Circuit Construction
Back to Electronics hub GlossaryThis is the Circuit construction area of the course: decoding resistor values, choosing the right board and wiring, soldering safely, and completing circuit and layout diagrams.
Resistor values: colour code & BS1852
- Colour code (three colour bands + a tolerance band): bands 1 & 2 are digits, band 3 is the multiplier, and the tolerance band gives the ± range.
- BS1852 / R-notation: the letter (R, K or M) does two jobs. It gives the size — R = ohms, K = ×1000, M = ×1 000 000 — and it marks where the decimal point goes. Examples: 270R = 270 Ω, 5K8 = 5.8 kΩ, 2M7 = 2.7 MΩ.
- Tolerance: the real resistor can be a little above or below its label. Tolerance tells you the highest and lowest it could be.
On a 4-band resistor, bands 1 and 2 are the and band 3 is the . For a circuit you expect to change many times, the best board is a .
The letter (R, K or M) replaces the decimal point and sets the size.
See the full answer
Write 4 700 Ω and 5 800 Ω in BS1852 notation, and give the resistance band sequence for 4.7 kΩ (yellow, violet, red).
Answer
Boards: prototype, stripboard & PCB
| Board | Connection | Best for |
|---|---|---|
| Prototype (breadboard) | push-fit, non-permanent | quick testing & changes |
| Stripboard | soldered onto copper strips (tracks cut where needed) | a permanent one-off build |
| PCB | soldered to a printed copper layout | reliable, repeatable production |
State which board you would use to test an idea you expect to change several times, and why.
Answer
Soldering safely & non-permanent methods
- Return the iron to its stand the moment you are not using it.
- Solder in a well-ventilated area or use fume extraction — never breathe the fumes.
- Hold work in a clamp or stand, not your fingers, and let joints cool before touching.
- Wear eye protection and wash your hands afterwards.
Permanent joints are made by soldering; non-permanent joints use prototype boards, screw terminals or plug/socket connectors.
State two safety measures you must follow when soldering a circuit.
Answer
Cables & wiring
| Cable | Use |
|---|---|
| Multi-strand | flexible connecting wire that bends without breaking |
| Ribbon | many parallel cores for neat multi-way connections between boards |
| Co-axial | a screened signal cable that resists interference |
| Fibre-optic | carries data as light — fast and immune to electrical interference |
Wiring conventions on a build
Follow these conventions so a board is neat, reliable and easy to fault-find:
| Convention | Why / where |
|---|---|
| Single-core wire | for Wye links and permanent connections on the board — holds its shape and lies flat. |
| Multicore (stranded) wire | for flying leads — connections that flex or move (to switches, off-board parts) without the core snapping. |
| Colour coding | identify each signal at a glance: red = +, black = 0 V, and a distinct colour for each other signal. |
| Joining separate boards | use crimped connectors or terminal blocks so boards can be linked and unplugged cleanly. |
| Spiral wrap & cable ties | gather wire bundles together and keep them tidy, so wires don't snag or pull on their joints. |
- Securing & looming: bundle wires together and hold them in place with cable ties, a spiral wrap, heat-shrink, cable lacing / a loom, or P-clips — so wires don't pull on their joints, snag or get trapped.
- Advantage of colour-coding or numbering: each wire is easy to identify, so it is quicker to connect correctly and to fault-find.
- Examples of where it's used: mains plug wiring (brown = live, blue = neutral, green-yellow = earth); the red stripe on ribbon cable marking pin 1; resistor colour bands; and numbered terminal blocks / network cables.
(a) A signal must travel a long way without picking up electrical interference — state a suitable cable. (b) State one valid way to secure a bundle of wires. (c) Give one advantage of colour-coding wiring and one example of where it is used.
Answer
Circuit ↔ layout diagrams & missing components
- A circuit diagram shows components as symbols and how they connect electrically.
- A layout diagram shows where each part physically sits on the board and how the tracks run.
- You must be able to convert between the two, and complete a circuit diagram from a specification by adding any missing components (using the symbol reference).
The conversion, one step at a time
The thing that makes this work is a net is a track. A net is everything joined together on the circuit diagram; a track is a strip of copper running the whole width of the board. Convert a diagram by giving each net a track and then putting every leg that belongs to that net into it. Everything else — the cuts, the link wires — follows from that one idea.
-
1 · Section the circuit and label it
Mark the printout into input, process and output. You are not converting one big circuit; you are converting three small ones that meet at known points.
-
2 · Give every part an item ID
R1, R2, IC1, LED1 — every part, no exceptions. The IDs are what the wiring schedule and your own layout refer to, and a part drawn but not labelled is a mark gone.
