バックパック・ディフェンダー

Backpack Defender

A design proposal for a fun motion-activated alarm that guards a kid's backpack or locker.

76 × 51 × 26 mm, deck-of-cards size 2 × AAA, pull one to disarm 5 alarm voices ≈ $5.40 electronics

Hands off my snacks! ♡

Who it's for

For kids whose stuff keeps "wandering off"

School-age kids, roughly 6 to 12. It's a toy, not a security system: a cute guard with a loud, silly voice that makes a sneaky hand think twice, and makes the owner grin.

8:12 am · Locker 214

Lies flat on the shelf

It sits on its big flat side, face up. When the door swings open and a warm arm reaches in, the purple dome sees it and the alarm goes off. If it's the owner, one slap of the big button shuts it up.

12:30 pm · Lunch line

Clipped to a backpack

A charm loop on one end takes a keyring, a carabiner or a zip tie. The face looks out, and anyone unzipping the front pocket gets a robot shouting "INTRUDER" at them.

1:05 pm · Mid-lesson

Teachers can stop it in one move

The silence button is huge on purpose, so anyone can see how to stop it. For a full disarm, pop the battery door and take out a AAA. No app, no tools, no password.

Spin it around

The real enclosure

This is the Stage 4 CAD model, not a render of the art. It's a soft pebble in mint, butter and lavender, sized around real, cheap parts. Drag to rotate, pinch or scroll to zoom.

1Silence buttonA Ø24 mm coral dome, held captive by a flange, that presses a 12 mm tact switch. It's also the setup key.
2Motion "eye"The AM312 PIR's lavender Fresnel dome pokes 4 mm through the lid and looks straight up.
3Sound holesSeven Ø1.6 mm holes sit right over the piezo's port, with a three-slot grille on the front edge.
4Charm loopA tab with a Ø4 mm hole for a keyring, carabiner or zip tie.

76 × 51 × 26 mm2.0 mm wall11 parts

The face is flush inlays (plum eyes, pink cheeks) and could become a pad print or a sticker in production.

Pick its voice

Five alarm voices, picked from a phone

Hold the big button while you put the batteries in. Backpack Defender starts its own little Wi-Fi network with a settings page, so there's no app and no account. Pick a voice, hear a preview, save it.

Try it: play a voice on the phone, then hit the button. These previews come from your browser's synthesiser. On the real thing they play through a 12 mm piezo, which sounds buzzier.

📶 BackpackDefender-7F3A192.168.4.1

Hi! Pick my voice

Tap to preview · battery 87%

Mock-up of the setup page the device serves. Nothing here leaves your browser.

How it behaves

Asleep almost all the time, loud when it counts

The firmware plan from Stage 5. Deep sleep is what lets two AAA cells last most of a school year.

ARMED

Snoozing

The ESP32-C3 sleeps at about 5 µA. The PIR or the button can wake it, and Wi-Fi stays off.

ALARM

Squeals

Motion wakes it. It turns the amp on, streams the chosen voice for about 10 s, then goes back to sleep.

SILENCE

Shh

One press stops the sound and ignores motion for a cool-down (about 60 s) so the owner can get their stuff.

SETUP

Hello, phone

Hold the button at power-on to start the hotspot and settings page. The choice is saved to flash.

What's inside

One small board, two AAA cells

A 70 × 45 mm two-layer board sits on four posts above the batteries. All 43 parts are on top, because the battery holder is just 0.5 mm below the board. It's fully routed, with zero ERC errors and zero DRC violations in KiCad 10.

~90 µA

from the cells while armed, about 12–15 months of pure standby

6–9 mo

per set of AAAs, at ~20 alarms a day plus a weekly setup

≈ $5.40

electronics at 1k units, before assembly

components, 111 pins, all checked against their intended nets

The board in 3D

Stage 5 · KiCad export

How it stacks up

PIR dome and bottom plate~30 mm
Silence button top28.5
Mint lid12–26
Piezo, switch, moduleto ~23
PCB, 1.6 mm14.5
2 × AAA, side by side2–14
Butter base0–12

Schematic side view, not to exact scale for every part. Heights in mm above the floor.

Brain + Wi-Fi

ESP32-C3-WROOM-02

U2 · 18 × 20 × 3.2 mm

Hosts the hotspot settings page, plays tones over I2S, and wakes from deep sleep on a GPIO with no extra parts. It has native USB, so there's no serial chip.

~$1.90

Voice

MAX98357A + PS1240 piezo

U3 · BZ1

A filterless class-D amp takes the digital audio and drives a Ø12 mm TDK piezo, loudest near 4 kHz. Two 100 Ω resistors tame the edges.

~$1.45

Eye

AM312 mini PIR

U4 · Ø10 Fresnel lens

Near-field (3–5 m) with a digital output, drawing about 15 µA. An RC filter stops boost and Wi-Fi ripple from setting it off. Radar was rejected because it sees through locker walls.

~$0.80

Heart

TPS61023 boost

U1 · L1 1 µH

Two AAAs sag from 3.2 V to 2.0 V. The boost holds a steady 3.3 V and copes with 650 mA Wi-Fi peaks.

