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.
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Students will be able to
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.
Timing
Predict and Try 8 min · Worked example 12 min · Vary 5 min · Tinkercad 10 min · Practise and Make 10 min
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
What you will see
A question to ask
The fix
A student divides 3.3 by the resistance.
How much of the 3.3 V does the LED use?
Use the 1.3 V left for the resistor.
A student's answer is 0.0059 for milliamps.
Is that in amps or milliamps?
Times 1000.
A student moves the resistor and expects a change.
Is it still one loop?
One loop, one current.
Answers to the checks
no resistor: You take the resistor out and join the LED straight to pin 0. What happens? Far too much current: about 100 mA, five times the LED's limit. The resistor is what keeps the current safe.
No resistor: Choose no resistor. What does the board say?
About 100 mA: far over 20 mA. Only the wires and the pin's own resistance are left to limit it.
Smallest safe resistor: Try 47 Ω, 100 Ω, 220 Ω and 1 kΩ. Which is the smallest resistor that keeps the current under 20 mA (in ohms)? 100 100 Ω gives 13 mA; 47 Ω gives about 28 mA, too much.
Ohm's law with an LED: Now 470 Ω. How many milliamps? (1 decimal place) 2.8 1.3 ÷ 470 = 0.00277 A ≈ 2.8 mA.
Work backwards: Work backwards: what resistance gives exactly 20 mA (0.02 A)? (ohms) 65 R = 1.3 ÷ 0.02 = 65 Ω. Anything bigger is safe; kits use 220 Ω to leave a margin.
Find the mistake: Sam is sure a resistor after the LED won't protect it. Click the line where Sam's reasoning first goes wrong. line 2 Line 2: in a single loop the current is the same everywhere. It doesn't arrive at the LED first and then slow down: the resistor limits the current in the whole loop, wherever it sits.
Move it: With 220 Ω, move the resistor after the LED. Does the current change?
No: 5.9 mA either way. One loop, one current.
Very big: Now try 10 kΩ. Is the LED still lit?
Only just: 0.13 mA. Too big a resistor is safe but too dim to be useful.
1 kΩ: 1 kΩ (1000 Ω): how many milliamps? (1 decimal place) 1.3 1.3 ÷ 1000 = 0.0013 A = 1.3 mA.
Choose for 10 mA: You want 10 mA (0.01 A). What resistance? (ohms) 130 1.3 ÷ 0.01 = 130 Ω.
Review: dark LED: Review: the LED is dark with a 220 Ω resistor in the loop. Which is NOT a likely cause? The resistor is too big 220 Ω still gives about 6 mA: plenty to see.
choose and place: A battery-saving night light: between 2 mA and 10 mA, with the resistor after the LED.
220 Ω (5.9 mA) or 470 Ω (2.8 mA), and either side of the LED works.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: change one thing › Move it
Open task: any program that does what the step asks.
Vary: change one thing › Very big
Open task: any program that does what the step asks.
choose and place
Open task: any program that does what the step asks.
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.
Exit ticket
With a 330 Ω resistor and a 2 V LED on a 3.3 V pin, how much current flows? Is it safe?
1.3 ÷ 330 ≈ 0.0039 A ≈ 3.9 mA: safe, under 20 mA.
What each step is for
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.
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.