With no resistor, far too much current flows and the LED is damaged. A resistor limits the current: current = voltage ÷ resistance (Ohm's law). Bigger resistor, less current. In a single loop it works on either side of the LED.
Every LED circuit in every kit has a small striped part next to the LED. Leave it out and the LED works brilliantly, for about a second. What is it doing?
Current is measured in milliamps (mA). An LED is damaged above about 20 mA. Resistance is measured in ohms (Ω): the more ohms, the harder it is for current to flow.
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.
No resistor
TryTry it on the board and compare the result with your prediction.
Try: choose a resistor
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Choose no resistor. What does the board say?
Use the board. This ticks when you get there.
Try 47 Ω, 100 Ω, 220 Ω and 1 kΩ. Which is the smallest resistor that keeps the current under 20 mA (in ohms)?
InvestigateWork out why it happens, one step at a time.
Worked example: how much current?
Worked exampleBefore each step, say what you think comes next.
3.3−2=1.3 VPin 0 gives 3.3 V. The LED uses about 2 V, so 1.3 V is left across the resistor.
I=RV=2201.3Ohm's law: current = voltage ÷ resistance.
I=0.0059 AIn amps.
0.0059×1000=5.9 mATimes 1000 for milliamps.
5.9<20✓Under the LED's 20 mA: safe.
Now do the same for 470 Ω.
Now 470 Ω. How many milliamps? (1 decimal place)
Work backwards: what resistance gives exactly 20 mA (0.02 A)? (ohms)
Sam is sure a resistor after the LED won't protect it. Click the line where Sam's reasoning first goes wrong.
ModifyChange the problem on purpose and see what happens.
Vary: change one thing
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With 220 Ω, move the resistor after the LED. Does the current change?
Use the board. This ticks when you get there.
Now try 10 kΩ. Is the LED still lit?
Use the board. This ticks when you get there.
Modify TypicalChange the problem on purpose and see what happens.
Build it in Tinkercad
Tinkercad's LED and pin are not exactly the board's model, so expect close, not identical, numbers.
PractiseA few on your own, to check you have it.
Ohm's law
1 kΩ (1000 Ω): how many milliamps? (1 decimal place)
You want 10 mA (0.01 A). What resistance? (ohms)
MakeMake your own problem using what you have learned. Expect a few tries; that is normal.
Choose and place
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Need a hint?
Work out the resistance for 2 mA and for 10 mA: the answer is between them.
Another hint
1.3 ÷ 0.01 = 130 Ω and 1.3 ÷ 0.002 = 650 Ω.
Another hint
220 Ω or 470 Ω, then Move the resistor.
A battery-saving night light: between 2 mA and 10 mA, with the resistor after the LED.
Use the board. This ticks when you get there.
For teachers
Students take the resistor out of an LED circuit, compare resistor sizes, use Ohm's law with the LED's 2 V taken off to find the current and the smallest safe resistor, test that the resistor works on either side of the LED, and choose one for a target current. About 45 minutes.
I can explain what a resistor does in an LED circuit.
I can use Ohm's law to work out the current.
I can choose a resistor for a safe current.
Before class
Model: pin 3.3 V, LED forward voltage 2 V, LED limit 20 mA; with no resistor the board shows about 100 mA (a stand-in for "limited only by the pin and wires"). Real LEDs vary (red about 2 V, blue and white about 3 V), and a micro:bit pin should really be kept to a few milliamps, so kits use 220 Ω or more. Say that the numbers are a model.
Ohm's law may be new: this lesson applies it, Year 9 Science (electric circuits) teaches it fully.
Running the lesson
Predict: ask what limits the current when there is no resistor.
Worked example: write each line live. Pause on 3.3 − 2 and ask why.
Find the mistake: ask a student to trace the loop with a finger and say where the current is bigger.
Tinkercad: expect close, not identical, numbers; ask why a model might differ.
The hard part (on purpose)
Two steps trip students: taking the LED's 2 V off before dividing, and turning amps into milliamps. Model both slowly, out loud.
When students get stuck
A student divides 3.3 by the resistance. Ask: How much of the 3.3 V does the LED use? Fix: Use the 1.3 V left for the resistor.
A student's answer is 0.0059 for milliamps. Ask: Is that in amps or milliamps? Fix: Times 1000.
A student moves the resistor and expects a change. Ask: Is it still one loop? Fix: One loop, one current.
Stretch and support
Stretch: A blue LED drops about 3 V. With 220 Ω on a 3.3 V pin, how much current? Why might a blue LED look dim on a micro:bit?
Support: A card with the three steps: 3.3 − 2 = 1.3; 1.3 ÷ R; × 1000.
Where it leads
Next: A button is an input.
Science: Year 9 electric circuits (voltage, current, resistance). Engineering › Power and motors: why a pin can't drive a motor.
no resistor: Committing to what happens without the resistor tests the new move before it is taught: the resistor is what limits the current.
Try: choose a resistor: Removing the resistor settles the prediction; trying the sizes in turn shows the pattern: more ohms, less current.
Worked example: how much current?: Ohm's law with the LED's 2 V taken off models the move with numbers; the faded tasks apply it to 470 Ω, then reverse it to find the smallest safe resistor. Sam's 'resistor first' idea is planted after the method.
Vary: change one thing: Moving the resistor tests Sam's idea on the board; a very large resistor shows the other limit: too little current, and the LED barely glows.
Build it in Tinkercad: In Tinkercad the same circuit gives a warning without the resistor, and a multimeter shows the current, so students check the model against a fuller simulator.
Ohm's law: Ohm's law forwards and backwards, with the LED's 2 V taken off, and a review of the loop.
choose and place: Choosing a resistor for a target current and placing it after the LED applies both ideas from the lesson at once.