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 Transformer –Primary and secondary are isolated –Provides safe low voltage from the mains

Three Pin Mains Plug

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

Logic Gates NOT, AND, OR, NAND, NOR, EX-OR Truth tables – every combination of inputs.

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.

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

4017 counter 10 decoded outputs

555 Monostable When the TRIGGER input goes below 1/3V s OUTPUT goes high and DISCHARGE switches off C charges through R Until capacitor voltage =2/3V s (THRESHOLD voltage) OUTPUT goes low and DISCHARGE switches on Capacitor discharged.

555 astable When first switched on, TRIGGER is less than 1/3V s OUTPUT goes high, DISCHARGE switches off Capacitor charges through R 1 and R 2 Until capacitor voltage = 2/3V s (THRESHOLD) OUTPUT goes low, DISCHARGE switches on Capacitor discharges through R 2 Until capacitor voltage = 1/3V s (TRIGGER) OUTPUT goes high, process repeats

Operational Amplifier Differential amplifier Large input resistance 10 9  Large open-loop voltage gain, A,  10 6 Low output resistance  100 Frequency compensated Gain-bandwidth product

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

The Audio Amplifier LM380, LM386, TBA820 Bandwidth At least half of its rated power At least 70% of its rated voltage gain

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× ()V, A, W, F nano× (n)nF pico× (p)pF

Voltage and Current Voltage ACROSS object – Volts Current THROUGH object – Amps Power = Voltage x Current – Watts Resistance = Voltage/Current - Ohms

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

Series and Parallel Series:- –Current is the same –Voltage is shared Parallel:- –Current is shared –Voltage is the same

Resistors In series – In parallel – Colour Code BS1852 Preferred values Tolerance

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

Rectification Full wave Half wave

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

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

Voltage Dividers This formula is not on the data sheet!

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

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

Three Terminal Regulators Provides constant output voltage 7805 = 5V 7812 = 12V 7815 = 15V

Audio Systems A domestic hi-fi installation: –Tuner –Amplifier –Microphone –Loudspeaker –Minidisk –MP3 player –CD player –Cassette tape recorder

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

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

Frequency Modulation (FM) Constant amplitude Varying frequency Broadcast bandwidth is 100kHz VHF wave band