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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 • 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 earth (green & yellow) neutral (blue) 3 A M P fuse live (brown) cable clamp cable ©IKES0902 Systems FEEDBACK INPUT PROCESS OUTPUT ©IPK01 • Complex systems broken down into sub-systems. • Identify sub-systems in circuit diagrams. Processes and Concepts • • • • • Timing Amplifying AND, OR, NOT NAND, NOR Memorising • • • • • • • • Bit Byte, KB, MB, GB Address Data Read Write Hardware Software Flow Charts No START END INPUT OUTPUT PROCESS COMPARE Yes©ikes1001 • Used to determine the sequence of operations required • Aids logical thought Logic Gates A INPUTS A B OUTPUT AND B OUT 0 0 0 1 0 0 1 1 0 1 0 INPUTS A B OUTPUT OR 1 A B OUT 0 0 1 1 0 1 0 1 1 1 0 1 ©IPK01 INPUT A OUTPUT NOT A OUT 0 1 1 0 ©IPK01 ©IPK01 INPUTS A B OUTPUT EX-OR A B OUT 0 0 1 1 0 1 0 1 1 0 0 1 ©IPK01 • NOT, AND, OR, NAND, NOR, EX-OR • Truth tables – every combination of inputs. D-Type Flip-Flop D S Q CK D Q Q 0 0 Q Q R Q 0 1 Q Q 0 0 1 1 1 0 >CK • 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. Frequency Divider • To make a flip-flop D Q toggle:output >CK • Both Set and input Reset are Q R connected to 0 • D is connected to Q • The D input is then always opposite to Q and so toggling occurs on each successive clock pulse. S input Q ©IKES0902 4017 counter 5 1 +Vs 1 • 10 decoded outputs 0 2 6 input 7 0 R 4 0 1 7 0V 2 3 ©IKES0902 CI Carry 9 4 3 1 CK ©IKES0902 8 555 Monostable +Vs R +Vs RESET T 1.1 R C OUT PUT T RIGGER T HRESHOLD DISCHARGE Vin Vout GND CONT ROL C 10nF 0V ©IPK01 • • • • • • 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. 555 astable tH 0.7R1 R 2C +Vs R1 +Vs RESET tL 0.7 R2 C DISCHARGE OUT PUT R2 T RIGGER T HRESHOLD GND CONT ROL C Vout 10nF 0V • • • • • • • • ©IPK01 1.44 f R1 2 R 2C 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 Operational Amplifier +Vs + V+ V– -Vs Differential amplifier • Large input resistance 109 • Large open-loop voltage gain, A, 106 V out 0V • Low output resistance 100 • Frequency compensated • Gain-bandwidth product Vout A(V V) Comparator +Vs R1 R2 V1 V2 + – V1 V 2 Vout Vs V 2 V1 Vout Vs R3 ©IPK01 0V • Can be used as a one-bit analogue to digital converter The Audio Amplifier • LM380, LM386, TBA820 • Bandwidth Vout Voltage gain (Gv) Vin • At least half of its rated power • At least 70% of its rated voltage gain voltage gain 100 80 70 60 40 bandwidth 20 10 10 10 2 10 3 10 4 10 5 10 6 frequency / Hz Prefixes • giga • mega • kilo ×1,000,000,000 ×1,000,000 ×1,000 (G) (M) (k) GHz MHz, M kHz, k, kV • milli • micro • nano ×0.001 ×0.000 001 ×0.000 000 001 (m) () (n) mV, mA, mW V, A, W, F 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 V I R Alternating Current voltage or current peak value • Sine wave rms value • Continuously reverses direction 0 amplitude time 1 cycle • Peak value = 1.4 x rms value • Frequency = 1 / time period ©IKES0902 Series and Parallel • Series:- 1.4 k – Current is the same – Voltage is shared 2.2 k 3.7 k ©IPK01 • Parallel:– Current is shared – Voltage is the same 10k 5k ©IPK01 Resistors • In series – R T R1 R 2 R 3 • In parallel – 1 RT • Colour Code • BS1852 • Preferred values • Tolerance 1 R1 1 R2 Diodes anode cathode -40 cathode symbol component outline current/mA 10 current/mA 10 reverse bias -60 breakdown anode -20 Figure 4.5(a) 8 forward bias reverse bias 8 6 6 4 4 2 2 0 +20 voltage/V -1 forward bias 0 1 voltage/V ©IPK01 Figure 4.5(b) • Conventional current flow. • Allows current to pass in one direction. • 0.7V across a forward biased silicon diode Rectification Half wave diode + Vin Vout ©IKES0902 V V load time time input waveform _ output waveform ©IKES0902 A D1 V time D4 D2 D3 + Full wave V B load Vout _ time ©IKES0902 LEDs +Vs R 0V 0V ©IPK01 • Find the voltage across R. • Note the maximum current through the LED • Calculate R • Choose the next largest preferred value Resistive Input Devices • LDR resistance / ohms 7 10 10 6 10 5 10 4 10 3 10 ©IPK01 2 0.1 1 10 10 2 10 3 10 4 illumination/lux • Thermistor resistance / ohms 10 5 10 4 10 3 10 2 10 ©IPK01 – as light level increases, the resistance decreases – Log-log graphs. 0 20 40 60 80 100o temperature / C – As the temperature increases, the resistance decreases – Log-lin graphs Voltage Dividers I R2 V in V out R1 R 2 R1 Vin R2 Vout ©IPK01 • This formula is not on the data sheet! Transistors and MOSFETs n-channel MOSFET npn transistor collector drain large current base large current gate small current emitter ©IPK01 Vgs source ©IPK01 • MOSFETs • Very high input resistance • Voltage operated • Transistors • Low input resistance – needs base series resistor • Current operated Output devices +Vs • Motor • Relay • solenoid protection diode input 0V ©IPK01 • Use diode with inductive devices • To remove large induced voltage • Diode protects semiconductor when the device is switched OFF Three Terminal Regulators top view • Provides constant output voltage • 7805 = 5V +Vin • 7812 = 12V • 7815 = 15V metal tab output common input mounting hole 78xx 470nF ©IKES0902 +Vout 100nF 1 - 10k 0V 0V ©IKES0902 Audio Systems • A domestic hi-fi installation: – Tuner – Amplifier – Microphone – Loudspeaker – Minidisk – MP3 player – CD player – Cassette tape recorder The Simple Receiver aerial • • • • • rf tuned circuit demodulator af amplifier loud speaker 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 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 Voltage Carrier time Voltage Information time Voltage Amplitude Modulated Carrier time ©ikes1201 Frequency Modulation (FM) Voltage • Constant amplitude • Varying frequency • Broadcast bandwidth is 100kHz • VHF wave band Carrier time Voltage Information time Voltage Frequency Modulated Carrier time ©ikes1201