A printed circuit board starts as parts on a grid and a list of nets: pads that must end up joined. Thin ratsnest lines show the joins still to make. Students find nets, then move and turn parts so the joins are short and the connector sits at the edge.
Your breadboard circuit works. To make it a real board, every wire becomes copper, and the first decision is where each part goes.
On a breadboard, wires go wherever you push them. On a printed circuit board (PCB), the wires are strips of copper called traces, printed onto the board. Before any copper, a designer places the parts.
The circuit is a button and an LED on a small shield. J1 is the connector to the micro:bit or Arduino: VCC (power), IN (the button), OUT (the LED) and GND. A net is a set of pads that must end up joined. The thin ratsnest lines show the joins still to make. One grid step is 2.54 mm, the gap between pins on most parts.
Predict: Read the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Try: Try it on the board and compare the result with your prediction.
Investigate: Work out why it happens, one step at a time.
Modify: Change the problem on purpose and see what happens.
Practise: A few on your own, to check you have it.
Make: Make your own problem using what you have learned. Expect a few tries; that is normal.
Make your prediction first. Help opens once you've locked it in.
PredictRead the problem and decide what you think before you try anything. A wrong guess is fine: it shows you what to look for.
Where to put the parts
TryTry it on the board and compare the result with your prediction.
Try: nets and moves
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Click any pad on the GND net (J1's bottom pin is one). Which pads light up?
Use the board. This ticks when you get there.
How many pads are on the GND net?
Click D1 to select it, then click an empty cell close to R1 to move it there. Watch the ratsnest total.
Use the board. This ticks when you get there.
InvestigateWork out why it happens, one step at a time.
Worked example: read the board
Worked exampleBefore each step, say what you think comes next.
1. Fix the connectors:J1 goes at the edge, where a cable can reach it. Mark it fixed.
2. Follow the signal:Place parts in the order the signal flows: J1, then the button and its resistor, then R1 and the LED.
3. Keep nets close:A part with many nets (like GND) goes near the middle of the pads it joins.
4. Leave a gap:At least one cell between parts, so copper can pass and the parts can be soldered.
5. Check the numbers:Total ratsnest down, nothing off the board, nothing touching.
A ratsnest line runs 10 cells straight across. How long is it, in mm?
ModifyChange the problem on purpose and see what happens.
Vary: meet the brief
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Need a hint?
Bring S1 and R2 near J1's top pins: they share VCC and IN with it.
Another hint
Turn R2 upright (Rotate) so its ends reach IN and GND.
Another hint
Put R1 and D1 together near J1's OUT and GND pins.
Get the total ratsnest to 100 mm or less, with every part on the board, none touching, and J1 at the edge.
Use the board. This ticks when you get there.
PractiseA few on your own, to check you have it.
Placement
Two pads are 4 cells apart. How far apart are they, in mm?
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
A smaller board
Need a hint?
Start with the parts that share the most nets with J1.
Another hint
Turn the resistors upright to save width.
Another hint
Check the facts line after every move.
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On a board 12 × 8 cells (about 30 × 20 mm), get the ratsnest to 70 mm or less, with every part on the board, none touching, and J1 at the edge.
Use the board. This ticks when you get there.
For teachers
Students find nets by clicking pads, move and rotate parts to shorten the ratsnest, learn a placement order (connectors first, then follow the signal), and meet a layout brief on a normal and a small board. About 40 minutes.
I can place parts so the joins are short and the connector is at the edge.
I can measure a board in millimetres from its grid.
Before class
Do Arduino › From simulator to breadboard (or any breadboard circuit) first: this course turns a breadboard circuit into a board.
The board is a teaching model of a real PCB tool (KiCad, EasyEDA): a 2.54 mm grid, through-hole parts, nets, a ratsnest. Real tools add footprints for every part, a schematic that makes the nets, and surface-mount parts on finer grids. The Capstone project Design a PCB uses a real tool.
Curriculum hooks: Design and Technologies, engineering principles and systems, and materials and technologies; Digital Technologies, digital systems (hardware).
Running the lesson
Predict: show a photo of a real PCB and ask where the copper goes.
Try: have students say out loud which pads light up for GND.
Worked example: model the order on the board: connector, signal path, the many-net part, gaps, numbers.
Make: compare layouts; whose was shortest, and what did they place first?
The hard part (on purpose)
Students move parts at random. Ask them to fix the connector first, then place one part at a time, watching the ratsnest total.
When students get stuck
A part won't move. Ask: Is it selected (gold outline)? Is the cell you clicked empty? Fix: Click the part, then an empty cell for its pad 1.
The board says parts are too close. Ask: Is there an empty cell between every pair of parts? Fix: Leave a gap for copper and soldering.
The ratsnest got longer after a move. Ask: Which nets does that part share, and where are their other pads? Fix: Move it towards the pads it joins.
Stretch and support
Stretch: Rotate J1 so it points out of a different edge. How does the best layout change?
Support: Give the placement order as a checklist, and the target as a number to beat by steps of 20 mm.
Where it leads
Next: Route the traces.
Real tools: KiCad (free, open source), EasyEDA (in the browser, linked to a board maker). The Capstone projects' Design a PCB uses one.
where to put the parts: Placement decides most of how hard routing will be. Committing first makes the point land: short, uncrossed ratsnest lines now mean short, easy copper later.
Try: nets and moves: Clicking pads shows what a net is; moving one part and watching the total ratsnest drop shows what placement is for.
Worked example: read the board: The facts line is the designer's checklist: ratsnest length, parts on the board, a gap between parts, connectors at the edge. Converting cells to millimetres keeps the grid honest.
Vary: meet the brief: A target turns placement into a design problem with a measure: students move and turn parts until the board meets every criterion.
placement: Millimetres from cells, why parts need a gap, and what a net is.
a smaller board: A smaller board forces real trade-offs: every part must still fit, with gaps, and the ratsnest must be shorter still.
Now you can
Explain what a net and a ratsnest line are.
Place parts so the joins are short and the connector is at the edge.