--- title: "Outputs that vary" subtitle: "Arduino · Writing C++ · Lesson 5" --- # Outputs that vary **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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? :::teacher-note **In one sentence:** 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. **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (predict, run, investigate and make). Typical adds the modify practice; wider adds the real-world contexts. They add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Predict :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 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. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: knob to brightness :::keep-together ### Scale it 1. $\text{analogRead(A0)} \to 0 \ldots 1023$ — The knob, as before. 2. $\text{analogWrite} \leftarrow 0 \ldots 255$ — Brightness: 0 off, 255 fully on. 3. $1024 \div 256 = 4$ — The input range is 4 times the output range. 4. $512 \div 4 = 128$ — Halfway on the knob gives 128. 5. $128 \div 255 \approx 0.5$ — On for about half of each cycle. Now the top of the knob. **The knob reads 1023. What does analogWrite get after level / 4?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 255 1023 / 4 = 255 in whole numbers: fully on. ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** About a quarter 64 ÷ 255 ≈ 0.25: on a quarter of the time, at the full 5 V. ::: :::keep-together ### 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 } ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** 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. ::: :::typical ## Vary: change one thing **Smooth.** **Not a PWM pin.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: :::typical ## Build it in Tinkercad Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## PWM :::keep-together ### 75% **You want the LED on for 75% of each cycle. What value? (whole number)** Answer: {{field-15}} ::: :::teacher-note **Answer:** 191 0.75 × 255 ≈ 191. ::: :::keep-together ### Which pin **Which Uno pin can fade an LED with analogWrite?** ☐ 2 ☐ 7 ☐ 10 ☐ 12 ::: :::teacher-note **Answer:** 10 PWM pins are 3, 5, 6, 9, 10 and 11. ::: ## Make: turn it round Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Why does analogWrite(9, 128) make an LED look half as bright, if the pin is only ever 0 V or 5 V?** {{lines-3}} ::: :::teacher-note **Answer:** The pin is on for half of each cycle and switches too fast to see, so the eye sees the average: half as bright. :::