half as much voltage as R1 y The voltage seen at the output is 3v y If supply = 12V and R1 =2K and R2=1k, what will the output voltage be????

Remembery The bigger the resistor, the bigger the voltage drop y Output voltage is always measured between 0V and

the connection point of R1 & R2y What is the difference between: y A: supply = 9V, R1 = 1K, R2=2K y B: supply = 9V, R1 = 100 , R2 = 200

Other ways of controlling voltagey The zener diode y This conducts only when

the voltage accross it reaches a certain voltage

DC Average Power

P I V

y How much energy is transferred in a unit of time y Burned or generated y Measured in Watts y Average power: P= IV y Things that affect how much power a circuit will use: y Supply voltage y Amount of time it is switched on for y Activity (amount of switching)

Why Is Power Important ?y Burning energy generates heat y If I have a resistor with 9V across it and 100mA flowing through it, I will burn

0.9 W y Resistors come in different power ratingsy Higher values are:y y y

Physically bigger Can dissipate more heat Cost a lot more

y If your resistor cant handle the power of your circuit, it will melt & fail y Most resistors in school are 0.25W, 0.5W or 0.6Wy Metal film / carbon

y Higher values (1W, 5W etc) are availabley Wire wound

Dry Cellsy A single Dry Cell size D y Battery = number of chemical cells connected together in series. y 9 Volt battery is a proper battery made up of 6 cells (6 x 1.5V) y Usually, the bigger the cell or battery, the more current you can draw from it.

What is a capacitor?y An electrical energy storage device. y Two plates that are not connected. y Separated by an insulator a dielectric. y Charge Q = C/V y Q = coulombs, C = farad, V = volts y Depends on plate area, separation and dielectric (dielectric constant)

What do they look like?

Capacitor ChargingVolts++ ++ ++ ++

T=RC Time constant the rate at which a capacitor charges through a resistor

++ ++ ++ ++ After one time constant, capacitor is at 0.6 of its full charge, and fully charged after 5 time constants

TimeT 5T

Capacitor DischargingVolts

T=RC Time constant the rate at which a capacitor discharges through a resistor

Time

General Ruley Switches are normally inserted into the most positive

part of the circuity They connect a circuit to power rather than connect the

circuit to ground

PTM: Push To Make A momentary switch The circuit is normally disconnected The circuit is connected only whilst pressure is applied to the button

PTB: Push To Break A momentary switch The circuit is normally connected The circuit is disconnected only whilst pressure is applied to the button

Momentary Switch Implementations

mouse

Pedal switch

Bell push

PCB Mount

microswitch

Computer keyboard

Chrome with LED backlight

SPST: Single Pole Single Throw A latching switch The circuit is normally disconnected The circuit stays connected when the switch position is changed Normally used for power on/off applications

SPST Switch ImplementationsPush button Rocker Rocker with neon Rocker with faceplate

Key switch

Toggle switch

Tactile switch

Slide switch

DPST: Double Pole Single Throw A latching switch Two independent circuits are turned on or off at the same time

SPDT: Single Pole Double Throw A latching switch Connects one of two circuits to a common point

DPDT: Double Pole Double Throw A latching switch Two independent circuits have two independent common points. Each common point is be connected to one of two circuits The circuits connected to the two common points are changed at the same time

Some Other Switch Implementations

Reed Switch Rotary Switch

Tilt Switch (Ball or mecury)

Summaryy A switch throw is the number of circuits that can

connect to a common point

y Single throw = 1 (on or off) y Double throw = 2 (one circuit or another)

y A switch pole is the number of independent circuits

that can be affected at the same timey Single pole = 1 y Double pole = 2

Relaysy Relays are electromechanical switches y Used to switch independent circuits

Relays Typically a low voltage circuit controls a high voltage or high current circuit The low voltage circuit energises a coil The coil draws two contacts together in the second circuit switching it on Electromechanical relays are becoming less common in modern electronics

Why is D1 Needed ?

Because of the possibility of back EMF damaging the Transistor.

Back EMF EMF = Electro Motive Force A coil of wire is like a waterwheel When the water (electricity) is turned off, the wheel keeps spinning The energy from the wheel has to go

somewhere

The diode stops it going back through the

transistor the wrong way and damaging it

Diodes are always used to protect circuits Speakers, relays, motors, solenoids

which have electro-mechanical components (AKA inductors) Diode, and any device connected to a transistor and containing a coil must be Clamped

y This arrangement is known as a Clamping

Reviewy Where might you use a SPST switch ? y Where might you use a DPST switch ? y Where might you find a PTM switch ? y What is the purpose of a relay ?

