Lesson at a glance
maze.education/microbit/lessons/fall-alarm
- Grades
- Grades 7 to 9
- Time
- 45 to 60 minutesThe CAPS gives Coding and Robotics 2 hours a week (Coding and Robotics CAPS, Grades 7 to 9, page 22).
- Level
- Confident codersFor confident classes: more steps to build, with variables, radio or data.
- Board
- Any micro:bit
- Code with
- MakeCode blocks, with Python as an extension
- CAPS
- Technology, Coding and Robotics, Natural SciencesFits: Grade 9, Term 2 · Where it fits
Learning intentions
Learners will learn to:
- Use the accelerometer's free fall gesture as an input.
- Design an alarm with OR logic: a fall OR a panic button sets it off.
- Send an alert by radio to a second board, and judge the system's strengths and weaknesses.
Success criteria
Each learner can say:
- I can use the free fall gesture as an input that sets off the alarm.
- I can complete the truth table for an OR alarm and point to the OR in my program.
- I can send an alert by radio to a second board, and test the system for false alarms.
- I can name a strength and a weakness of our alarm, with evidence from our tests.
- I can trace alarm and the radio messages through a test run.
Competencies
From the Coding and Robotics CAPS places this lesson cites.
- R.3Outline the different components of a robot
- R.5Design a simple artefact based on a set of design instructions
- R.6Mimic the operations of a robot
- D.7Present a basic understanding of the concept of input processing and output
Key words
- accelerometer
- A sensor that measures movement and tilt. The micro:bit uses it to recognise gestures.
- free fall
- Falling with nothing holding you up. The accelerometer reads close to zero force.
- OR logic
- The output is on if one input OR the other OR both are on.
- panic button
- A button someone presses to call for help.
- false alarm
- An alarm that goes off when nothing is wrong.
- truth table
- A table that shows the output for every combination of inputs.
- trace table
- A table that follows the value of each variable, step by step, as a program runs.
What you need
- Two micro:bits for each pair, with USB cables and battery holders
- A computer with Chrome or Edge for each pair
- A cushion, a pillow or a soft bag on the floor for each pair, for drop tests from no higher than a desk
From our shop: BBC micro:bit V2 Club Pack, BBC micro:bit V2.21 Next Gen Classroom Pack
Safety
- Drop the micro:bit only onto a cushion or a soft bag, from no higher than a desk. Never throw it, and never drop it with the USB cable attached.
- Keep the battery holder taped or held to the board for drop tests, so it cannot swing and pull on the wires.
- Keep the volume low and the micro:bit away from ears. Someone may be sensitive to sudden sounds, so warn the class before the first sound plays.
The lesson plan
One lesson of 45 to 60 minutes in three parts: introduction, lesson development and consolidation. The CAPS gives Coding and Robotics 2 hours a week, so it fits one of the week's lessons. Times are a guide.
Introduction
5 to 10 minutesRetrieval practiceHow do two micro:bits hear each other's radio messages and nobody else's? (Both use the same radio group number, and other pairs use different ones.)
Problem statementA gogo who is home alone while the family is at work or school may lie for hours after a fall before anyone knows. Design a prototype alarm that senses a fall or a panic button and alerts a carer's board across the room by radio, then test how far you would trust it.
- Ask: who in a community might need an alarm if they fell when nobody was home? Think of older people living alone and people with some medical conditions. Show a picture of a personal alarm pendant or a smartwatch with fall detection: what does it sense, and who does it tell?
- Read the problem statement. Explain that the class will build a prototype: a first version to test and learn from, not a product to trust with someone's safety.
Lesson development
35 to 40 minutes- Unplugged first. Draw the truth table for the alarm: two inputs, fall and panic button, each yes or no, and the output alarm. Fill in all four rows. Which row is a false alarm risk? Which row matters most?
- Plan on paper first. Give each pair a radio group number from 0 to 255. Pairs write the algorithm and fill in the IPO table, with a row for each input on either board. Then they complete the trace table for a test run.
- Predict. Ask: when the wearer's board falls onto the cushion, what will each board show? What happens if the wearer then presses B? What would an AND alarm do instead? Pairs write their predictions beside the trace table.
- Build in blocks. Pairs share one computer. The driver builds the alarm from the Blocks tab, with the pair's group number instead of 9; the navigator checks each block against the IPO table. Swap roles halfway, after step 4, so the new driver builds on radio received and the alarm loop. Download it to both boards: every board can be the wearer or the carer.
- Test and debug. Test the panic button first: A and B together start the alarm on the wearer's board and send HELP to the carer's board, and B stops it. Then test the fall: hold the board over the cushion, no higher than a desk, and let go. Compare each result with the trace table and fix the first place they differ.
- Test for false alarms: walk, sit down hard, jump, wave the board. Does anything set it off? Record every test in a table: what you did, what happened and whether it was a false alarm.
Plan on paper: a model answer
In pairs, write the alarm's algorithm and fill in the IPO table for both boards, then complete the trace table for a test run.
