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GCSE

GCSE. REVISION 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

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GCSE

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  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 <700W 13A>700W • 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. Timing Amplifying AND, OR, NOT NAND, NOR Memorising Bit Byte, KB, MB, GB Address Data Read Write Hardware Software Processes and Concepts

  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. 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. D-Type Flip-Flop

  11. 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 Frequency Divider

  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. Differential amplifier Large input resistance 109 Large open-loop voltage gain, A,  106 Low output resistance  100 Frequency compensated Gain-bandwidth product Operational Amplifier

  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. Sine wave Continuously reverses direction Peak value = 1.4 x rms value Frequency = 1 / time period Alternating Current

  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. Full wave Half wave Rectification

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

  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

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