~$0.45

Kid-proofing

AO3401A guard FET

Q1 · SOT-23

Kids will put batteries in backwards. This one-cent P-FET shrugs it off and costs only ~40 mV of drop.

~$0.01

Big button

12 × 12 × 7.3 tact switch

SW1 · THT

Sits under the coral cap. A 0.4 mm pocket in the cap keeps it from pressing the switch all the time.

~$0.08

Full bill of materials (18 lines)

Ref	Qty	Value	Description	MPN
U2	1	ESP32-C3-WROOM-02-N4	ESP32-C3 Wi-Fi/BLE module, 4 MB flash, PCB antenna	ESP32-C3-WROOM-02-N4
U3	1	MAX98357A	I2S class-D mono amplifier, TQFN-16 3 × 3 mm	MAX98357AETE+T
U1	1	TPS61023	Synchronous boost converter, 0.5–5.5 V in, 3.7 A switch, SOT-563	TPS61023DRLR
U4	1	AM312	Mini PIR motion sensor, Fresnel dome through the lid	AM312
BZ1	1	PS1240P02BT	Piezo transducer, 12.2 mm, ~4 kHz resonance	PS1240P02BT
SW1	1	SILENCE	Tactile switch 12 × 12 × 7.3 mm THT, under the big button	TS-1212-7.3
Q1	1	AO3401A	P-MOSFET −30 V −4 A SOT-23, reverse-battery guard	AO3401A
L1	1	1 µH	Power inductor, 9 A sat, 7 × 6.6 mm	FXL0624-1R0-M
C1 C5 C8 C9	4	10 µF	Ceramic, 25 V X5R 0805	CL21A106KAYNNNE
C2 C3	2	22 µF	Ceramic, 25 V X5R 0805	CL21A226MAQNNNE
C4 C6 C10	3	100 nF	Ceramic, 50 V X7R 0603	CC0603KRX7R9BB104
C7	1	1 µF	Ceramic, 50 V X5R 0603	CL10A105KB8NNNC
R1 R3 R4	3	1 M	Resistor 1 % 0603	0603WAF1004T5E
R2	1	220 k	Resistor 1 % 0603	0603WAF2203T5E
R5–R8	4	10 k	Resistor 1 % 0603	0603WAF1002T5E
R9 R11	2	100 k	Resistor 1 % 0603	0603WAF1003T5E
R10 R12 R13	3	100 Ω	Resistor 1 % 0603	0603WAF1000T5E
BT1	1	2 × AAA holder	Side-by-side, wire leads (~52 × 25 × 12 mm), soldered to J1	generic

bom.csv schematic.png backpack-defender.kicad_pcb model.step model.glb

From doodle to board

How we got here in one episode

STAGE 2

Concept art

The vibe: a pastel pebble, a kawaii face, a playfully huge button.

STAGE 3

Layout sheet

Views, a 76 × 51 × 25 mm envelope, batteries under a board.

STAGE 4

Enclosure

Parametric CAD, 11 parts, clearance-checked with no overlaps.

STAGE 5

Electronics

KiCad schematic and routed board, fitted back into the shell.

What's next

What we still don't know

This is a first pass from a speedrun. It's credible and buildable, but nothing has been printed, ordered or heard by an actual kid yet. Here's the honest list.

test first

Does a piezo sound fun enough?

A 12 mm piezo is loud near 4 kHz, but "musical" and "goofy" will come out thin and buzzy. A 20–28 mm micro-speaker fits the same pads for the same cost. We need an A/B test with real kids' ears.

test first

Does the PIR behave in a metal locker?

The dome looks straight up. It should trip on a door opening and a warm arm, but not when the locker warms up. Firmware could require two triggers within a few seconds.

before ordering

Check pinouts and part numbers

Check the AM312 pin order (it varies by vendor), the MAX98357A TQFN pinout and every LCSC number. Clean up the autorouted boost loop by hand, and confirm the fab can drill the 0.2 mm thermal vias.

before ordering

Fix the switch height mismatch

The enclosure is sized for a 7.3 mm tact switch, but the KiCad 3D model is an 8.5 mm part. Order the right one, or the plunger will dig 1.2 mm into the button.

enclosure

How does it stay together?

The lid, base and bottom plate aren't fastened yet (snap lip or screws?), and the battery door has no latch. Both matter for something that lives on a backpack.

enclosure

Inlays, pad print or sticker?

The face is printed as flush colour inlays for the prototype. Pad printing or a sticker would be much cheaper at volume.

product

Does it need a power switch?

Pulling a battery disarms it, as the brief asked. A $0.05 slide switch on the end is easy to add if teachers want one.

product

AAA or LiPo, and how cheap can it get?

We chose AAA so teachers can disarm it and nothing needs charging. Swapping the amp for an NS4168 saves about $1. A chip-down ESP32-C3 saves more, but needs RF work.