-
3 · Number every net on the diagram
Ring each junction and number it. Count them: that is how many tracks the board needs. Two legs on the same number end up in the same track; two legs on different numbers must not.
-
4 · Place the DIL socket, and cut every track it crosses
The chip sits across the tracks, so each pin lands on the pin opposite it. Cut every track between the two rows of pins — all seven for a 14-pin chip — or pin 1 is joined to pin 14 and pin 7 to pin 8. Draw the cuts as X's inside the chip outline. Solder a socket, never the chip itself.
-
5 · Give each net a track
Write the net numbers down the edge of the sheet, one per track. Start with the ones the chip has already decided for you — whichever track pin 7 is on has to be 0 V, whichever pin 14 is on has to be +5 V — then fit the rest around them.
-
6 · Place the parts, each leg in its net's track
Now the layout is bookkeeping: put each leg in the track its net was given. Legs bend, so a resistor will reach four holes to seven and a sensor two to four — use that to make a part reach the track it needs.
-
7 · Link the nets that could not be next to each other
Some legs of one net end up on different tracks — a chip's output and the next gate's input always do, because adjacent pins are not connected. Join them with a link wire. Number each link and give it a colour, and keep red for +5 V and black for 0 V so nothing else can use them.
-
8 · Mark and number the test points
One on each sub-system input and output, marked on the layout before the board is built, with the reading you expect written beside it. Added after soldering they are not planning — they are poking.
-
9 · Check it before you solder
Read your own board back: take each track in turn and list what is in it, then compare that list with the numbered nets on the diagram. They either match or they do not. Learn to plan a stripboard schematic does this marking for you, and Plan your own stripboard schematic does it on a board of your own design.
The layout rules, and where the mark comes from
| Rule | Why it is a mark |
|---|---|
| Every track the chip crosses is cut, between the two rows of pins | Detailed component layout diagrams — missing track cuts are the fault the external verification reports name most often |
| Every part carries its item ID, and the IDs match the wiring schedule | Detailed component layout diagrams — a part drawn but not labelled cannot be marked |
| Link wires are numbered, and coloured red for +5 V, black for 0 V, one colour per signal | Detailed component layout diagrams |
| A test point on each sub-system input and output, planned before building | Use of test points — and the expected reading written beside each one |
| Every gate input held: pulled down to 0 V through a resistor, or tied straight to 0 V if the gate is spare | The board works. A TTL input joined to nothing is not a logic 0 — it floats |
| Polarised parts the right way round; nothing shorted across the supply | The board survives being switched on |
The two headings in the right-hand column are the assessment's own — the course and assessment material for National 5 Electronics (C884 75) and the external verification reports. © Qualifications Scotland (SQA), reproduced for educational use.
Mark your own layout
Print this, or work down it on screen, before you hand a layout in.
- Every net on the circuit diagram is numbered, and the board has that many tracks in use.
- Every part has an item ID, and it is the same ID on the layout and the wiring schedule.
- Every track the chip crosses is cut, and the cuts are drawn inside the chip outline.
- Every leg is in the track its net was given — checked track by track, not part by part.
- Every link wire is numbered and coloured, red and black used for nothing but the supply.
- Every gate input is held down to 0 V unless another gate is driving it.
- Polarised parts — LED, electrolytic capacitor, diode, transistor — are the right way round.
- A test point on each sub-system input and output, numbered, with its expected reading.
- Sheets attached to the booklet. They cannot be marked if they are not there.
A specification says an LED must be protected from too much current. Its circuit diagram shows only the LED and supply. State the missing component and where it goes.
Answer
Solder Master — read the colour code
Read each resistor's four bands and pick its value. Bands 1 & 2 are digits, band 3 is the multiplier, the gold band is ±5% tolerance. Score 8 / 10 to earn the 🪛 Solder Master badge.
Check your understanding
Answer the multiple-choice questions, then mark yourself with the RAG self-check.
RAG self-check
Tap Red / Amber / Green for how confident you feel.
Sources & credits: the layout method, the fault list and the assessment requirements quoted on this page come from the National 5 Electronics (C884 75) course and assessment material and from the external verification reports — © Qualifications Scotland (SQA), reproduced for educational use. The worked example is drawn by the same renderer as Learn to plan a stripboard schematic, on the real boards used in class (Rapid Electronics 34‑0410 and 34‑0411, datasheet 583629_v1); its drawing conventions — tracks across the sheet, a cut marked as an X on the hole, ICs as a labelled rectangle, link wires coloured and numbered — follow the Stripboard Layout Planning Sheet © 2002 Electronics in Meccano (eleinmec.com), the conventions reproduced and not the artwork. Circuits, diagrams and code © R Stewart, 2026. Full site credits are on the credits page.