Types of Transistorsy There are different types of transistors y MOSFETS (Metal Oxide Semiconductor Field Effect Transistor) y Bipolars (BJT Bipolar Junction Transistor) y Firstly we are just going to look at bipolars

What do transistors do ?y They can either be used as y Automatic Switches y Amplifiers

COLLECTOR

BASE

EMITTERy In both cases, a small signal is made bigger

Transistor as a switch

In order to switch on the transistor the voltage at the base must be 1.2V or above

A NPN transistor is like a sandwich The bread conducts electricity (N material) The filling is a SEMICONDUCTOR (P material) It is normally an insulator Conducts when a voltage > 0.6V is applied to it

N P N

The Electric Sandwich

The sandwich normally just sits there doing nothing If I plug it in, nothing happens If I connect the filling to a battery ..

The sandwich will start to conduct when the filling has a voltage > 0.6V As voltage increases, resistance acrross the sandwich decreases (turns more on ) At some point, the sandwich s filling will be fully on (saturated) and its resistance wont decrease any more

N P N

The NPN Sandwichy The collector is the most positive bit y The base is the input signalWe can control the sandwich with this input y Too much current though & we burn the fillingy

COLLECTOR

y The emitter is the most negative bit

N P N

BASE

EMITTER

Transistor AmplifierThe current entering the base controls the current that flows through the collector and emitter, with a fixed relationship called the gain (hfe)

Ice ib

Gain (hfe) = Ice Ib Or Ib x Gain Eg Ice = Ice

Gain 100, Ib =1mA = ?

Questionsy What do we mean by a transistor in saturation ? y In what state would you expect the transistor to be able

to source the most load ? y What could the load be ?

PNP Transistorsy These have the filling on the outside and the bread in

the middle y Electricity can flow right the way through by default y Applying a voltage to the base turns it off

Using NPN Transistor as switch.y NPN Transistor - sinking. y When as the PIC pin goes high current flows through R1 turning the transistor ON, effectively connecting the bottom end of the lamp to ground. y Can have separate power supplies for lamp and PIC as long as common ground.

Using PNP Transistor as switch.y PNP Transistor sourcing y The 'source' example is effectively the same but upside down, while the PIC output is high no current flows through R2, so the transistor is turned OFF, as the PIC pin goes low current flows through R2 as before turning the transistor ON. y This effectively connects the top of the lamp to the +ve supply. Unlike the 'sink' example, this simple 'source' circuit must use the same supply as the PIC - because for the PIC to turn the transistor OFF it's output must go as high as the +ve supply to the emitter of the transistor.

Darlington Transistor or Pairy Compound structure consisting of

two bipolar NPN transistors (either integrated or separated devices) connected in such a way that the current amplified by the first transistor is amplified further by the second one y Why do this? This structure gives a much higher current gain for driving devices.

Transistor Latchy Causes a circuit to stay in a fixed

state. y What are the transistors? y What happens when I press the PTM switch? y How do I turn the circuit off again?

Astable Multi-vibrator Circuity R2, R3, C1 and C2 control the timing of the circuit. y C1 = C2 and R2 = R3 produces two equal but opposite square waves, as shown. y Lots of applications for this circuit, flashing, oscillators, timing, etc.

Field Effect Transistor (FET)y Also made from a combination of

n-type and p-type semiconductor material. y Legs are called Drain, Gate and Source. y FET amplifies the voltage at the gate to gain an increase in voltage or current. y The size of current on the gate does not affect the current flowing between the drain and source.

How FETs work.y When the gate receives an input y y y y

voltage of at least 2 V, it switches on fully. If it is less than 2 V it will be fully switched off. The gate has high impedance and is not dependent on current for switching. FETs are like digital switches (unlike the Bipolar Transistors). Normally used as transducer drivers for high current outputs.

Sensors

PTC = positive temp. coefficient NTC = negative temp. coefficient

Sensors

Strain Gaugey Pattern on gauge is

important. y Used to measure tension and compression. y Can also be used to measure weight? How?

Voltage Dividers as Sensors

Vout =

R2 x Vsupply R1+R2 So, if R2 >> R1, Vout is close to Vsupply

Transistor plus sensor (voltage divider)Vout = R2 x Vsu R1 R2ly

In cold Vbase

= 1/11 x 9V = 0.81V Transistor is off, bulb off In warm Vbase

=2/12 x 9V =1.5V Transistor on, bulb on

System Diagram to explain circuit

Systems Electronicsinput process output

Systems ElectronicsinputSwitch LDR Thermistor Moisture Sensor Variable Resistor Microphone Piezo

processTransistor Delay Oscillator Counter Latch Amplifier Comparator Logic Gates PIC

outputBuzzer Speaker Bulb LED Motor Relay Solenoid Piezo

Example of a system with feedback.

INPUT Dark Sensor

PROCESS Transistor Switching

RELAY Reversing Switch

OUTPUT Motor

FEEDBACK Limit Switches

A development of the automatic curtain-opening/closing system

What is a latch ?y A mechanical latch y Causes a door to stay in one position (open or shut)

An electronic latch Causes a circuit to stay in a fixed state (logic 0 or logic 1) until reset Why is this useful ? If you want your circuit to remember that something has happened (until you tell it to forget) Example: A steady hand game The light comes on when the wand touches the steel pattern What if you only touch it briefly ?

