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.
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Students will be able to
I can explain why current needs a complete loop.
I can wire an LED the right way round.
I can find why an LED won't light.
Timing
Predict and Try 8 min · Worked example 8 min · Vary 6 min · Tinkercad 10 min · Practise and Make 8 min
Before class
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
What you will see
A question to ask
The fix
A student thinks one wire is enough.
Where does the current go back into the micro:bit?
Into GND: it needs a loop.
The LED is dark in Tinkercad but every wire looks fine.
Which leg goes to the resistor?
The long leg (anode) towards pin 0; the bent leg in Tinkercad.
Answers to the checks
unclip a wire: Pin 0 is on and the LED is lit. You unclip the wire from the LED to GND. The LED… It goes out. With the GND wire off the loop is broken, so no current flows anywhere in it.
Unclip GND: Unclip the GND wire. What happens to the LED?
It goes out: the current has no way back to GND, so it stops everywhere, even in the pin 0 wire.
Unclip pin 0: Clip GND back on, then unclip the pin 0 wire instead.
It goes out again. It doesn't matter where the loop is broken.
Switch the pin off: Clip both wires on, then switch pin 0 off.
The loop is complete, but pin 0 is at 0 V, the same as GND, so nothing pushes current round. Code switches the pin; wires make the loop.
Trace it: The loop is complete and pin 0 is on. Where is the current smallest? Nowhere: it is the same all the way round the loop In a single loop the same current flows everywhere. Parts use energy (voltage), not current.
Find the mistake: Sam's LED never lights. Click the line where Sam's plan first goes wrong. line 2 Line 2: pin 0 doesn't send power one way like a tap. Current flows round a loop, out of pin 0 and back into GND, so the GND wire is needed.
Turn it round: Turn the LED round, with both wires clipped and pin 0 on.
It goes out. An LED only lets current through one way: the long leg (the triangle's flat side) towards pin 0.
Check first: Every wire is clipped and pin 0 is on, but the LED is dark. Which is the best first thing to check? Which way round the LED is With the loop complete and the pin on, a backwards LED is the most likely slip.
Which lights?: Which of these lights the LED? Pin 0 on → resistor → LED (long leg first) → GND Only the first is a complete loop, the right way round, with the pin on.
A torch: A torch has a battery, a switch and a bulb. Turning the switch off… Breaks the loop, so no current flows anywhere A switch is a deliberate break in the loop.
fix the circuit: This LED won't light. Find and fix every fault.
A wire off, the LED backwards and the pin off: three different reasons for the same dark LED. Check the loop first, then the parts, then the code.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: the LED the wrong way round › Turn it round
Open task: any program that does what the step asks.
Vary: the LED the wrong way round › Check first
Open task: any program that does what the step asks.
fix the circuit
Open task: any program that does what the step asks.
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.
Exit ticket
Pin 0 is on, every wire is clipped, and the LED is dark. Give two possible reasons.
The LED is the wrong way round, or a wire isn't really connected (the loop is broken). Also possible: the pin isn't actually on.
What each step is for
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.
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).