GCSEREVISION NOTES

Dangers of Electricity

• An electric current can cause:– Electric shock, muscle spasms– The heart may be stopped– Burning

• Do NOT touch the casualty until the electricity is disconnected

• Keep the casualty warm• Get assistance

Preventing Accidents

• Never work alone• Carry out a risk assessment for all

activities• Know how to summon help• Do not touch the inside of electrical

equipment for some time after it is switched off– May be hot– Capacitors may hold a lethal charge

Protective Measures

• Circuit breakers– Thermal– Magnetic– Fuse – 3A <700W 13A>700W

• Transformer– Primary and secondary are isolated– Provides safe low voltage from the

mains

Three Pin Mains Plug

3AMP

fuse

cable clamp

cable

neutral(blue)

earth(green & yellow)

live(brown)

©IKES0902

Systems

• Complex systems broken down into sub-systems.

• Identify sub-systems in circuit diagrams.

INPUT PROCESS OUTPUT

FEEDBACK

©IPK01

Processes and Concepts

• Timing• Amplifying• AND, OR, NOT• NAND, NOR• Memorising

• Bit• Byte, KB, MB, GB• Address• Data• Read• Write• Hardware• Software

Flow Charts

• Used to determine the sequence of operations required

• Aids logical thought

START END INPUT OUTPUT PROCESS COMPARE

Yes

No

©ikes1001

Logic Gates

• NOT, AND, OR, NAND, NOR, EX-OR

• Truth tables – every combination of inputs.

OUTA B

0

1

0

1

0

1

0

1

0

1

0

0OUTPUT

AB

INPUTS

AND

©IPK01

OUTA B

0

1

0

1

0

1

0

1

0

1

1

1AB OUTPUT

INPUTS

OR

©IPK01

A OUT

0

1

1

0A OUTPUTINPUT

NOT ©IPK01

OUTA B

0

1

0

1

0

1

0

1

0

0

1

1OUTPUT

INPUTSAB

EX-OR

©IPK01

D-Type Flip-Flop

• S sets Q to 1, R resets Q to 0. Not dependent on the state of the clock, CK.

• On the rising edge of CK, Q is set to the logic state of D.

Q

Q

>CK

D DCK Q Q

1

0

0

0

0 0

0 1 1

1

Q

Q

Q

Q

S

R

Frequency Divider

• The D input is then always opposite to Q and so toggling occurs on each successive clock pulse.

• To make a flip-flop toggle:-

• Both Set and Reset are connected to 0

• D is connected to

Q

Q

>CK

D S

R

input

output

Q

input

Q

©IKES0902

4017 counter

• 10 decoded outputs

0V

+Vs

8

CI

Carry

9

R

4

1

CK

2

5

0

7

6

3

1

4017

©IKES0902

input

0

1

2

3

©IKES0902

555 Monostable

• When the TRIGGER input goes below 1/3Vs

• OUTPUT goes high and DISCHARGE switches off• C charges through R• Until capacitor voltage =2/3Vs (THRESHOLD voltage)• OUTPUT goes low and DISCHARGE switches on• Capacitor discharged.

Vin

Vout

0V

C

R RESET +Vs

TRIGGER

DISCHARGE

THRESHOLD

OUTPUT

GND CONTROL

+Vs

10nF

©IPK01

T R C 1 1.

555 astable

• When first switched on, TRIGGER is less than 1/3Vs

• OUTPUT goes high, DISCHARGE switches off• Capacitor charges through R1 and R2

• Until capacitor voltage = 2/3Vs (THRESHOLD)• OUTPUT goes low, DISCHARGE switches on• Capacitor discharges through R2

• Until capacitor voltage = 1/3Vs (TRIGGER)• OUTPUT goes high, process repeats

0V

C

RESET

TRIGGER

DISCHARGE

THRESHOLD

OUTPUT

GND CONTROL

10nF

R1

R2

+Vs

Vout

+Vs

©IPK01

CRR7.0t 21H CR7.0t 2L

CR2R

44.1f

21

Operational Amplifier

Differential amplifier• Large input resistance

109• Large open-loop voltage

gain, A, 106

• Low output resistance 100

• Frequency compensated• Gain-bandwidth product

V+ V–

Vout

+Vs

-Vs

0V

+

)VV(AVout

Comparator

• Can be used as a one-bit analogue to digital converter

+

–

R1 R2

R3

V1

V2

0V

+Vs

©IPK01

VVVV

VVVV

sout12

sout21

The Audio Amplifier• LM380, LM386, TBA820

• Bandwidth• At least half of its rated power• At least 70% of its rated voltage gain