Algorithm
- Set the radio group to the pair's number, and set alarm to false.
- If a free fall is sensed, set alarm to true and send FALL by radio.
- If A and B are pressed together, set alarm to true and send HELP by radio.
- If B is pressed, set alarm to false and clear the screen.
- If a message arrives, scroll it and play ba ding.
- Over and over: if alarm is true, flash the cross and beep.
IPO table
Input Processing Output A free fall, sensed by the accelerometer Set alarm to true and send FALL by radio The wearer's board flashes a cross and beeps A and B pressed together (the panic button) Set alarm to true and send HELP by radio The wearer's board flashes a cross and beeps Button B is pressed Set alarm to false and clear the screen The flashing and beeping stop A message arrives by radio Keep it in receivedString and play ba ding in the background FALL or HELP scrolls on the carer's board, with a ba ding Trace table
A test run, with the wearer's board (W) and the carer's board (C) in the pair's radio group. Event alarm on W Sent by radio W shows C shows Start false Nothing Nothing Nothing The wearer walks and sits down false Nothing Nothing Nothing W falls onto the cushion true FALL A flashing cross, with beeps FALL, with a ba ding B pressed on W false Nothing Nothing Nothing new A and B pressed together on W true HELP A flashing cross, with beeps HELP, with a ba ding B pressed on W false Nothing Nothing Nothing new Consolidation
5 to 10 minutes- Ask: where is the OR in our program? What would an AND alarm do differently, and why would it be dangerous here?
- Ask: what are the weaknesses of our system? (The radio's range, flat batteries, a fall onto the floor that is not a free fall.)
- Back to the problem: would you trust this prototype to look after a gogo at home alone? What should a real product do after a false alarm, and who should be told? Learners check themselves against the success criteria.
Build it, step by step
Pairs build the program in blocks, following the numbered steps, with the same program as text beside them to check against or paste. Test in the simulator before sending it to a board.
These are the MakeCode steps. The other routes are on the lesson's web page.
Build it in MakeCode blocks
- Start a New Project called Fall alarm. Put radio set group 9 in on start, with your pair's number. In Variables, make alarm.
- From Input, add on shake and change shake to free fall. Inside: set alarm to true and radio send string "FALL".
- Add on button A+B pressed: set alarm to true and radio send string "HELP". This is the panic button.
- Add on button B pressed: set alarm to false and clear screen.
- From Radio, add on radio received receivedString: show string receivedString and play melody ba ding in the background.
- In forever, add if alarm then: show icon with the cross, play tone 880 Hz for 1/4 beat until done, clear screen and pause (ms) 200.
- Test the panic button in the simulator (it shows two boards). The simulator cannot drop a board, so test the fall on real micro:bits after you Download to both.
let alarm = false
input.onGesture(Gesture.FreeFall, function () {
alarm = true
radio.sendString("FALL")
})
input.onButtonPressed(Button.AB, function () {
alarm = true
radio.sendString("HELP")
})
input.onButtonPressed(Button.B, function () {
alarm = false
basic.clearScreen()
})
radio.onReceivedString(function (receivedString) {
basic.showString(receivedString)
music.play(music.builtInPlayableMelody(Melodies.BaDing), music.PlaybackMode.InBackground)
})
radio.setGroup(9)
basic.forever(function () {
if (alarm) {
basic.showIcon(IconNames.No)
music.play(music.tonePlayable(880, music.beat(BeatFraction.Quarter)), music.PlaybackMode.UntilDone)
basic.clearScreen()
basic.pause(200)
}
})To use the text instead: open MakeCode, start a new project, click JavaScript at the top, replace what is there with this code, then click Blocks to see it as blocks.
To send it to the board, click Download. In Chrome or Edge, MakeCode can connect to the micro:bit by USB so the program goes straight onto it. Otherwise a .hex file downloads: drag it onto the MICROBIT drive.
No Purple Mash route for this lesson
We could not find a Purple Mash unit or project that matches this lesson, so it is MakeCode blocks, with Python as an extension. These lessons have a Purple Mash route:
Extension: text-based coding, beyond the CAPS
The same program in MicroPython, for the micro:bit Python Editor. It suits Grade 7 to 9 learners who are ready for text-based code. The CAPS itself follows a block-based approach (Coding and Robotics CAPS, Grades 7 to 9, page 24).
# Fall alarm: a fall OR the panic button (A and B) sets off the alarm and tells the carer's board. B stops it.
from microbit import *
import radio
import music
radio.on()
radio.config(group=9)
alarm = False
while True:
if accelerometer.was_gesture("freefall"):
alarm = True
radio.send("FALL")
if button_a.is_pressed() and button_b.is_pressed():
alarm = True
radio.send("HELP")
sleep(500)
button_a.was_pressed()
button_b.was_pressed()
elif button_b.was_pressed():
alarm = False
display.clear()
message = radio.receive()
if message:
display.scroll(message)
music.play(music.BA_DING)
if alarm:
display.show(Image.NO)
music.pitch(880, 150)
display.clear()
sleep(100)After the panic button, the Python version clears the button presses, so the B that was part of A and B does not switch the alarm straight off again. Both boards need the same version.
