Current only flows around a complete loop: out of pin 0, through the resistor and the LED, and back to GND. Break the loop anywhere, or turn the LED round, and the current stops everywhere at once.
You switch pin 0 on, but the LED stays dark. The code is right. What else has to be true for current to flow?
In micro:bit Foundations the LEDs were already on the board. Now you wire your own: an LED (a light that lets current through one way), a resistor (it protects the LED; next lesson says how) and two wires to the micro:bit's pins. Pin 0 can be switched on (3.3 V) by code. GND is the micro:bit's ground, 0 V.
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.
WidthMore contexts, same idea.
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.
Unclip a wire
TryTry it on the board and compare the result with your prediction.
Try: break the loop
Was your prediction right?
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Unclip the GND wire. What happens to the LED?
Use the board. This ticks when you get there.
Clip GND back on, then unclip the pin 0 wire instead.
Use the board. This ticks when you get there.
Clip both wires on, then switch pin 0 off.
Use the board. This ticks when you get there.
InvestigateWork out why it happens, one step at a time.
Worked example: follow the loop
Worked exampleBefore each step, say what you think comes next.
pin 0 on: 3.3 VCode switches pin 0 on. It is 3.3 V above GND.
pin 0→resistor→LEDCurrent flows out of pin 0 through the resistor and the LED.
LED→GND (0 V)It flows back into the micro:bit at GND.
one loop, one currentThe same current flows everywhere in the loop.
any break⇒0Unclip any wire, or switch the pin to 0 V, and the current is 0 everywhere.
Now apply it: where in the loop is the current smallest?
Sam's LED never lights. Click the line where Sam's plan first goes wrong.
ModifyChange the problem on purpose and see what happens.
Vary: the LED the wrong way round
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Turn the LED round, with both wires clipped and pin 0 on.
Use the board. This ticks when you get there.
Modify TypicalChange the problem on purpose and see what happens.
Build it in Tinkercad
Your teacher will give you the class code. Long leg of the LED towards the resistor and pin 0.
PractiseA few on your own, to check you have it.
Loops
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
Fix the circuit
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Need a hint?
There are three faults. Check each part of the loop in turn.
Another hint
Is every wire clipped? Is the LED the right way round? Is the pin on?
Another hint
Clip GND, turn the LED round and switch pin 0 on.
This LED won't light. Find and fix every fault.
Use the board. This ticks when you get there.
For teachers
Students break and remake a loop of pin 0, a resistor, an LED and GND on a board, see that current stops everywhere when the loop breaks anywhere, find that an LED only works one way round, then build the same circuit in Tinkercad. About 40 minutes.
Try the boards, and set up a Tinkercad class (Classes › New class; students join with the class code and a nickname, no email). Check Tinkercad loads on student devices.
Simplifications: the board's model is a 3.3 V pin, an LED that drops 2 V, and wires with no resistance. The resistor's job is the next lesson; here it is just part of the loop.
If you have kits, one LED, a 220 Ω resistor and two crocodile clips on a real micro:bit make the Try even better.
Running the lesson
Predict: ask those who chose 'stays lit' to say where the current would go.
Try: ask why unclipping the pin wire and unclipping GND have the same effect.
Vary: the backwards LED is the slip students will make in Tinkercad. Point at the long leg.
Make: ask students to list the faults in the order they checked them.
The hard part (on purpose)
Students picture the pin pushing power out like a tap. Keep asking 'where does the current go back in?'
When students get stuck
A student thinks one wire is enough. Ask: Where does the current go back into the micro:bit? Fix: Into GND: it needs a loop.
The LED is dark in Tinkercad but every wire looks fine. Ask: Which leg goes to the resistor? Fix: The long leg (anode) towards pin 0; the bent leg in Tinkercad.
Stretch and support
Stretch: Add a second LED and resistor on pin 1. Is that one loop or two? What happens to the first LED if the second's GND wire comes off?
Support: Give a printed loop to trace with a finger: pin 0, resistor, LED, GND, back to the micro:bit.
Where it leads
Next: Why the resistor (how much current, and what limits it).
Code links: micro:bit Foundations (pins and digital write); Engineering › Power and motors (why a pin can't run a motor).
unclip a wire: Committing to what happens when the GND wire comes off tests the new move: current needs a complete loop, so breaking it anywhere stops it everywhere. The wrong options are the 'pin pushes power one way' picture.
Try: break the loop: Unclipping each wire in turn settles the prediction and shows it doesn't matter where the loop breaks; switching the pin off is a third way to stop the current.
Worked example: follow the loop: Tracing the loop from pin to GND models the move; the questions apply it to a wire, the pin and a torch. Sam's 'one wire is enough' is planted after the method.
Vary: the LED the wrong way round: Turning the LED round is a break that looks like a complete loop: an LED is a one-way valve. It is the most common wiring slip.
Build it in Tinkercad: Building the same loop in Tinkercad Circuits checks the idea against a fuller simulator, and gets students used to the tool before the Arduino lessons.
loops: Spotting complete and broken loops in words, including the backwards LED and a pin at 0 V.
fix the circuit: Fixing a circuit with three faults reverses the process: students diagnose a dark LED from the loop rule, the one-way LED and the pin.