How Can We make an Electronic Latch ? Thyristor Simplest way Latch can source high currents Logic gates Cross coupled NAND or NOR gates Low current IC 555 in mono-stable mode Software Programming for PIC s

Thyristory Also known as a SCR (Silicon Controlled Rectifier) y 3 pins: y Anode (positive) y Cathode (negative)y

C or K

y Gate (control pin)

Thyristor Operation Basically a controllable diode. The thyristor starts to conduct when the gate has a current pulse on it. Continues to conduct when the gate pulse is removed and anode is more positive than cathode Forward biased Stops conducting when cathode becomes more positive than anode Reverse biased

Thyristor Latch

Similar to a transistor, but here a signal/current at the gate latches the thyristor on for as long as current flows through it, interrupting this (breaking the circuit) resets it to off. [once ON, it stays on until reset: e.g. car alarm]

Thyristor ApplicationWhat happens when SW1 is pressed? What happens when SW2 is pressed? What is R1 there for?

Using Logic GatesCross coupled NOR gatesy R & S are active high

R = Reset S = Set (latch) Q = latched output Qbar = inverted latched output

Cross coupled NAND gatesy R & S are active low

Using NOR Gates When there is a 1 pulse on S, Q goes to 1 and stays

there until a 1 pulse is put onto R The circuit remembers the pulse on S and shows this on Q even when the pulse on S has been removed

Using NOR GatesWaveforms for a NOR based RS latch

Questionsy What does a latch do ? y What makes a thyristor latch ? y What makes a thyristor stop conducting? y What is the difference between a NOR based RS latch

and NAND based RS latch ?

Miniature D. C. Motorsy Usually small DC motor used. y Contains two permanent

magnets on other side of a rotating armature. y Armature has number of coils that get current from two brushes pressed against the commutator. y Current flowing in the armature coils produces a magnetic field opposed to that of the magnets and the armature rotates.

DC Motorsy Change polarity to change y y y y

direction of rotation. DC Motors do not stop spinning the moment that current is switched off. Use shunt switching to stop motor very quickly. Behaves like a strong brake has been applied. The motor has been changed into a generator!

Stepper Motorsy Instead of armature, stepper

motors have a rotor consisting of several permanent magnets. y These are surrounded by fixed coils that, when switched on and off in the right combination, causes the rotor to rotate a small angle or step. y These motors can be started, run and stopped with great precision. y What uses can you think of?

A device used to amplify small differences between two input voltages. Gain can be as high as 100,000.

741 Op-Amp 41 OpIC = integrated circuit DIL = Dual in line 741 op amp = 8 pin DIL IC

OP AMP as a comparatorthe OP AMP compares the inverting input voltage to the noninverting input voltage, and gives a HIGH or LOW output depending upon which is the greater input voltage. The OP AMP detects very small changes in voltage and multiplies the difference by the GAIN (typically 100,000). Because the output is either HIGH or LOW, it is used as an analogue to digital converter (ADC) so is suitable for connecting analogue sensors (E.g LDR, THERMISTOR) to logic circuits.

Analogue and Digital ElectronicsAnalogue si als are c sta tly varia le, f r exa le te erat re, li i te sity, s aves etc Digital lectr ics c erical val es, si er syste ase a ff) t

verts si als i t t e i ary 1sa s(

Ad antages of Digital a e re relia ly re r tra s itte a e r cesse

ce a

Rules For comparingy If something in the + column is bigger than the -

column then we produce a logic 1y If something in the + column is smaller than the -

column then we produce a logic 0 or if something in the - column is bigger than the + column then we produce a logic 0

Task: Ta l Cr ati+ Output Logic 0 Logic 1

Logic 0

Logic 1

Comparing two Waveformsy What waveform would I expect at the output ?9 8 7 6 5 4 3 2 1 0 1 2 3 4 5 6 7 8 9 10

Waveforms

A circuit To Do the comparing for usy The guts that does the work is an Op amp (the triangle) y R2 and VR1 form a _________________

How It Works: It compares the input from a sensor against a target value. It provides an output based on the difference that can trigger another process block or part of a circuit. Clipping distortion to the signal caused when it is amplified beyond the voltage of the power supply. Combination of high switching speed and the input and output states means that the op-amp can be used as an analogue-to-digital converter (ADC).

Keywords So Fary y y y

Op amp (operational amplifier) Inverting Non-inverting comparator

Op Amp Connections+VInverting inputNon-inverting input

+ -V

Vout

y An op amp has 5 terminals y 2 inputs y 1 output y 2 power supply

Op Amp Power supplyy Electronic circuits we have seen so far 0V and supply voltage y Op amps require a supply y Why ? y Because the signal going into them can be positive or negative ?

