Electronics & Analogue Control
How electronic systems sense and respond — circuit symbols, Ohm's law, power, input and output transducers, voltage dividers, and the transistor as a switch.
Electricity and circuits Exam
Engineers design circuits to control how energy flows. The three quantities that matter in every circuit are voltage (V, volts — the energy given to each unit of charge), current (I, amperes — the rate of flow of charge) and resistance (R, ohms — how strongly a component opposes current).
Circuit symbols — from the data booklet
Engineers use the standard British Standard (BS) symbols listed in the N5 Engineering Science data booklet. Learn to recognise and draw every one.
Series and parallel circuits
| Series | Parallel | |
|---|---|---|
| Connection | one after the other in a single loop | on separate branches |
| Current | the same at every point (It everywhere) | splits between branches and adds back together (It = I1 + I2 + …) |
| Voltage | supply voltage shared between components (Vt = V1 + V2 + …) | supply voltage is the same across each branch |
| Total resistance | Rt = R1 + R2 + … | 1/Rt = 1/R1 + 1/R2 + … |
Measuring voltage and current
| Meter | Measures | How to connect | Resistance of meter |
|---|---|---|---|
| Voltmeter | voltage (V) | in parallel with the component | very high |
| Ammeter | current (A) | in series with the component | very low |
- Is the supply voltage set to the correct value?
- Are all the wires and components connected securely?
- Are the resistor values correct?
- Is the polarity of LEDs, diodes and the supply correct?
Ohm's law and resistance Exam
Ohm's law links voltage, current and resistance — it is the most important formula in electronics. Your marks come from showing the working: substitute the numbers, rearrange if needed, then give the answer with a unit.
Ohm's law — V = I R
Worked example — substitute & solve
Worked example — rearrange with a prefix
Resistors in series — Rt = R1 + R2 + …
Worked example — substitute & solve
Resistors in parallel — the reciprocal formula
Worked example — substitute & solve
Sense check: 66.7 Ω is less than 100 Ω — the parallel total is always less than the smallest resistor.
Electrical power Exam
Electrical power can be calculated three ways, depending on which quantities you know. All three are on the data sheet — choose the one that matches the question's givens.
Worked example — P = IV (substitute & solve)
Worked example — P = I²R with a prefix conversion
Worked example — P = V²/R (substitute & solve)
Worked example — rearrange (find the current)
Input and output devices Exam
Engineered systems follow input → process → output. An input transducer changes a real-world input into an electrical signal; an output transducer changes an electrical signal into a real-world output.
| Input transducer | Reacts to… | Engineering example |
|---|---|---|
| Switch | being pressed or moved | start button on a machine |
| Variable resistor | being turned by hand | volume control |
| LDR | the level of light falling on it | automatic street lighting |
| Thermistor (NTC) | temperature | temperature sensor in an oven |
| Output transducer | Output | Engineering example |
|---|---|---|
| Lamp / LED | light | warning indicator |
| Buzzer | sound | alarm system |
| Motor | movement (kinetic energy) | fan, pump, robot arm |
| Relay | switches a separate circuit | switches a high-power motor |
How the two sensors behave
| Sensor | Condition | Resistance |
|---|---|---|
| LDR | bright light | low (a few hundred ohms) |
| darkness | high (often hundreds of kilohms) | |
| NTC thermistor | high temperature | low |
| low temperature | high |
Voltage dividers and sensors ExamAssignment
A voltage divider is two resistors in series that split the supply voltage. The output V2 is taken across the lower resistor. Because the resistors are in series, the larger resistance always has the larger voltage across it — the voltages are in the same ratio as the resistances.
An analogue signal can take any value within a range and changes smoothly — like the output of an LDR divider as the light slowly fades at sunset. Replace one resistor with an LDR or thermistor and the divider becomes a sensor circuit: as the sensor's resistance changes, V2 changes.