Vin

Vout(Gv)gainVoltage

10

20

10 2 10 3 10 4 10 5

40

60

80

100

10 6 frequency / Hz10

voltagegain

70

bandwidth

Prefixes

• giga ×1,000,000,000 (G) GHz• mega ×1,000,000 (M) MHz, M• kilo ×1,000 (k) kHz, k, kV

• milli ×0.001 (m) mV, mA, mW• micro ×0.000 001 () V, A, W, F• nano ×0.000 000 001 (n) nF

• pico ×0.000 000 000 001 (p) pF

Voltage and Current

• Voltage ACROSS object – Volts• Current THROUGH object – Amps

• Power = Voltage x Current – Watts• Resistance = Voltage/Current - Ohms

VI R

Alternating Current

• Sine wave• Continuously

reverses direction

• Peak value = 1.4 x rms value• Frequency = 1 / time period

0

1 cycle

voltage or current

time

amplitude

peak value

rms value

©IKES0902

Series and Parallel

• Series:-– Current is the same– Voltage is shared

• Parallel:-– Current is shared– Voltage is the same

k k k1.4 2.2 3.7©IPK01

10

5

k

k ©IPK01

Resistors

• In series – • In parallel –

• Colour Code• BS1852 • Preferred values• Tolerance

RRRR 321T

R

1

R

1

R

1

21T

Diodes

• Conventional current flow.• Allows current to pass in one direction.• 0.7V across a forward biased silicon diode

anode cathode anode cathode

symbol component outline

0

2

4

6

8

10

1-1

forward biasreverse bias

voltage/V

current/mA

0

2

4

6

8

10

voltage/V-20-40-60

current/mA

breakdown

forward biasreverse bias

+20

Figure 4.5(a) Figure 4.5(b) ©IPK01

Rectification

Full wave

Half wave

load

diode+

_

Vin Vout

©IKES0902

V V

time time

input waveform output waveform ©IKES0902

load

+

_

A

B

V

time

V

time

D1

Vout

D4 D3

D2

©IKES0902

LEDs

• Find the voltage across R.

• Note the maximum current through the LED

• Calculate R• Choose the next

largest preferred value

R

+Vs

0V0V©IPK01

Resistive Input Devices• LDR

– as light level increases, the resistance decreases

– Log-log graphs.

• Thermistor– As the temperature

increases, the resistance decreases

– Log-lin graphs

1 1010 2

10 2

10 3

10 4

10 3 104

10 5

resistance / ohms

illumination/lux

10 6

0.1©IPK01

10 7

1020

10 2

10 3

10 4

10 5

resistance / ohms

0 40 60 80 100temperature / Co©IPK01

Voltage Dividers

• This formula is not on the data sheet!

Vin

Vout

R1

R2

I

©IPK01

outinV

V R

R R

2

1 2

Transistors and MOSFETs

• MOSFETs• Very high input resistance• Voltage operated

• Transistors• Low input resistance – needs base series resistor• Current operated

collector

emitter

base

small current

large current

npn transistor

©IPK01

large current

n-channel MOSFET

drain

gate

sourceVgs©IPK01

Output devices

• Use diode with inductive devices• To remove large induced voltage• Diode protects semiconductor when the

device is switched OFF

input

+Vs

protection diode

0V©IPK01

• Motor• Relay• solenoid

Three Terminal Regulators

• Provides constant output voltage

• 7805 = 5V• 7812 = 12V• 7815 = 15V

inputcommonoutput

©IKES0902

top view

metal tab

mounting hole

78xx

0V0V

+V

1 - 10k470nF 100nF

©IKES0902

in +V out

Audio Systems

• A domestic hi-fi installation:– Tuner– Amplifier– Microphone– Loudspeaker– Minidisk– MP3 player– CD player– Cassette tape recorder

The Simple Receiver

• Aerial/earth – changes em waves into electrical signal• Tuned circuit – filters out required signals• Demodulator – removes bottom half of the AM signal• Rf filter – removes the remaining rf signal• Output – recovered information signal

aerial demodulator afamplifier

loudspeaker

rf tunedcircuit

The Simple Receiver-Limitations

• Poor selectivity:– Only one tuned circuit– Increase the number of tuned circuits– Difficult to tune several tuned circuits together

• Poor sensitivity:– No amplification – uses energy received by the

aerial– Add rf amplifier– Add af amplifier

Amplitude Modulation (AM)• Frequency

constant• Amplitude

varies• Broadcast

bandwidth is 9kHz

• Long and medium wave bands Amplitude

ModulatedCarrier

time

Carrier

time

Information

time

Voltage

Voltage

Voltage

©ikes1201

Frequency Modulation (FM)

• Constant amplitude

• Varying frequency

• Broadcast bandwidth is 100kHz

• VHF wave band

time

Carrier

time

Information

FrequencyModulatedCarrier

time

Voltage

Voltage

Voltage

©ikes1201