Circuits with the micro:bit

Current only flows around a complete loop: out of pin 0, through the resistor and the LED, and back to GND. Break the loop anywhere, or turn the LED round, and the current stops everywhere at once.

1. Unclip a wire
2. Try: break the loop › Unclip GND
3. Try: break the loop › Unclip pin 0
4. Try: break the loop › Switch the pin off
5. Worked example: follow the loop › Trace it
6. Worked example: follow the loop › Find the mistake
7. Vary: the LED the wrong way round › Turn it round
8. Vary: the LED the wrong way round › Check first
9. Practise: loops › Which lights?
10. Practise: loops › A torch
11. Fix the circuit

With no resistor, far too much current flows and the LED is damaged. A resistor limits the current: current = voltage ÷ resistance (Ohm's law). Bigger resistor, less current. In a single loop it works on either side of the LED.

1. No resistor
2. Try: choose a resistor › No resistor
3. Try: choose a resistor › Smallest safe resistor
4. Worked example: how much current? › Ohm's law with an LED
5. Worked example: how much current? › Work backwards
6. Worked example: how much current? › Find the mistake
7. Vary: change one thing › Move it
8. Vary: change one thing › Very big
9. Practise: Ohm's law › 1 kΩ
10. Practise: Ohm's law › Choose for 10 mA
11. Practise: Ohm's law › Review: dark LED
12. Choose and place

A pin set as an input reads the voltage on it: joined to 3V it reads 1, joined to GND it reads 0. A button joins pin 0 to 3V only while it is held, so a pull-down resistor holds the pin at 0 when it is up. Without one the pin floats and reads 0 or 1 at random.

1. Press it
2. Try: hold and let go › Hold
3. Try: hold and let go › Let go
4. Worked example: two paths to pin 0 › What is pin 0 joined to?
5. Worked example: two paths to pin 0 › Find the mistake
6. Vary: no pull-down › Float
7. Vary: no pull-down › Pull it down
8. Vary: no pull-down › A press counter
9. Practise: inputs › Pin to GND
10. Practise: inputs › A pull-up
11. The doorbell program

Analog read turns the voltage on a pin, 0 to 3.3 V, into a number from 0 to 1023. A knob or a light sensor in a divider changes that voltage, so the number changes. A program compares the number with a threshold to decide, as a night light does.

1. Halfway
2. Try: turn the knob › Halfway
3. Try: turn the knob › All the way
4. Worked example: number and volts › Halfway, in numbers
5. Worked example: number and volts › Number to volts
6. Worked example: number and volts › Find the mistake
7. Vary: a light sensor › Dark
8. Vary: a light sensor › The 300 mark
9. Vary: a light sensor › Swap them
10. Practise: readings › Volts to a reading
11. Practise: readings › A threshold
12. A night light