SQA National 5 Engineering Science Area 2 Electronics and control: systems, analogue and digital electronics, and programmable control

Electronics and control is the electronics half of National 5 Engineering Science. It runs from analogue circuit calculations through digital logic to programmable control, all framed by the universal systems model. This guide maps the key areas; each has its own answer page with worked questions and cross-links.

The universal systems model

Any electronic or control system can be modelled as three sub-systems: an input (a sensor that detects a condition and produces a signal), a process (the part that decides or operates, such as a logic gate, comparator or microcontroller) and an output (a device that acts, such as a lamp, motor or buzzer). A block diagram draws these as boxes joined by arrows showing the signal flowing input to process to output. Feedback sends information about the output back to keep the system under control.

Analogue electronics

Analogue signals vary continuously. Ohm's law, $V = IR$, links voltage, current and resistance, and power is $P = IV$ (also $P = I^2R$ and $P = V^2/R$). Resistors in series add, $R_T = R_1 + R_2 + \dots$, while resistors in parallel combine by the reciprocal rule and give a total less than the smallest resistor. These calculations, all in the data booklet, underpin the rest of the area.

The voltage divider and transducers

A voltage divider shares the supply voltage in proportion to two resistances, $V_{out} = \frac{R_2}{R_1 + R_2} V_s$. An LDR (resistance falls as light rises) or thermistor (resistance falls as temperature rises) in a divider makes a sensing circuit whose output voltage tracks the condition - the input stage of almost every automatic control circuit.

Transistors and output devices

Output transducers - the lamp, LED (always with a current-limiting series resistor), buzzer and motor - usually need more current than a sensor can provide. A transistor acts as an electronic switch: when its base voltage rises above about 0.7 V it turns on and conducts a large current to the output. An inductive load such as a motor needs a protective diode across it.

The operational amplifier

An op-amp has two inputs. As a comparator it switches its output depending on which input voltage is higher, giving threshold switching from a sensor. As an inverting amplifier it boosts a small signal with gain $= -R_f / R_1$, and output $V_{out} = \text{gain} \times V_{in}$, where the minus sign means inversion.

Digital logic and combinational logic

Digital signals are logic 1 or logic 0. AND outputs 1 only when all inputs are 1; OR outputs 1 when any input is 1; NOT inverts its single input. A truth table lists the output for every input combination. Combinational logic combines gates to meet a requirement; a NAND is an inverted AND and a NOR an inverted OR. A Boolean expression writes the logic in shorthand (dot for AND, plus for OR, bar for NOT).

Programmable control

A microcontroller is a programmable process sub-system: it reads inputs, follows a stored program and switches outputs. Its behaviour is changed by reprogramming rather than rewiring, and one chip can replace many gates. A flowchart plans the program with a start/stop terminal, input/output and delay steps, a decision diamond and loops that repeat steps for continuous control.

How Electronics and control is examined

This area mixes calculation (Ohm's law, power, the divider, op-amp gain) with explanation (systems, transistor switching) and digital reasoning (truth tables, flowcharts). Select the right relationship from the data booklet, substitute and quote the unit; for logic, work through gates one at a time and never forget the loop in a control flowchart.

How to study Electronics and control

1. Drill the calculations. Ohm's law, power, series and parallel resistors, the divider equation and op-amp gain should be automatic.
2. Know the sensors. LDR resistance falls in light; thermistor resistance falls with heat.
3. Master the transistor switch. Base voltage above ~0.7 V turns it on; remember the LED resistor and the motor diode.
4. Learn the gate rules and symbols. AND = all, OR = any, NOT = invert; NAND and NOR are the inverted versions.
5. Practise flowcharts. Input, decision, output, delay and loop, with the correct symbols.

For the official course specification

The SQA publishes the full National 5 Engineering Science course specification, data booklet and past papers at sqa.org.uk. Always revise from the current specification and SQA past papers, because question style and terminology are board-specific.