gcse revision notes. dangers of electricity an electric current can cause: –electric shock, muscle...
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GCSEREVISION NOTES
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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
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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
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Protective Measures
• Circuit breakers– Thermal– Magnetic– Fuse – 3A <700W 13A>700W
• Transformer– Primary and secondary are isolated– Provides safe low voltage from the
mains
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Three Pin Mains Plug
3AMP
fuse
cable clamp
cable
neutral(blue)
earth(green & yellow)
live(brown)
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Systems
• Complex systems broken down into sub-systems.
• Identify sub-systems in circuit diagrams.
INPUT PROCESS OUTPUT
FEEDBACK
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Processes and Concepts
• Timing• Amplifying• AND, OR, NOT• NAND, NOR• Memorising
• Bit• Byte, KB, MB, GB• Address• Data• Read• Write• Hardware• Software
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Flow Charts
• Used to determine the sequence of operations required
• Aids logical thought
START END INPUT OUTPUT PROCESS COMPARE
Yes
No
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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
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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
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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
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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
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4017 counter
• 10 decoded outputs
0V
+Vs
8
CI
Carry
9
R
4
1
CK
2
5
0
7
6
3
1
4017
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input
0
1
2
3
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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.
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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
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CRR7.0t 21H CR7.0t 2L
CR2R
44.1f
21
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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
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Comparator
• Can be used as a one-bit analogue to digital converter
+
–
R1 R2
R3
V1
V2
0V
+Vs
©IPK01
VVVV
VVVV
sout12
sout21
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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
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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
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Voltage and Current
• Voltage ACROSS object – Volts• Current THROUGH object – Amps
• Power = Voltage x Current – Watts• Resistance = Voltage/Current - Ohms
VI R
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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
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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
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Resistors
• In series – • In parallel –
• Colour Code• BS1852 • Preferred values• Tolerance
RRRR 321T
R
1
R
1
R
1
21T
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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
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Rectification
Full wave
Half wave
load
diode+
_
Vin Vout
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V V
time time
input waveform output waveform ©IKES0902
load
+
_
A
B
V
time
V
time
D1
Vout
D4 D3
D2
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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
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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
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Voltage Dividers
• This formula is not on the data sheet!
Vin
Vout
R1
R2
I
©IPK01
outinV
V R
R R
2
1 2
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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
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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
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Three Terminal Regulators
• Provides constant output voltage
• 7805 = 5V• 7812 = 12V• 7815 = 15V
inputcommonoutput
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top view
metal tab
mounting hole
78xx
0V0V
+V
1 - 10k470nF 100nF
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in +V out
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Audio Systems
• A domestic hi-fi installation:– Tuner– Amplifier– Microphone– Loudspeaker– Minidisk– MP3 player– CD player– Cassette tape recorder
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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
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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
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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
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Frequency Modulation (FM)
• Constant amplitude
• Varying frequency
• Broadcast bandwidth is 100kHz
• VHF wave band
time
Carrier
time
Information
FrequencyModulatedCarrier
time
Voltage
Voltage
Voltage
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