+9V

0V

-9V

Op Amp Input SignalVoltage

Time

y The input signal can be both positive & negative y The size of the input waveform changes over timey Alternating current (AC) y It could come from an MP3 player or a microphone

Why do we have 2 inputs ?y Reason 1: to get rid of noisey We will put our good signal onto 1 input y Any noise in the circuit will be seen by both inputs y We only amplify the difference in voltage between the twoVout

inputs (V1-V2) Example: V1

V2

Vout

The Outputy Without any other components, the gain of the input signal seen at the

output is massive ! y However, this is not good because:y We want the shape of the input waveform at the output

just made bigger

(higher => more voltage) y We can t have an output voltage bigger than supply y Anything bigger than supply becomes a flat line y This is known as clipping or distortion Input (mV) +2 -2 Gain = 10,000 Supply = 9V +9 -9

So How Do We Control the Output ?y Use Feedback y Negative feedback takes some of the output and takes it away from

the input

y We place a feedback resistor (RF) between the output and the

inverting input. This helps adjust the amount of feedback happening.

The Inverting Amplifier (continued)y Gain with feedback:y We can choose the gain that we want with the ratio of 2

resistors

y As shown previously, open loop gain:y Is enormous and we have no control over its value

Keywords

y Gain y clipping y Feedback y Amplify y Inverting y Non-inverting y comparator

QuizWhat is open-loop gain ? What are the 5 inputs and outputs on an op-amp called ? What is gain ? How can we reduce open-loop gain ? What is the equation for gain ? Besides being bigger, what is the relationship between the input and output waveforms on an inverting amplifier ? y Describe two applications for a comparator circuit ? y In an inverting amplifier, what is it that we are amplifying ?y y y y y y

Pull-up Resistory A resistor used to ensure an input

receives the required voltage of a supply either zero or close to Vcc. y Based on the concept of a resistor bridge V0 = Vcc * [R2/(R1+R2)] y In this case V0 will be very close to Zero. y What happens if we swap the switch and the 10k resistor?

555 timersIC = integrated circuit DIL = Dual in line 555 timer = 8 pin DIL IC

ICs have three big advantages over conventional circuits with discrete components: they take up very little space they are extremely reliable, and they are extremely cheap to make8 1

4

555 timers - MONOSTABLER

RC timing: T=1.1xRC T (seconds) R (resistance - ohm) C (capacitance - Farad) Careful with units!!! Monosta le state hi h, pin 3 low pin 2

CPins 6 and 7 are connected, through PIN 3 = output pin

R to +V

Pin 2 then tri ere (taken low), so pin 3 oes hi h, apacitor char es up. hen it reaches 2/3 V+ ( the timing period) pin 3 then goes low and the capacitor discharges. 0k pull up resistor ensures Pin 2 is PTM is pressed.

when

555 timers ASTABLE (Pulse Generator)C1 charges through R1 and R , until voltage across C1 is > supply voltage. At this point pin goes from high to low. C1 then discharges into pin 7, until voltage across C1 is < 1 supply voltage. At this point pin goes from low to high. Pin = low = current flows into it (sinking current) Pin = high = current flows out (sourcing current)

Pins 6 and 2 are connected, through C1 to 0V PIN 3 = output pin

Astable Waveformy Mark (time on) = y Space (time off) = y If R1 = R

.7 x (R1

R ) x C1

Mark

Space

.7 x R1 x C1 Period T

then Mark = Space Space

y Period T = Mark

Electronic Countingy Lots of different counting

integrated circuits exist

y What things do you thinky Think of at least 3 reasons y You have 2 minutes

affect which one we would choose ?

Specificationy y y y y y y

Must work from a 9V battery Must count number of presses on a switch Must be able to work at 25oC Must be low cost Must be low power Must be compact Must work first time when committed to PCB

Counters

4017 is a decade counter but still needs a driver to drive a 7-segment display. 4026 is a decade counter with a built-in 7-segment counter

4026 countery A chip that can count pulses y Display the count on a 7 segment display

PULSE (CLOCK)

4026

The 7 Segment Displayy Each segment has a letter y A combination of segments

forms a letter, number or symboly What number would you get with:

A=1, B=1,C=1, D=1, G=1y All shapes, sizes & colours available y And you can form words as well

Common Cathode vs Common Anode 7-segmenty Common Anode y Logic zero causes a segment to light y Common is logic high y Common Cathode y Logic one causes a segment to light y Common is logic low

Other examples of Decade Counters

Design Rulesy All chip inputs must have a well defined logic state all of the time y Unused inputs can and should be tied to power or ground y Switches:y

Tie inputs to ground through a 10K pull-up resistor

Modellingy How Can we ensure that our design works correctly

first time ?y Circuit drawn out clearly with component rating y Simulation (Yenka) y Breadboarding (make sure wires are good fit) y Drawing a Veroboard template

Insulated gap in the centre

Breadboardingy Uses real components y Components can be

reused y No solderingInner rows conduct across

y Components connected

using bell wireOuter columns conduct downwards

Breadboarding Exampley Place major components y define power & ground y Don t connect the battery !

