--- title: "A complete loop" subtitle: "Circuits with the micro:bit · Lighting an LED · Lesson 1" --- # A complete loop **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** You switch pin 0 on, but the LED stays dark. The code is right. What else has to be true for current to flow? :::teacher-note **In one sentence:** Students break and remake a loop of pin 0, a resistor, an LED and GND on a board, see that current stops everywhere when the loop breaks anywhere, find that an LED only works one way round, then build the same circuit in Tinkercad. **Students should be able to say:** I can explain why current needs a complete loop. I can wire an LED the right way round. I can find why an LED won't light. **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 **Pin 0 is on and the LED is lit. You unclip the wire from the LED to GND. The LED…** ☐ Gets dimmer ☐ Stays lit: pin 0 still sends power to it ☐ Goes out ☐ Gets brighter My prediction, and why: {{lines-3}} ::: :::teacher-note **What happens:** It goes out. With the GND wire off the loop is broken, so no current flows anywhere in it. ::: ## Run Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Worked example: follow the loop :::keep-together ### Trace it 1. $\text{pin 0 on: } 3.3\text{ V}$ — Code switches pin 0 on. It is 3.3 V above GND. 2. $\text{pin 0} \to \text{resistor} \to \text{LED}$ — Current flows out of pin 0 through the resistor and the LED. 3. $\text{LED} \to \text{GND } (0\text{ V})$ — It flows back into the micro:bit at GND. 4. $\text{one loop, one current}$ — The same current flows everywhere in the loop. 5. $\text{any break} \Rightarrow 0$ — Unclip any wire, or switch the pin to 0 V, and the current is 0 everywhere. Now apply it: where in the loop is the current smallest? **The loop is complete and pin 0 is on. Where is the current smallest?** ☐ In the GND wire, because the LED used it up ☐ In the resistor ☐ In the pin 0 wire, before it reaches the LED ☐ Nowhere: it is the same all the way round the loop ::: :::teacher-note **Answer:** Nowhere: it is the same all the way round the loop In a single loop the same current flows everywhere. Parts use energy (voltage), not current. ::: :::keep-together ### Find the mistake **Sam's LED never lights. Click the line where Sam's plan first goes wrong.** ``` 1 Sam wants to save a wire. 2 Pin 0 sends power out to the LED. 3 So one wire from pin 0 to the LED is enough. 4 The GND wire is not needed. ``` Line: {{field-15}} Why is it wrong? {{lines-2}} ::: :::teacher-note **Answer:** line 2 Line 2: pin 0 doesn't send power one way like a tap. Current flows round a loop, out of pin 0 and back into GND, so the GND wire is needed. ::: :::typical ## Vary: the LED the wrong way round **Turn it round.** **Check first.** Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: :::typical ## Build it in Tinkercad Your teacher will give you the class code. Long leg of the LED towards the resistor and pin 0. Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ::: ## Loops :::keep-together ### Which lights? **Which of these lights the LED?** ☐ Pin 0 on → resistor → LED (short leg first) → GND ☐ Pin 0 on → resistor → LED (long leg first) → GND ☐ Pin 0 on → resistor → LED (long leg first), with no wire to GND ☐ Pin 0 off → resistor → LED (long leg first) → GND ::: :::teacher-note **Answer:** Pin 0 on → resistor → LED (long leg first) → GND Only the first is a complete loop, the right way round, with the pin on. ::: :::keep-together ### A torch **A torch has a battery, a switch and a bulb. Turning the switch off…** ☐ Only stops current in the bulb ☐ Stops the battery making electricity ☐ Keeps the current in the battery until you need it ☐ Breaks the loop, so no current flows anywhere ::: :::teacher-note **Answer:** Breaks the loop, so no current flows anywhere A switch is a deliberate break in the loop. ::: ## Make: fix the circuit Work in the tutorial. Plan first: what will you change, and how will you test it? {{box-4}} ## Exit ticket :::keep-together **Pin 0 is on, every wire is clipped, and the LED is dark. Give two possible reasons.** {{lines-3}} ::: :::teacher-note **Answer:** The LED is the wrong way round, or a wire isn't really connected (the loop is broken). Also possible: the pin isn't actually on. :::