Everyone in the group hears
A radio group is a channel, not a lock. Every micro:bit in the group hears every message, so anything sent in plain text is public to the group.
A school bus radio, a walkie-talkie, a baby monitor: if your radio is on the same channel, you hear it. That is how radio works, and it is why secret messages need more than a channel number.
Radio is how micro:bits talk without a cable. Each one is set to a radio group (a number from 0 to 255). A micro:bit sends to every other micro:bit in its group and to nobody outside it.
This course is about a question that follows: if a message goes out to everyone in the group, how can any of it be private? You will build the answers: secret codes, a way to share a key, a way to spot a changed message, and a way to prove who sent it.
- Predict: Read the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.
- Run: Run it and compare the result with your prediction.
- Investigate: Trace how each line of code connects to what you see.
- Modify: Change the code on purpose and check the result.
- Make: Build something new using what you have learned. Expect a few tries; that is normal.
Make your prediction first. Help opens once you've locked it in.
Predict
Show as code
radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
radio.sendString("MEET AT 3")
})
radio.onReceivedString(function (receivedString) {
basic.showString(receivedString)
})
Run
The simulator runs the program on two micro:bits. Press A on one and watch the other.
Show as code
radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
radio.sendString("MEET AT 3")
})
radio.onReceivedString(function (receivedString) {
basic.showString(receivedString)
})
Investigate
Show as code
radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
radio.sendString("MEET AT 3")
})
radio.onReceivedString(function (receivedString) {
basic.showString(receivedString)
})
Match each block with what it does.
Who hears a message sent from a micro:bit in group 5?
Modify
Make B send a second message of your own. Every micro:bit in the group would read it.
Need a hint?
Copy the A block, change it to Button B and change the text.
A sends one message and B sends a different one.
This ticks by itself when your blocks do what is asked.
Make
Make a doorbell. Pressing A on one micro:bit sends RING; the other shows an icon and plays a tone. You will meet this doorbell again: by the end of the course it will be a lock that is hard to fool.
Need a hint?
Pressing A sends the word RING.
Another hint
On radio received string: if the text is RING, show an icon and play a tone.
A doorbell: A sends RING; a micro:bit that receives RING shows an icon and plays a tone.
This ticks by itself when your blocks do what is asked.
For teachers
Students learn that a radio group is a shared channel, so plain-text radio messages are public to everyone in the group. About 40 minutes.
Open the teacher guide: answers, solutions, slides and a worksheet.
Students will be able to
- I can say who hears a message sent by radio.
- I can explain why a group number does not make a message private.
- I can send and receive a text message by radio.
Before class
The idea: radio goes to everyone. A radio group is a channel number; it decides who is listening, not who is allowed to listen. Anything sent in plain text can be read by any micro:bit set to that group.
Why start here: the whole course answers one question: if everyone in the group hears everything, how do we keep a message private, know it was not changed, and know who sent it? Students should leave this lesson feeling that question is real.
Hardware: the course works in the simulator throughout. Real micro:bits make it better: give every pair its own group and write the numbers on the board.
Running the lesson
- Predict: most say 3 or 4. Ask: does the sender hear itself? Is group 6 listening?
- Investigate: Mia's 'secret' group is the key misconception. Ask what a stranger would have to do to hear it.
The hard part (on purpose)
A group number is a channel, not a lock. Ask who else can hear the message before they assume it is private.
When students get stuck
- Nothing arrives. Ask: Are both micro:bits in the same group? Is the other one running the program? Fix: Use the same radio set group number on both.
- Messages from other pairs appear. Ask: Which group number are you using? Fix: Pick your own number and download again.
Stretch and support
Stretch: Add a third micro:bit in the same group and see that both hear A's message.
Support: Give the doorbell with the receiving part complete.
Where it leads
Next: hide the message from anyone who is listening, starting with a simple scrambling method.
What each step is for
- Investigate: A group number picks a channel. It does not hide anything: every micro:bit set to that group hears every message, in plain text.
- Modify: Adding a message is easy; so is anyone's reading it. Nothing in the program hides the text.
- Say who hears a message sent by radio.
- Explain why a group number does not make a message private.
- Send and receive a text message by radio.