Breadboarding Example (2)y Distribute power & ground using differently coloured

bell wire (single strand)

y Wire should be stripped for approx 5mm

Breadboarding Example (3)y Consider the connection from each chip pin y Work one pin at a time y Work around the chip anti-clockwise y Add the switch & resistor when you come across them

What You Should End Up With

Wrap Up Questionsy What are the advantages & disadvantages of

simulation and Breadboarding ?y Why do switched inputs to digital systems need to be

connected in series with a (pull-up) resistor ?

Switch Bouncey When mechanical switches make multiple (unintended) contacts whilst being used. y See diagram on right. Each spike seen as an input! y Can use a 555 monostable with approximately 1 second delay to clean the input

Switch bounce

Use a SCHMITT TRIGGER

Microcontroller and PIC Chips A microcontroller is a computer on a chip. A common form used in schools is a Peripheral Interface Controller (PIC) Come in various numbers of pins which limit the number of inputs and outputs Easiest way to imagine a PIC is like a programmable process block. Uses BASIC programming language to program chips.

Writing ProgrammesPr ra es ca e ritte as fl c arts Start/E d Pr cess

Flas i

Li

t

I put 1 On?

/p 1 n Decisi Output Wait 1 /p 1 ff Wait 1

Circuit modelling and CAD CAM

CAD = Computer Aided Design Advantages = quick to model a circuit, voltages and currents can be measured, no damage to components, design can be exported into a PCB layout design program, use of programmable chips (PICs) Disadvantages = unable to test the circuit in real conditions so you have to make a pcb to test it properly, software can be expensive

1. What does an electrical conductor do? An electrical conductor allows electricity to flow through it easily. An electrical conductor prevents electricity from flowing. An electrical conductor is a type of plastic material. All of the above

2. What is a material that can exhibit both insulating and conducting properties? Double conductor Insulator Conductor Semi conductor

3. An insulator has what sort of value? A high resistivity value A high conductance value A high material value All of the above

4. What is the main element used for making semiconductors? Carbon Aluminium Silicon Sodium Potassium

5. When describing integrated circuits, what do the initials DIL stand for? Dual Input Logic Dual In Line Digital Input Line

6. Which of the components below is an input component? Relay 555 timer Push-to-make switch Light-Emitting Diode [LED] Motor

7. Which of the components below is an output component? Relay 555 timer Push-to-make switch LED Motor

8. Which of the following components would you say is a process component? Relay 555 timer Push-to-make switch LED Motor

9. When an astable timer is switched on, it will give a constant on/off output. The output will flash at a particular rate. Is this statement true or false? True False

10. A common operational amplifier is the: 4017 Op-Amp 747 Op-Amp 741 Op-Amp

Introductiony Structures designed to resist loads or forces e.g. skeleton and roof. y Loads can be static (not moving) or dynamic (moving). y Two of the most common sorts of forces are compression (pushed together) or tension (stretched). y Some materials can withstand compression and not tension (concrete beams) and others can withstand tension and not compression (steel wire).

Compression and Tension

Example

Adding Strength Ribs, folds, etc.y Some seemingly weak materials can be strengthened by using ribs, folds or curves e.g. steel sheet, plastic sheet, steel beams. y These give the material a high stiffness-to-weight ratio think of airplanes.

Force

Other Forces

Frames

Strengthening Frames

The Trussy Is a framed structure that is very familiar in roof and bridge construction. y Used to span across wide gaps. y Aim is to do the job with the least amount of material.

Frame Elements: Struts and Ties

Forces in Action

Forces in Action

Example Problem

Example Problem

Force and Turning Moments




Example Problem

Example Problem

How Structures can Faily Failure in use can be cause by many things poor design, overloading and faulty manufacture. y Good design needs to consider appropriate materials and adequate jointing techniques. y Even if materials and fastenings correctly selected, problems can still arise e.g. stress concentrations. y Example: Trying to pull apart a straight strip of paper can be difficult. If, however, a small nick is cut along one edge and the paper pulled again, it tears easily starting at the nick and across the strip.

How to reduce stress concentrations.

Properties of Materialsy Tensile Strength the resistance the material has to being pulled apart. Steel rod has high TS and concrete is very weak in tension. y Compressive Strength the resistance of the material to being squashed or compressed. Concrete has a very high CS. y Impact Resistance Resistance of a material to sudden impacts such as a hammer blow. Glass has poor and polypropylene has high IR.

Behaviour of Materialsy When a material is first loaded in tension or

compression, it behaves like a spring. Up to a certain load the material will return to its original length. y If the load is further increased, there will come a point when the material becomes permanently deformed.

Stress and Strainy Stress the load the material is undergoing. y Strain the deformation of the material due to that load. y Elastic Material will deform under load but will return to its

original size and shape. y Plastic Material will deform under load and will remain deformed when the load is removed. y Hooke s Law = Strain/Stress is a constant within the elastic limit. y The constant is known as Young s Modulus. Different for different materials.

Young s Modula (Stress/Strain Curve)y1

2 Elastic range (where Hooke s law applies) y 3 = Elastic limit or yield point y 4 = Ultimate Tensile Strength y 5 = Fracture Point

Questionsy What is a truss? y What is the main purpose of having trusses? y Where do structures usually fail? y What do we mean when we say that a material is under

tension or compression? y What is Stress? And Strain? y Explain Hooke s Law.

