--- title: "A button is an input" subtitle: "Circuits with the micro:bit · Reading inputs · Lesson 3" --- # A button is an input **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** The micro:bit has buttons A and B built in. Wire your own button to a pin, and it can be a doorbell, a game controller or a sensor on a door. How does a pin know it has been pressed? :::teacher-note **In one sentence:** Students hold and release a button wired from 3V to pin 0, see digital read give 1 and 0, trace what pin 0 is joined to in each state, remove the pull-down resistor to see the pin float, and put the reading-and-reacting program in order. **Students should be able to say:** I can explain what digital read gives for 3.3 V and 0 V. I can explain why a button needs a pull-down (or pull-up) resistor. I can put a program in order that reacts to a button. **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 **The button joins 3V to pin 0 while you hold it. While you hold it, digital read pin P0 gives…** ☐ 0 ☐ 1 ☐ 3.3 ☐ 1023 My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** 1. Holding the button joins pin 0 to 3.3 V, and digital read reports that as 1. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: two paths to pin 0 :::keep-together ### What is pin 0 joined to? 1. $\text{held: } 3\text{V} \to \text{button} \to \text{pin 0}$ — The button joins pin 0 straight to 3.3 V. 2. $\text{held: reads } 1$ — 3.3 V reads as 1. 3. $\text{up: pin 0} \to 10\text{ k}\Omega \to \text{GND}$ — The button is open; the pull-down joins pin 0 to GND. 4. $\text{up: reads } 0$ — 0 V reads as 0. Now: while the button is held, pin 0 is joined to both. Why does 3V win? **While the button is held, pin 0 is joined to 3V through the button and to GND through the 10 kΩ pull-down. Why does it read 1?** ☐ Pin 0 adds the two voltages ☐ The button joins it straight to 3V; the 10 kΩ only lets a tiny current leak to GND ☐ It doesn't: it reads 0 ☐ The pull-down switches off when the button is pressed ::: :::teacher-note **Answer:** The button joins it straight to 3V; the 10 kΩ only lets a tiny current leak to GND The button is a direct join to 3.3 V. The big resistor to GND only lets about 0.3 mA through, so pin 0 stays at 3.3 V. ::: :::keep-together ### Find the mistake **Sam's doorbell sometimes rings when nobody has pressed it. Click the line where Sam's reasoning first goes wrong.** ``` 1 When the button is up, pin 0 is joined to nothing. 2 Joined to nothing means 0 volts. 3 So pin 0 reads 0. 4 So the pull-down resistor is not needed. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 2 Line 2: a pin joined to nothing isn't at 0 V. It floats, picking up electrical noise from nearby wires and your hand, and reads 0 or 1 at random. That is the doorbell ringing by itself. ::: :::typical ## Vary: no pull-down **Float.** **Pull it down.** **A press counter.** 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}} ::: ## Inputs :::keep-together ### Pin to GND **Pin 1 is joined straight to GND. Digital read pin P1 gives…** ☐ 1023 ☐ 0 ☐ 1 ☐ It changes at random ::: :::teacher-note **Answer:** 0 Joined to 0 V, it reads 0. ::: :::keep-together ### A pull-up **Another way to wire it: the button joins pin 0 to GND, and a 10 kΩ pull-up joins pin 0 to 3V. What does pin 0 read while the button is held?** ☐ 0 ☐ It floats ☐ 0.5 ☐ 1 ::: :::teacher-note **Answer:** 0 Held: joined to GND, reads 0. Up: the pull-up holds it at 1. The mirror image of a pull-down, and how many boards wire their buttons (Arduino's INPUT_PULLUP). ::: ## Make: the doorbell program Then build it in Tinkercad, or on a real micro:bit with a button and a resistor on a breadboard. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Why does a button wired from 3V to a pin need a resistor from the pin to GND?** {{lines-3}} ::: :::teacher-note **Answer:** So the pin is held at 0 V (reads 0) when the button is up. Without it the pin floats and reads 0 or 1 at random. :::