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 <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