ElectronicsQ&A by dot point

The silicon diode and its one-way (rectifying) behaviour: using a diode to protect a circuit against reverse polarity and against inductive spikes, and half-wave rectification to convert AC into DC.

The Zener diode and its reverse breakdown behaviour, and its use with a series resistor to provide a stable regulated output voltage.

Electric charge and current as the rate of flow of charge, the charge equation, voltage (potential difference) as energy per unit charge, standard circuit symbols, and the test equipment used to measure electrical quantities (multimeter, oscilloscope, logic probe).

Electrical power and energy, the power equations linking power to voltage, current and resistance, the energy equation, and using them to choose a suitable power rating for a component.

Resistance and Ohm's law, the equation linking voltage, current and resistance, and the current-voltage (I-V) characteristics of an ohmic resistor, a filament lamp and a silicon diode.

The current and voltage (potential difference) rules for series and parallel circuits, including the conservation of current at a junction and the sharing of voltage around a loop, and using them to analyse simple circuits.

Boolean expressions and the basic Boolean identities, simplifying combinational logic, building any logic function from NAND gates (NAND universality), and designing a logic system to meet a requirement using a data sheet to select ICs.

Logic levels (logic 1 and logic 0 as high and low), the NOT, AND, OR, NAND, NOR and XOR gates, their logic symbols, and constructing and using truth tables to describe combinational logic.

The systems approach to electronics: the input (sensing), process and output sub-systems, the use of system block diagrams, common input sensors, processing units and output devices, and why transducer drivers are needed between a processing sub-system and an output device.

The microcontroller as a programmable integrated circuit, how sensors and output devices are interfaced to it, designing and analysing a control program as a flowchart, and why microcontrollers are now used so widely.

The Schmitt inverter and its switching with hysteresis, used to debounce switches and to give a clean digital output from a slow or noisy analogue input, and comparing transistors, comparators and Schmitt inverters as interface circuits.

Amplification as increasing the voltage (or power) of a signal, the voltage gain equation, the gain-frequency response and bandwidth, and clipping distortion when the output is limited by the supply.

The operational amplifier in inverting and non-inverting configurations and their gain equations, the summing amplifier (mixer) that adds several inputs, and the amplifier system block diagram from signal source to loudspeaker.

Input sensors as variable resistors: the light-dependent resistor (LDR) and the NTC thermistor, how their resistance varies with light and temperature, and using them in a potential divider so the output voltage responds to the physical quantity.

The potential divider: how two resistors share a supply voltage, the potential divider equation, and designing and analysing a divider to produce a required output voltage.

Pull-up and pull-down resistors that give a switch a defined logic level, and the current-limiting (series) resistor that protects an LED, including calculating its value.

Calculating the total resistance of resistors in series and in parallel, and identifying a resistor's value, tolerance and power rating from its colour code or the E24 preferred-value series.

Building a 1-bit and a 2-bit binary up-counter from rising-edge-triggered D-type flip-flops, how each stage divides the clock frequency by two, and reading the count from a timing diagram.

The rising-edge-triggered D-type flip-flop: how it copies its D input to its Q output on the rising edge of the clock, its use as a data latch and for data transfer, and reading its behaviour from a timing diagram.

BCD and decade counters, the seven-segment display with its decoder/driver to show decimal digits, the 4017 decade counter as a sequencer with one output high at a time, and resetting a counter early to make a custom count length.

The voltage comparator as an integrated-circuit switching circuit that compares an input voltage with a reference, the use of a potential divider to set the reference and threshold, and interface circuits (including a relay or transistor driver) to switch an output.

The npn bipolar transistor and the n-channel enhancement MOSFET used as switches: how a small input controls a larger output current, the meaning of saturation and cut-off, and the differences between the two devices.

An overview of Component 3, the extended system design and realisation task (the non-exam assessment): analysing a problem, writing a design specification, developing and testing sub-systems, building and testing the complete system, and evaluating it against the specification.

The 555 timer in astable mode: how it produces a continuous square-wave output, the equations for frequency and period, and the meaning and calculation of the mark-space ratio.

The 555 timer in monostable mode: how it produces a single output pulse of fixed duration when triggered, and the equation for the pulse duration in terms of the timing resistor and capacitor.

The capacitor and the resistor-capacitor (RC) network: how a capacitor charges and discharges through a resistor, the shape of the voltage-time curves, and how the RC combination produces a time delay.