Open the Python editor, replace the code that is there with this program (or open the downloaded file), then click Send to micro:bit. MakeCode also has a Python view, which is written a little differently.
Extension or homework
Challenges for learners who finish early, or to take further in a club or as homework.
- Challenge 1
Write a design brief for a real fall alarm: who it is for, where it is worn, how long the batteries must last and who it should alert.
- Challenge 2
Draw the system as a block diagram: inputs, process, outputs and the radio link between the two boards.
- Challenge 3
Measure the radio's range: how far apart can the boards be, and through how many walls, before the message stops arriving?
Support and extension
Support
For learners who need more help
- Build the panic button and the radio message first; add the free fall once those work.
- Give pairs the truth table half filled in, and the finished program with only the radio group to change.
Extension
For learners who are ready for more
- Cut false alarms: after a free fall, wait two seconds and only send FALL if the board is now still (use acceleration (mg) strength).
- Add a repeat: the wearer's board sends the message every five seconds until a carer presses A on their board to say help is coming, and that reply stops it.
Informal assessment
Ask these as learners work. Informal assessment happens every day and is not recorded.
- Name the two inputs that can start the alarm, and the logic that joins them.
- Fill in the truth table: no fall and no button gives what? A fall and no button gives what?
- What is a false alarm, and what in your tests caused one?
Give one reason our system would not be good enough for a real person yet.
Observation checklist
Watch pairs as they plan, build and test. Write each pair's initials under the level you see.
| Watch for | Beginning | Developing | Accomplished | Exemplary |
|---|---|---|---|---|
| Completes all four rows of the truth table, and says which row matters most. | ||||
| Tests the panic button and the fall separately, and records each result. | ||||
| Compares the results with the trace table and fixes the first difference. | ||||
| Judges the system's weaknesses with evidence from the tests. |
- Beginning:
- not yet, even with help
- Developing:
- with help
- Accomplished:
- on their own
- Exemplary:
- on their own, and explains it or takes it further
The four levels are the CAPS rubric levels (Coding and Robotics CAPS, Grades 7 to 9, page 163). The short descriptions are ours.
This lesson is not a formal task, but it is good practice for the mini-PATs in Grades 7 to 9 (Coding and Robotics CAPS, Grades 7 to 9, page 157).
Common mistakes and fixes
- The fall does not set off the alarm.Let the board fall freely, without throwing it or holding the cable. The drop must be long enough to sense: from desk height onto a cushion works.
- The alarm stops as soon as the panic button is pressed.Press A and B at exactly the same moment. In Python, keep the lines that clear the button presses after HELP is sent.
- The carer's board shows nothing.Check both boards share a radio group and have the new program. Test with the boards a few metres apart first: walls, doors and people shorten the radio's range indoors.
Where it sits in CAPS
- Coding and RoboticsFitsGrade 9, Term 2: use the micro:bit's accelerometer as an input.Source: Coding and Robotics CAPS, Grades 7 to 9, Grade 9, Term 2, Digital Concepts, D.7 Present a basic understanding of the concept of input processing and output, page 145:
Using the accelerometer to obtain readings
- Coding and RoboticsFitsGrade 9, Term 2: AND, OR and NOT logic in a system with sensors, with a truth table.Source: Coding and Robotics CAPS, Grades 7 to 9, Grade 9, Term 2, Robotics, R.5 Design a simple artefact based on a set of design instructions, with R.6, page 140:
Logical gates (AND, OR and NOT) and how it relates to the design, programming, and functioning of robotic systems.
- Coding and RoboticsFitsGrade 7, Term 3: sending a message from one micro:bit to another by radio, here the alert.Source: Coding and Robotics CAPS, Grades 7 to 9, Grade 7, Term 3, Robotics, R.3 Outline the different components of a robot, page 79:
The learners are introduced to the concept of message passing between two devices.
- TechnologyGrade 8 Technology: OR logic and where it is used, in the term that designs a panic button system.Source: CAPS Technology, Grades 7 to 9, Grade 8, Term 4: Electrical systems and control (Mini-PAT, logic gates):
Investigation: OR logic gate and simple cases where it is used.
- Natural SciencesFits Natural Sciences: forces and motion, and what happens to the forces we feel when something falls freely.
Coding and Robotics quotes come from the Department of Basic Education's final CAPS, with the page number of the 2024 printed edition (the Department's October 2025 library copies number their pages differently). Schools may teach a topic in another term: the CAPS leaves the order to each school's annual teaching plan.
micro:bit is a trade mark of the Micro:bit Educational Foundation, and BBC is a trade mark of the British Broadcasting Corporation. Maze Education is an independent reseller of genuine BBC micro:bit products. Our teacher training and our other products and services are our own, and are not endorsed by the Foundation or the BBC.