Mechanismsy A mechanism is simply a device which takes an

input motion and force, and outputs a different motion and force. The point of a mechanism is to make the job easier to do (leverage/advantage). But there is a trade-off y The mechanisms most commonly used in mechanical systems are levers, linkages, cams, gears, and pulleys.

Five basic types of Mechanisms

Leversy These are simple mechanisms which create an advantage for the user. y A leaver is a long ridged object or beam with a pivot (sometimes called

a fulcrum) somewhere along its length. y The beam rotates around the pivot (this is a fixed point), this is generally used to apply effort to move a load y Leavers can be used to increase or decrease movement. y There are 3 key parts to all levers:

y Effort The force exerted by the user y Load The force exerted by the object being acted upon y Fulcrum (or pivot) The point at which the lever revolves around

There are three different classes of Leversy Effort on one end, y Pivot or fulcrum in the

middle and y Load on other end.Learn the position of load, effort and fulcrum for each class of lever. Linking it to a practical example will help you to remember them.

There are three different classes of Leversy Effort on one end, y Pivot or fulcrum at the

other end and y Load in the middle.

There are three different classes of Leversy Effort in the middle, y Pivot or fulcrum at the

one end and y Load at the other end. y No Mechanical advantage just convenience.

Important Ratiosy Mechanical Advantage (MA): the ratio of the output force to

the input force in a mechanism - in other words, the factor by which the mechanism multiplies the force put into it y Velocity Ratio (VR): the ratio of the distance travelled by the effort to the distance travelled by the load. Also sometimes called the distance ratio y Gear Ratio (GR): the ratio of the number of teeth on a driving gear to the number of teeth on the driven gear. A form of velocity ratio, the gear ratio determines the number of revolutions made by each gear and speed.

Mechanical Advantage Class 1 and 2 only.y Load and effort are forces and are measured in

Newtons (N). y Mechanical advantage is calculated as follows:y Mechanical advantage = load effort y If the load=500N and the effort=100N, the mechanical

advantage would be: y 500N 100N = 5

Velocity Ratioy The mechanical advantage gained with levers makes it seem like you are getting something for nothing: moving a large load with a small effort. The catch is that to make the effort smaller, you have to move a greater distance. y This trade-off is calculated by the velocity ratio: Velocity ratio = distance moved by effort distance moved by load y Efficiency = MA/VR (Always less than 100%) WHY?

Linkagesy Linkages are mechanisms which allow force or motion to beThe direction of motion y The type of motion y The size of a forcey

directed where it is needed. Linkages can be used to change:

y A linkage consists of a system of rods or other rigid materials

connected by joints or pivots. The ability of each rod to move will be limited by moving and fixed pivots. The input at one end of the mechanical linkages will be different from the output, in place, speed, direction and other ways.

Reverse Motion Linkagey A reverse-motion linkage

changes the direction of motion. In the diagram, note how the linkage looks a little like a "Z". See how the central rod moves around a central fixed pivot. By pulling (or pushing) the linkage in one direction, it creates an exact opposite motion in the other direction. If the fixed pivot was not central, it would create a larger or smaller motion in the opposite direction.

Parallel Motion Linkagey A parallel-motion linkage creates

an identical parallel motion. In the diagram on the right, note how the linkage looks a little like an "n". This time, it is the two side rods that move around two central fixed pivots, while the top of the "n" moves freely. By pulling (or pushing) the linkage in one direction, it creates an identical parallel motion at the other end of the linkage.

Bell-Crank Linkagey A bell-crank linkage changes the

direction of movement through 90. A bell-crank linkage tends to look a little like an "L" or a mirror image of an "L". By pulling (or pushing) the linkage in one direction, it creates a similar motion at the other end of the linkage. For example, a bellcrank linkage could be used to turn a vertical movement into horizontal movement, as in a bicycle braking system.

Crank and Slider Linkagey The rods move forwards

and backwards in slider. The fixed pivot anchor the linkages to one place.

Some uses.y Considerable MA obtained by using linkages. y Can be used to apply pressure and for clamping. y E.g. y Toggle clamp y Geometrical locking

Springsy There are a lot of different types of spring which are

used in a variety of ways to resist different forces. y Four broad groups. Springs which:y Resist extension y Resist compression y Resist radial movement y Resist twisting

Camsy Cams are mechanisms which convert rotary motion to linear motion (up and down or left and right motion). y The cam is fixed to a rotation shaft. A follower rests on the edge of the cam and as the shaft is turned the follower moves up and down (reciprocates). y Dependant on the shape, size and position of the cam or follower the motion in which the follower reciprocates can be altered. y It is also possible that the movement on the other end of the follower can be used to control another mechanism.

