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# Outputs that vary
Arduino · Writing C++
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## Today
**We are learning to** analogWrite(9, value) sets a brightness from 0 to 255 on a PWM pin (marked ~ on the Uno: 3, 5, 6, 9, 10, 11). The pin doesn't get a lower voltage: it switches fully on and off very fast, and the value sets how much of the time it is on. analogRead gives 0 to 1023, so divide by 4 to drive it from a knob.
- 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.
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## Why it matters
> A digital pin is only ever 0 V or 5 V. So how does a dimmer switch, a fading LED or a motor speed control work from one?
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💬 Discussion
## Predict
**analogWrite(9, 128); — what does pin 9 do?**
- Switches between 5 V and 0 V very fast, on about half the time
- Switches off
- Stays at 5 V
- Stays at 2.5 V
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✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Farduino%2Fpwm%2Fstep-2&code=widget%3A+arduino%0Aled%3A+9%0Awave%3A+on%0Aedit%3A+off%0Acode%3A+void+setup%28%29+%7B%0Acode%3A+%7D%0Acode%3A%0Acode%3A+void+loop%28%29+%7B%0Acode%3A+analogWrite%289%2C+128%29%3B%0Acode%3A+delay%28100%29%3B%0Acode%3A+%7D%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/arduino/pwm/?full=step-2#step-2)
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✅ Check
## What happened?
* 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.
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💬 Discussion
## Scale it
Now the top of the knob.
$$\text{analogRead(A0)} \to 0 \ldots 1023$$
1) $\text{analogWrite} \leftarrow 0 \ldots 255$ — Brightness: 0 off, 255 fully on.
2) $1024 \div 256 = 4$ — The input range is 4 times the output range.
3) $512 \div 4 = 128$ — Halfway on the knob gives 128.
4) $128 \div 255 \approx 0.5$ — On for about half of each cycle.
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✅ Check
## Scale it
**The knob reads 1023. What does analogWrite get after level / 4?**
* Answer: 255
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✅ Check
## On how long?
**analogWrite(9, 64). For how much of each cycle is the pin on?**
- 64 cycles out of 100
- About half
- All of it, at a quarter of the voltage
- About a quarter
* Answer: About a quarter
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✅ Check
## 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.**
```
1 int level = 0;
2
3 void setup() {
4 }
5
6 void loop() {
7 level = analogRead(A0);
8 analogWrite(9, level);
9 delay(50);
10 }
```
* Answer: line 8
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✍️ Activity
## Vary: change one thing
- **Smooth:**
- **Not a PWM pin:**
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/arduino/pwm/?full=step-4#step-4)
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✍️ Activity
## Build it in Tinkercad
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✅ Check
## 75%
**You want the LED on for 75% of each cycle. What value? (whole number)**
* Answer: 191
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✅ Check
## Which pin
**Which Uno pin can fade an LED with analogWrite?**
- 2
- 7
- 10
- 12
* Answer: 10
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✍️ Activity
## Make: turn it round
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/arduino/pwm/?full=step-7#step-7)
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✅ Check
## 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.