Everyone in the group hears

Classroom resources

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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.

Timing

Predict and Run 8 min · Investigate 10 min · Modify 8 min · Make 12 min

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

1. Predict: most say 3 or 4. Ask: does the sender hear itself? Is group 6 listening?
2. 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

Answers to the checks

1. Predict: Four micro:bits are switched on: three in group 5 and one in group 6. A is pressed on one micro:bit in group 5. How many micro:bits show MEET AT 3?
   2. The message reaches the other two micro:bits in group 5. The sender does not hear itself, and group 6 is a different channel.
2. The group number: Mia says: 'Group 99 keeps our messages private, because nobody knows the number.' Is she right?
   No: any micro:bit set to group 99 hears everything, and there are only 256 groups to try
   A group is like a radio station number. Anyone who tunes to it hears everything on it, and with only 256 groups, finding the busy one is quick.
3. The blocks: Match each block with what it does.
   radio set group 5 → C. choose the channel to talk on; radio send string → B. send text to every micro:bit in the group; on radio received string → A. run when text arrives from another micro:bit
4. Who hears it: Who hears a message sent from a micro:bit in group 5?
   Another micro:bit in group 5, across the room → Hears it; A micro:bit in group 6 → Does not hear it; A micro:bit in group 5 in the next classroom → Hears it; The micro:bit that sent it → Does not hear it

Solutions

One possible solution for each task. Other correct programs are fine.

Modify

radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
    radio.sendString("MEET AT 3")
})
input.onButtonPressed(Button.B, function () {
    radio.sendString("BRING SNACKS")
})
radio.onReceivedString(function (receivedString) {
    basic.showString(receivedString)
})

Make

radio.setGroup(5)
input.onButtonPressed(Button.A, function () {
    radio.sendString("RING")
})
radio.onReceivedString(function (receivedString) {
    if (receivedString == "RING") {
        basic.showIcon(IconNames.Heart)
        music.play(music.tonePlayable(659, music.beat(BeatFraction.Whole)), music.PlaybackMode.UntilDone)
        basic.clearScreen()
    }
})

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.

Exit ticket

Why does a radio group number not make a message private?

Every micro:bit set to that group hears every message in it, so the text is public to the group.

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.

Where it leads

Next: hide the message from anyone who is listening, starting with a simple scrambling method.