Example of Cams

More examples of Cams

Calculating Lifty It is very easy to calculate the amount of lift by simply taking

the measurement from the centre of the drive shaft to the lowest point of the cam and subtracting this from the measurement to the highest point. This calculation will give the amount of lift the cam will produce. y The concentric cam, is a circle with an offset centre. By offsetting the centre you produce the lift. The further you move away from the centre point the greater the amount of lift you will produce. It is better to make a larger cam that rises gently than a small one that rises rapidly. They will both do the same job but the smaller cam is more likely to jam. y If you need to produce lift to a specific height, the following formula is simple and shows you how to work out the fixing point for the drive shaft: Every millimetre that you move away from the cam's centre point, you must double, in order to calculate the amount of lift generated by the cam.

Pear Shaped Camy During one-half of its

rotation, the cam does not lift the follower. y There is a dwell period. y Used in engines to open and close valves.

Crank Mechanismsy Converts Rotary motion into linier motion. y A quick return mechanism such as the one seen opposite is used where there is a need to convert rotary motion into reciprocating motion. As the disc rotates the black slide moves forwards and backwards.

Piston Cranksy As the slider moves to the right the

connecting rod pushes the wheel round for the first 180 degrees of wheel rotation. When the slider begins to move back into the tube, the connecting rod pulls the wheel round to complete the rotation. y One of the best examples of a crank and slider mechanism is a steam train. Steam pressure powers the slider mechanism as the connecting rod pushes and pulls the wheel round. The cylinder of an internal combustion engine is another example of a crank and slider mechanism

Gearsy Gears are comparable to continuously applied

levers; as one tooth is engaging, another is disengaging. The amount of teeth on each gear wheel affects the action on the gear wheel it engages or meshes with. The gear wheel being turned is called the input gear and the one it drives is called the output gear. Gears with unequal numbers of teeth alter the speed between the input and output. This is referred to as the Gear Ratio. y Gears also alter the direction of rotation. In the above example gear wheel A is rotating clockwise, but as it turns, gear wheel B is moved anti-clockwise.

Stepping Downy Stepping down has the

advantage of producing more power although at a slower rate.

Stepping Upy This Stepping up

produces a much faster output speed, but mechanically delivers less power

Gear Ratios Controls Speedy If the input gear (A) has 10 teeth and the output gear (B) 30

teeth, then the ratio is termed 3 to 1 and is written down as 3:1 Ratio = No. of teeth on the output gear B (30) No. of teeth on the input gear A (10) = 3/1 and is written down as 3:1

y Simply divide the amount of teeth from the output by the input gear to

work out the ratio. In the above example, for every complete revolution of the input gear the output turns 1/3 of the way round. In other words it takes three turns of A to rotate B once. This means you are slowing down the action and is referred to in engineering terms as Stepping Down. y If B were the input gear and A the output gear, then the opposite happens and we Step Up. Then with one turn of the input gear the output gear would turn three revolutions, giving a ratio of 1:3.

Compound Gear Trainsy Where very large speed reductions are

required, several pairs of gears can be used in a compound gear train. A small gear drives a large gear. The large gear has a smaller gear on the same shaft. This smaller gear drives a large gear. With each transfer, the speed is significantly reduced. y A gearbox usually houses a gear train it is an assembly of gears that connect input and output shafts.

Worm Gears change of motiony Another method of making large speed

reductions is to use a worm gear. This is a shaft with a thread like a screw. This connects at 90 to a large gear (the thread shaft points along the outside edge of the larger gear). Each time the shaft spins one revolution, the gear turns forward by only one tooth. If the gear has 50 teeth, this creates a gear ratio of 50:1. y The worm can drive the worm gear round, but the worm gear cannot drive the worm. This means that worm gears are good to use in hoists, the load will not fall back when the motor stops. Worm gears are a good option when you wish to alter direction or rotary motion through 90 and reduce the speed. The photograph to the left shows a worm gear powered by a motor.

Bevel Gearsy Bevel gears, like worm

gears, change the axis of rotation through 90. The teeth have been specially cut so the gears will mesh at right-angles to each other, where spur gears must be parallel.

Rack and Piniony A pinion is a round cog and the rack is a flat bar with teeth. y The driver cog either moves along the rack, as in a rack and pinion railway - or else the driver cog moves the rack, as in the steering system in cars. Rack and pinion changes rotary motion into linear motion as shown in the diagram.

Pulleysy Pulleys are used to change the speed, direction of rotation, or

turning force or torque. y A pulley system consists of two grooved pulley wheels each on a shaft, connected by a belt. This transmits rotary motion and force from the input, or driver shaft, to the output, or driven shaft.

Velocity Ratioy If the pulley wheels are different sizes, the smaller one will spin faster than the larger one. The difference in speed is called the velocity ratio. This is calculated using the formula: y Velocity ratio = diameter of the driven pulley diameter of the driver pulley y If you know the velocity ratio and the input speed of a pulley system, you can calculate the output speed using the formula: y Output speed = input speed velocity ratio

Exampley Work out the velocity ratio and the output speed of the pulley shown in the diagram. y Velocity ratio = 120mm 40mm = 3 y Output speed = 100rpm 3 = 33.3 revolutions per minute (rpm)

Workshop Pillar Drilly Effort is lost as the belt heats

up. y When the belt is at the top driver fast and driven slow the driven shaft has high torque or turning power y When the belt is at the bottom driver slow and driven fast the driven shaft has low torque or turning power.

