Students see analogWrite(9, 128) as a fast on-off signal rather than a lower voltage, scale a knob's 0–1023 down to 0–255, meet the real board's behaviour when they don't (it wraps round), find that only the ~ pins can do PWM, and turn the mapping round.
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Students will be able to
I can explain what analogWrite does to a pin.
I can scale a reading from 0–1023 to 0–255.
I can say which Uno pins can do PWM.
Timing
Predict and Try 6 min · Worked example 12 min · Vary 8 min · Tinkercad 9 min · Practise and Make 10 min
Before class
Model: PWM pins 3, 5, 6, 9, 10, 11; analogWrite keeps the lowest 8 bits (so 300 acts like 44), as the Uno's timers do; on other pins it sets HIGH for 128 and above. The signal is drawn as six cycles with the on part as wide as value/255. Real PWM on pin 9 runs at about 490 Hz.
The same idea controls motor speed (Power and motors) and dims LEDs in screens.
Running the lesson
Predict: ask what an LED would look like at 2.5 V (it may not light at all), versus flickering faster than the eye can see.
Find the mistake: turn the knob slowly so students see the four ramps.
Tinkercad: the oscilloscope is the best moment of the lesson.
The hard part (on purpose)
'Half brightness means half voltage' is intuitive and wrong. Keep the drawn signal in view: always full height, only the width changes.
When students get stuck
What you will see
A question to ask
The fix
A student uses analogWrite on pin 8.
Is there a ~ next to 8?
Use 3, 5, 6, 9, 10 or 11.
Brightness jumps as the knob turns.
What is the biggest number analogWrite takes?
level / 4.
Answers to the checks
128: analogWrite(9, 128); — what does pin 9 do? It switches fully on and off, about 490 times a second, on for 128/255 of each cycle. Your eye averages it, so the LED looks about half as bright.
Try: see the signal: Upload and run. Look at the signal drawn beside the LED.
The signal is either at the top (5 V) or the bottom (0 V); it is at the top for half of each cycle.
Scale it: The knob reads 1023. What does analogWrite get after level / 4? 255 1023 / 4 = 255 in whole numbers: fully on.
On how long?: analogWrite(9, 64). For how much of each cycle is the pin on? About a quarter 64 ÷ 255 ≈ 0.25: on a quarter of the time, at the full 5 V.
Find the mistake: As Sam turns the knob up, the LED gets brighter, then suddenly dark, then brighter again, four times over. Click the line where Sam's sketch goes wrong. line 8 Line 8: analogRead gives 0–1023 but analogWrite takes 0–255. The board keeps only the lowest 8 bits, so 256 acts like 0 and it starts again. Use level / 4.
Smooth: Fix the sketch so the knob sets the brightness smoothly, all the way to fully on.
analogWrite(9, level / 4): 0–1023 becomes 0–255.
Not a PWM pin: This LED is on pin 8 (no ~). Upload: what does analogWrite(8, 128) do? Then try 100.
Pin 8 can't do PWM: analogWrite just sets it HIGH for 128 or more and LOW below. 100 is off.
75%: You want the LED on for 75% of each cycle. What value? (whole number) 191 0.75 × 255 ≈ 191.
Which pin: Which Uno pin can fade an LED with analogWrite? 10 PWM pins are 3, 5, 6, 9, 10 and 11.
turn it round: Change it so the LED is brightest with the knob at 0 and off at the top. Test it at a few positions.
255 - level / 4 turns the scale round.
Solutions
One possible solution for each task. Other correct programs are fine.
Vary: change one thing › Smooth
Open task: any program that does what the step asks.
Vary: change one thing › Not a PWM pin
Open task: any program that does what the step asks.
turn it round
Open task: any program that does what the step asks.
Stretch and support
Stretch: Fade in and out by itself with a for loop: for (int b = 0; b <= 255; b++) { analogWrite(9, b); delay(5); }
Support: Draw three signals for 64, 128 and 191 and shade the on parts.
Exit ticket
Why does analogWrite(9, 128) make an LED look half as bright, if the pin is only ever 0 V or 5 V?
The pin is on for half of each cycle and switches too fast to see, so the eye sees the average: half as bright.
What each step is for
128: Committing to what analogWrite(9, 128) does to the pin tests the new move before the signal is shown: on and off fast, not a lower voltage.
Try: see the signal: The drawn signal settles the prediction: full height, half the time.
Worked example: knob to brightness: Scaling 0–1023 down to 0–255 models the move with numbers; Sam's missing ÷ 4 is planted after the method, with what the real board does (it keeps only the low 8 bits).
Vary: change one thing: Fixing Sam's range, then moving to a non-PWM pin, shows both limits of analogWrite on the board.
Build it in Tinkercad: Tinkercad's oscilloscope shows the real PWM signal, so students see the same square wave on a fuller simulator.
PWM: Values and fractions both ways, and which pins can do it.
turn it round: Reversing the mapping (brightest at the bottom of the knob) makes students reason about the calculation rather than copy it.
Where it leads
Next: Real hardware (breadboard and upload), then Power and motors (the same PWM sets motor speed through a driver).