Build on it

Everything behind Backpack Defender is open: the 3D design, the KiCad boards and the notes. Download it, change it, make it, even sell it. All we ask is a credit and a link back.

Open the project on GitHub Later revisions The build tools

3D model

Enclosure, ready to edit or print

model.stepopens in any CAD tool (FreeCAD, Fusion, Onshape…)
model.stlmesh for 3D printing
model.solidcraftthe editable Solidcraft design
build_model.pythe parametric script that builds it
NOTES.mddimensions, decisions and open questions

Download .zip Folder on GitHub

Circuit board

KiCad project, schematic and BOM. Saved with KiCad 10: open it in 10 or newer.

backpack-defender.kicad_proKiCad project
backpack-defender.kicad_schschematic
backpack-defender.kicad_pcbrouted 2-layer board
bom.csvparts list with manufacturer part numbers

Download .zip Folder on GitHub

Continue it with an AI agent

Paste this into Claude Code, Codex, Cursor or any agent that can run commands. It fetches just this project and the tools, reads the notes in the right order, and asks you before it changes anything.

I'd like to keep developing Backpack Defender, an open-source hardware concept from Product Speedrun episode 001 (https://donaldbell.github.io/productspeedrun/episodes/001-demo-1/6-showcase/). Its files are in the GitHub repository donaldbell/productspeedrun: the episode in episodes/001-demo-1/, and later revisions in products/backpack-defender/ (the latest is Rev B).

1. Get the files. First ask me whether to fork the repository on GitHub (so my work shows up as a fork and can go back as a pull request) or just clone it. Then fetch only this project and the shared tools:
   git clone --filter=blob:none --sparse <my fork's URL, or https://github.com/donaldbell/productspeedrun.git>
   cd productspeedrun
   git sparse-checkout set episodes/001-demo-1 products/backpack-defender tools kit

2. Read these, in order:
   - CLAUDE.md (how Product Speedrun works: the stages, the folder layout and the tools)
   - episodes/001-demo-1/episode.json (the brief: what it is, what it does, who it's for, what's inside, design language)
   - products/backpack-defender/rev-b/REV.md (what changed in this revision, and what's still open)
   - products/backpack-defender/rev-b/4-model/NOTES.md (the enclosure: dimensions and decisions)
   - products/backpack-defender/rev-b/5-electronics/NOTES.md (the electronics: parts, power, open questions)
   - products/backpack-defender/rev-b/FIT.md and fit.json (how the board sits in the enclosure, shared by both)
   - the concept art in episodes/001-demo-1/2-concept/ and the model screenshot products/backpack-defender/rev-b/4-model/iso.png

3. How it's built, so you can change it rather than start over:
   - The enclosure is products/backpack-defender/rev-b/4-model/build_model.py, a parametric script for Solidcraft (solidcraft-cli). If I don't have Solidcraft, work from model.step in a CAD tool I do have and tell me what can't be rebuilt.
   - Each circuit board is a KiCad project (saved with KiCad 10) generated from src/design.py (parts and nets) and src/layout.py (outline and placement) by the kit in tools/kicad/ (read tools/kicad/README.md: build.sh rebuilds the schematic, places, routes with Freerouting and runs DRC). Change the .py files and rebuild rather than editing the KiCad files by hand.
   - fit.json is shared by the enclosure and the board: change it there and both follow, then run the fit check in tools/fit/. Firmware is a PlatformIO project in products/backpack-defender/rev-b/7-firmware/.

4. Before changing anything, give me a short summary of the project and the open questions from the notes, and ask what I want to change. Work on a branch or in a new folder of my own, so the original stays as it aired.

5. Licences: hardware CERN-OHL-P v2, words and images CC BY 4.0, code MIT. Anything I make or publish from this should credit Product Speedrun episode 001 and link to https://donaldbell.github.io/productspeedrun/episodes/001-demo-1/6-showcase/.

6. When we're done, ask me how I'd like to share it: a pull request back to donaldbell/productspeedrun, a link to my fork, or keep it to myself.

Hardware under CERN-OHL-P v2, words and images under CC BY 4.0, code under MIT.

For kids whose stuff keeps "wandering off"

Lies flat on the shelf

Clipped to a backpack

Teachers can stop it in one move

The real enclosure

Five alarm voices, picked from a phone

Hi! Pick my voice

Asleep almost all the time, loud when it counts

Snoozing

Squeals

Shh

Hello, phone

One small board, two AAA cells

The board in 3D

How it stacks up

ESP32-C3-WROOM-02

MAX98357A + PS1240 piezo

AM312 mini PIR

TPS61023 boost

AO3401A guard FET

12 × 12 × 7.3 tact switch

How we got here in one episode

Concept art

Layout sheet

Enclosure

Electronics

What we still don't know

Does a piezo sound fun enough?

Does the PIR behave in a metal locker?

Check pinouts and part numbers

Fix the switch height mismatch

How does it stay together?

Inlays, pad print or sticker?

Does it need a power switch?

AAA or LiPo, and how cheap can it get?

Build on it

3D model

Circuit board

Continue it with an AI agent

Get each run by email