Torquey The velocity ratio of a pulley system also determines

the amount of turning force or torque transmitted from the driver pulley to the driven pulley. The formula is:Output torque = Input torque Velocity ratio.

Drive Beltsy Drive belts are usually made

of synthetic fibres such as neoprene and polyurethane, with a Vshaped cross section. Common to see toothed belts and pulley wheels as well. y It is possible to reverse the direction of the driven pulley by twisting the belt as it crosses from input to output. Pulley belts have the advantage over chains that they do not need lubrication (though unlike a chain, a belt can slip).

Chain and Sprockety Usually seen in bicycles

and motorbikes. y A form of pulley system in which there is no possibility of the belt slipping. y Same calculations apply.

Chain and Sprockety A chain and sprocket changes rotary motion to linear motion - or vice versa. y A wheel-and-axle, rackand-pinion, rope-andpulley, screw thread, or chain-and-sprocket could also be used for this.

Simple to Complexy Small systems can be combined to make more complex

systems. y For example: A cam which is turned by an electric motor can operate a micro switch which could be used to turn a light on or off. Two mechanical systems can be connected together to give complex movements.

Introductiony The screw can be defined as a

threaded rod that engages in a similar internal thread y Screw threads vary in their shape or profile for different applications. y The diagram on the right shows various thread designs.

Uses of Screwsy Converting rotary to

linear motion. y Obtaining a mechanical advantage. y As fastenings.

Other forms of Control Mechanismsy Cable Control

bicycles. Flexible for transmitting force and movement. y Pneumatic Control usually compressed air. Safe, fast and flexible. y Hydraulic Control usually oil or water. Little compression of medium, hence good for heavy duty applications e.g. vehicle brakes. Can obtain significant MA using different size pistons.

Bearingsy Wherever a moving surface is in contact with a fixed

one, a form of bearing is needed to reduce friction and wear. y To support rotating shafts, two types of bearings are commonly used.

Plain Bearingsy Journal bearing consists of

a bush (or sleeve) fitted into a housing. Replaced when worn. y Made from softer metals, plastics and other materials. y Need lubrication.

Ball Bearing Racesy Uses steel balls or rollers

trapped in a cage between inner and outer rotating shells. y Shaft is supported in the inner shell. y Very little friction. y If loading is severe, roller bearings are used instead, to provide additional support.

1. Which of these are levers used for?Levers are used for applying a force. Levers are used for changing the amount of force. Levers are used for changing the direction of movement. Levers are used for changing the amount of movement. Levers are used for all of the above.

2. What class of lever is a wheelbarrow? Class-one lever Class-two lever Class-three lever

3. How is the velocity ratio of a lever calculated?Distance divided by load Effort multiplied by load Load divided by effort Distance moved by load multiplied by distance moved by effort Distance moved by effort divided by distance moved by load

4. A load of 600N is moved by an effort of 200N. What is the mechanical advantage? 1/ 3 3 400 800 1,200

5. Linkages can be used to change: The direction of motion The type of motion The size of a force All of the above.

6. A bicycle breaking system is an example of a reverse-motion linkage: true or false? True False

7. Windscreen wipers are an example of a treadle linkage: true or false? True False

8. A cam mechanism has three basic parts: cam, slide and follower: true or false? True False

9. What type of cam is required to make the follower dwell for half the cycle, then rise and fall smoothly? An eccentric cam A snail cam A pear-shaped cam A dwelling cam A drop cam

10. A number of gears connected together are called a what? Gear system Gear tram Gear train Worm gear

11. The input, perhaps a motor, is connected to the what? Driven gear Primary gear Gear train Driver gear

12. What type of gears change the direction of rotary motion by 90 degrees? Compound gear trains Worm gears Bevel gears Worm gears or bevel gears Compound gear trains or worm gears

13. A pulley system has two pulley wheels. The driven wheel is 90mm wide and the driver wheel is 30mm wide. What is the velocity ratio? 1/ 3 3 60 120 180

14. What type of motion is a pendulum in a clock? Oscillating motion Linear motion Rotary motion Reciprocating motion

15. Which of the mechanisms listed below would be used to change linear motion to rotary motion, or rotary to linear?Wheel and axle Rack and pinion Rope and pulley Chain and sprocket Any of the above

16. An open driving belt will cause two pulleys to turn in the same direction. What effect will a crossed driving belt have on two pulleys?A crossed driving belt will cause two pulleys to turn in opposite directions A crossed driving belt will cause two pulleys to turn in the same direction. A crossed driving belt will cause two pulleys to cancel each other out.

IGCSE Systems and Control Presentation

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Transcript of IGCSE Systems and Control Presentation