Worked example — substitute & solve
Worked example — sensor divider
Where you put the sensor decides what the circuit does
| Sensor position | In bright light / when hot… | In darkness / when cold… |
|---|---|---|
| LDR as R₁ (top) | LDR resistance low → V2 high | LDR resistance high → V2 low |
| LDR as R₂ (bottom) | LDR resistance low → V2 low | LDR resistance high → V2 high |
The transistor, relay and protection diode Exam
A transistor is a fast, automatic electronic switch: a small input voltage at the base controls a much larger output current. An NPN transistor switches on when the base voltage reaches about 0.7 V.
| Connection | Letter | Function |
|---|---|---|
| Base | B | the input — a small signal here switches the transistor on or off |
| Collector | C | connected to the output device, through the supply |
| Emitter | E | the path back to 0 V |
Relays and the protection diode
A relay is an electromagnetic switch: a small current through its coil produces a magnetic field that closes a separate set of contacts — so a small electronic circuit can switch a much larger one. When the coil switches off it produces a voltage spike that can destroy the transistor, so a protection diode is connected across the coil, in reverse to the supply.
A full control circuit
Every control circuit in this topic is built from the same four blocks: an input transducer in a voltage divider, a transistor that switches when V2 reaches about 0.7 V, and an output device — or a relay to switch a separate, larger circuit.
Choosing the sensor position
| Circuit | Position of sensor | Output switches on when… |
|---|---|---|
| Light-sensing | LDR as R₁ (top) | the light level is high |
| Dark-sensing | LDR as R₂ (bottom) | the light level is low |
| High-temperature | thermistor as R₁ (top) | the temperature is high |
| Low-temperature | thermistor as R₂ (bottom) | the temperature is low |
Common mistakes — watch out for these in the exam
- Giving an answer with no unit, or the wrong unit (V, A, Ω, W).
- Connecting meters wrongly — an ammeter goes in series, a voltmeter in parallel.
- Drawing the battery the wrong way round — the long line is +.
- Describing a sensor circuit without using Vout — say what happens to the resistance, then Vout, then the transistor/output.
- Forgetting the transistor is the switch, and the relay lets it switch a larger current.
Check your booklet work
Try each task in your booklet first, then open the matching answer. Calculations show the final value with the key steps.
Try This — final answers
Try This — Ohm's law Booklet p.9
- R = 12 ÷ 0.5 = 24 Ω
- I = 5 ÷ 220 = 0.023 A (23 mA)
- R = 24 ÷ 2 = 12 Ω
Try This — Series resistance Booklet p.10
- 220 + 330 = 550 Ω
- 1000 + 2000 + 4700 = 7700 Ω (7.7 kΩ)
- 4 × 100 = 400 Ω
Try This — Parallel resistance Booklet p.11
- Two equal resistors halve: 470 ÷ 2 = 235 Ω
- 1/Rt = 1/100 + 1/1000 = 0.011 → 90.9 Ω ≈ 91 Ω
- 1/Rt = 1/100 + 1/200 + 1/400 = 0.0175 → 57.1 Ω
Try This — Power (P = VI, P = I²R, P = V²/R) Booklet p.12–14
P = VI:
- 230 × 5 = 1150 W
- 24 × 0.8 = 19.2 W
- I = 18 ÷ 12 = 1.5 A
P = I²R:
- 0.2² × 100 = 4 W
- 0.04² × 470 = 0.75 W
- 50 mA = 0.05 A; 0.05² × 1000 = 2.5 W
P = V²/R:
- 12² ÷ 220 = 0.65 W
- 9² ÷ 4700 = 0.017 W (17 mW)
- V = √(P × R) = √(1 × 100) = 10 V
Try This — Voltage divider & sensor divider Booklet p.21 & p.23
Voltage divider:
- V₁ = 6 V, R₁ = 2 kΩ, R₂ = 1 kΩ → V₂ = 6 × 1/2 = 3 V
- V₁ = 4 V, R₁ = 470 Ω, R₂ = 940 Ω → V₂ = 4 × 940/470 = 8 V
- V₂ = 4 V, R₁ = 1 kΩ, R₂ = 2 kΩ → V₁ = 4 × 1/2 = 2 V
Sensor divider:
- LDR R₁ = 2 kΩ (V₁ = 4 V), R₂ = 3 kΩ → V₂ = 4 × 3/2 = 6 V
- R₁ = 10 kΩ (V₁ = 2 V), thermistor R₂ = 20 kΩ → V₂ = 2 × 20/10 = 4 V
- R₁ = 2 kΩ (V₁ = 3 V), LDR R₂ = 6 kΩ → V₂ = 3 × 6/2 = 9 V
Section practice — final answers
Practice — Electricity and Circuits Booklet p.7
- Volt (V).
- Ampere (A).
- Ohm (Ω).
- In parallel with (across) the component.
- In series with the component.
- The current is the same at every point.
- The voltage is the same across each branch (equal to the supply).
