Routing turns every ratsnest line into copper. Two traces of different nets can never touch, so where they must cross, one goes round or drops to the other side of the board. Students route the shield on one layer, then on two, and compare the copper.
Two wires on a breadboard can cross in the air. Two copper traces that touch are a short circuit. How does GND get past OUT?
Routing draws copper along each ratsnest line. Click a pad to start a trace, then click one cell at a time to the next pad of the same net. The board refuses a step that would touch another net: that would be a short circuit.
Most boards have copper on two layers: the top and the bottom. The pads of these parts go right through the board, so a trace on either side can reach them. A via is a small plated hole that takes a trace from one side to the other.
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.
Crossing
TryTry it on the board and compare the result with your prediction.
Try: route on the top
Try it yourselfYour edits are saved in this browser.
Route VCC: click J1's top pad, then step to S1's left pad.
Use the board. This ticks when you get there.
Route OUT (J1's third pad to R1's left pad), then try to route GND (J1's bottom pad) towards R2 on the top layer.
Use the board. This ticks when you get there.
InvestigateWork out why it happens, one step at a time.
Worked example: round or under?
Worked exampleBefore each step, say what you think comes next.
Round (one layer):GND goes down J1's side, along the bottom edge, and back up to D1 and R2.
≈47 cells×2.54≈119 mm of copperLong, but the board needs copper on one side only, which is cheaper.
Under (two layers):GND drops to the bottom layer at J1's pad and runs straight under OUT.
≈37 cells×2.54≈94 mmShorter and tidier; two-layer boards are standard and cost only a little more.
A detour adds 9 cells of copper. How many extra mm is that, to 1 decimal place?
ModifyChange the problem on purpose and see what happens.
Vary: one side only
Try it yourselfYour edits are saved in this browser.
Need a hint?
Route the short nets first: VCC, LEDA, OUT.
Another hint
IN can finish at S1's pad, then start again from it to reach R2.
Another hint
GND can run down J1's side, along the bottom row and up the right.
Route every net on the top layer only.
Use the board. This ticks when you get there.
PractiseA few on your own, to check you have it.
Routing
A board's traces are 52 cells long in all. How much copper is that, in mm, to the nearest mm?
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
Route it short
Need a hint?
Switch to bottom layer before you start GND.
Another hint
Keep each trace as straight as the parts allow.
Another hint
Undo removes the last step if a trace wanders.
Try it yourselfYour edits are saved in this browser.
Route every net, using both layers, with 100 mm of copper or less.
Use the board. This ticks when you get there.
For teachers
Students route copper traces cell by cell, find that a trace can't touch another net (a short circuit), route the shield on one layer with a detour, then on two layers with a trace on the bottom, and compare the copper. About 45 minutes.
I can route copper traces that join every net without a short circuit.
I can explain when a trace goes round and when it goes to the other layer.
I can explain what a via is for.
Before class
The board model: one trace per grid cell per layer, through-hole pads on both layers, steps of one cell (no diagonals). Real tools route at angles of 45°, check gaps (clearance) between copper, and route at finer grids; KiCad and EasyEDA both have an interactive router that pushes other traces aside.
Keyboard: the board can be focused (Tab); arrow keys move a cursor and Enter acts on the cell.
Curriculum hooks: Design and Technologies, engineering principles and systems; Digital Technologies, digital systems (hardware).
Running the lesson
Predict: hold up two wires and ask what happens if two bare copper strips touch.
Try: when GND is blocked, stop and ask for both answers (round, or under).
Vary: the one-layer route takes a while; let early finishers try a different GND path.
Make: compare copper lengths across the class.
The hard part (on purpose)
Routing takes patience: one cell at a time, and some nets block others. Suggest routing the short nets first and using Undo freely.
When students get stuck
A trace won't start. Ask: Did you click a pad (or a trace) of a net? Fix: Start on a pad.
A step is refused. Ask: Is that cell next to the last one, and free of other nets on this layer? Fix: Go round, or switch layer.
A net still has a thin line after a trace finished. Ask: How many pads does the net have? Fix: Start a new trace from a joined pad to the next one.
Stretch and support
Stretch: Route the shield so GND is a wide area of copper on the bottom (a ground plane, many cells). Why do real boards do this?
Support: Give the VCC and OUT paths as cell lists to follow, then let students route GND.
Where it leads
Next: Design rules and ordering.
Real tools route at 45°, check clearance between copper, and pour ground planes; the Capstone project Design a PCB uses one.
crossing: Students know wires can cross; copper can't. Committing to a plan for a crossing sets up the two answers routing gives: go round, or go under.
Try: route on the top: Routing a simple net shows the mechanics; routing GND on the top only shows the cost of a crossing: a long detour.
Worked example: round or under?: The worked example compares the two answers by copper length and by layers. The tasks turn cells into millimetres and say when a via is needed.
Vary: one side only: Routing everything on one layer forces the detours, and makes the value of a second layer obvious.
routing: What a short is, what a via does, and measuring copper.
route it short: With both layers allowed, students aim for the shortest copper: the two answers to a crossing, chosen net by net.
Now you can
Route copper traces that join every net without a short circuit.
Explain when a trace goes round and when it goes to the other layer.