GCSE

1
GCSE

• REVISION NOTES

2
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

3
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

4
Protective Measures

• Circuit breakers
• Thermal
• Magnetic
• Fuse 3A lt700W 13Agt700W
• Transformer
• Primary and secondary are isolated
• Provides safe low voltage from the mains

5
Three Pin Mains Plug
6
Systems

• Complex systems broken down into sub-systems.
• Identify sub-systems in circuit diagrams.

7
Processes and Concepts

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

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

8
Flow Charts

• Used to determine the sequence of operations
  required
• Aids logical thought

9
Logic Gates

• NOT, AND, OR, NAND, NOR, EX-OR
• Truth tables every combination of inputs.

10
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.

11
Frequency Divider

• To make a flip-flop toggle-
• Both Set and Reset are connected to 0
• D is connected to

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

12
4017 counter

• 10 decoded outputs

13
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.

14
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

15
Operational Amplifier

• Differential amplifier
• Large input resistance ?109?
• Large open-loop voltage gain, A, ? 106
• Low output resistance ? 100?
• Frequency compensated
• Gain-bandwidth product

16
Comparator

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

17
The Audio Amplifier

• LM380, LM386, TBA820
• Bandwidth
• At least half of its rated power
• At least 70 of its rated voltage gain

18
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

19
Voltage and Current

• Voltage ACROSS object Volts
• Current THROUGH object Amps
• Power Voltage x Current Watts
• Resistance Voltage/Current - Ohms

20
Alternating Current

• Sine wave
• Continuously reverses direction

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

21
Series and Parallel

• Series-
• Current is the same
• Voltage is shared
• Parallel-
• Current is shared
• Voltage is the same

22
Resistors

• In series
• In parallel
• Colour Code
• BS1852
• Preferred values
• Tolerance

23
Diodes

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

24
Rectification

• Half wave

• Full wave

25
LEDs

• Find the voltage across R.
• Note the maximum current through the LED
• Calculate R
• Choose the next largest preferred value

26
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

27
Voltage Dividers

• This formula is not on the data sheet!

28
Transistors and MOSFETs

• MOSFETs
• Very high input resistance
• Voltage operated
• Transistors
• Low input resistance needs base series resistor
• Current operated

29
Output devices

• Motor
• Relay
• solenoid

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

30
Three Terminal Regulators

• Provides constant output voltage
• 7805 5V
• 7812 12V
• 7815 15V

31
Audio Systems

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

32
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

33
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

34
Amplitude Modulation (AM)

• Frequency constant
• Amplitude varies
• Broadcast bandwidth is 9kHz
• Long and medium wave bands

35
Frequency Modulation (FM)

• Constant amplitude
• Varying frequency
• Broadcast bandwidth is 100kHz
• VHF wave band