- 6 V — the 12 V supply is shared equally between two identical lamps.
- Any two: supply voltage correct; wires/components secure; resistor values correct; polarity of LEDs/diodes/supply correct.
- The ammeter is in series, so its resistance adds to the circuit — a high resistance would reduce the very current it is trying to measure.
Practice — Electrical Calculations Booklet p.9–14
- V = I R.
- I = 9 ÷ 470 = 0.019 A (19 mA)
- 220 + 470 + 1000 = 1690 Ω (1.69 kΩ)
- Two equal in parallel halve: 220 ÷ 2 = 110 Ω
- P = 24 × 1.5 = 36 W
- P = I²R → I = √(1 ÷ 100) = 0.1 A
- P = 12² ÷ 1000 = 0.144 W (144 mW)
- 1000 ÷ 2 = 500 Ω
- 1000 + 2200 + 4700 = 7900 Ω (7.9 kΩ)
- V = 0.05 × 220 = 11 V; P = 11 × 0.05 = 0.55 W
Task 3 & Practice — Input and Output Devices Booklet p.17–20
Task 3 — match the sensor: dark → LDR · warm room → thermistor · oven door opened → switch · dimming knob → variable resistor · floor sensor → switch (pressure switch).
- A component that changes a real-world input into an electrical signal.
- A component that changes an electrical signal into a real-world output.
- The LDR's resistance increases as the light level decreases.
- The NTC thermistor's resistance increases as the temperature decreases.
- Input: thermistor. Output: motor (fan).
- Read the value your class recorded for normal room light in your data table.
- Read your table — interpolate between the recorded temperatures if needed.
- Lamp and LED.
- A relay lets a small electronic circuit switch a high-power circuit — e.g. switching a mains-powered motor from a low-voltage sensor circuit.
- A switch (limit / guard switch).
Practice — Voltage Dividers and Sensors Booklet p.25–26
- A signal that can take any value within a range and changes smoothly.
- To produce a smaller voltage from the supply voltage.
- V₁ = 3 V, R₁ = 220 Ω, R₂ = 440 Ω → V₂ = 3 × 440/220 = 6 V
- The LDR's resistance increases, so more voltage is dropped across the LDR (R₁) — V₂ decreases.
- The thermistor's (R₂'s) resistance decreases, so V₂ decreases.
- LDR R₁ = 2 kΩ (V₁ = 2 V), R₂ = 3 kΩ → V₂ = 2 × 3/2 = 3 V
- The output is across only one of two series resistors — the two voltages add up to the supply, so each part must be less than the supply.
- R₁ = 1 kΩ, R₂ = 4 kΩ, V₂ = 8 V → V₁ = 8 × 1/4 = 2 V
- e.g. a volume control or lamp dimmer — turning the knob changes the output voltage smoothly.
- Bottom position (R₂) — in darkness the LDR's resistance is high, so V₂ is high and the output switches on.
Practice — Transistor Switching, Relays and Output Control Booklet p.29–30
- It acts as a fast electronic switch — a small base voltage switches a larger output current.
- Base, collector, emitter.
- About 0.7 V.
- An electromagnetic switch: a small current through the coil closes separate contacts, switching a larger circuit.
- It protects the transistor from the voltage spike produced when the relay coil switches off.
- Bottom (R₂) — so V₂ rises as it gets dark.
- As it gets darker the LDR's resistance increases (1), so the voltage across the LDR / V₂ increases (1); when it reaches about 0.7 V the transistor switches on and the lamp lights (1).
- The transistor circuit is low-voltage; the relay's contacts are electrically separate, so its small coil current can safely switch the 230 V mains circuit.
- Thermistor as R₁ (top). When the machine gets hot the thermistor's resistance falls, so V₂ (across R₂) rises; at about 0.7 V the transistor switches on the fan.
- Any two with fixes, e.g.: LED/battery the wrong way round — check polarity before power-up; missing protection diode across the relay coil — always fit one, reversed to the supply; meters connected wrongly — voltmeter in parallel, ammeter in series; loose breadboard wires — push components fully home and test step by step.
Check yourself
Sources & credits: The Topic 3 booklet © R Stewart, 2026. The Past Paper Finder is compiled by Mr McDonald, 2024; past-paper questions © Qualifications Scotland (SQA). The N4/N5 data booklet is reproduced for educational use, © Qualifications Scotland (SQA).