A design proposal for a motion alarm that guards a kid's backpack or locker, with a face, a voice and a big glowing off button.
The idea: a pastel pebble with a kawaii face clips onto a backpack or sits on a locker shelf. When a hand reaches in, a motion sensor wakes it, the eyes flash and it plays a ringtone until someone hits the big button. The kid picks the ringtone and the button's colour from a phone, and writes their name on the back.
The kid owns it. The teacher has to be able to switch it off.
It's designed to clip onto the backpack with a real carabiner, or lie flat on a locker shelf. They'd pick the ringtone and the button colour from a phone, then write their name on the back in marker so everyone knows whose guard this is.
A slide switch on the end cuts the power completely, so nobody has to pry out a battery. The big glowing button silences an alarm in one press, so anyone can see how to make it stop.
"Hands off my snacks!"A hand reaches into the backpack, the eyes start flashing, and Für Elise plays until someone hits the big button.
Try it here. Slide the power on, pick a ringtone and a button colour, then reach for the backpack. Tap the big button to silence it.
Tap the switch on the right end, or the big button.
It's switched off. Slide the power on.
These are the firmware's real sounds: Phil Burk's Mobileer ringtone engine, rendered on a Mac at 22.05 kHz with the same code and data the board plays. Through the real 20 mm speaker they'll sound thinner. The lights are drawn here and follow the sound's loudness, as the firmware does. The real quiet time after a silence is 60 s; here it's 5 s.
The enclosure, modelled in Solidcraft around the real circuit board inside. Drag to spin it.
One 70 × 45 mm two-layer board. The ESP32 sits on the underside so the top is free for the eye LEDs, and the motion sensor drops through a hole in the board so its dome clears the thin lid.
A right-angle slide switch in series with the battery, so off means off. Its lever comes out through the end wall.
An I2S class-D amp drives a 20 mm, 8 Ω micro speaker, so it can play real ringtones instead of piezo beeps.
Two white LEDs under the eye light pipes, switched together by one MOSFET. A pull-down keeps them dark at boot.
A 2 mm RGB LED under the clear button. A P-FET cuts its power while it sleeps, since a dark WS2812 still draws about 1 mA.
Hosts the setup page over its own Wi-Fi, plays the ringtones over I2S and sleeps between alarms, waking on the PIR or the button.
Bulk capacitance on the battery rail, asked for by the firmware, so Wi-Fi bursts and speaker peaks don't brown out the chip on old AAAs.
A PlatformIO project for this board. It builds cleanly, but it hasn't run on real hardware yet.
The hosts didn't like our first homemade synth, so the alarms are six ringtones they picked from Phil Burk's Mobileer set. They play on his General MIDI engine (Apache-2.0) at 22.05 kHz with 16 voices, and the four chirps (awake, silenced, saved, low battery) are tiny MIDI files on the same engine.
pio run builds with 0 warnings for the board and for a plain ESP32-C3 dev board stand-in. That's 43.5 % of the app slot and 15 % of RAM, with room to spare.
pins.h is generated from the KiCad design script before every build, and the build stops if a pull-up the code relies on disappears from the board.
Hold the button while switching on and it starts a BackpackDefender-XXXX hotspot with a captive page: ringtone tiles that preview on the device, colour swatches or Rainbow, brightness, volume, alarm length and quiet time.
Backpack Defender was the first product on Product Speedrun, a show where two hosts take a randomly rolled product idea from brainstorm to a showcase-ready concept.
Episode 001 ended with a first version, Rev A: concept art, a CAD model and a routed board. The hosts voted to keep building, and set the goals for the second version, Rev B (this page): slimmer and slicker, with a carabiner loop, a real off switch, a speaker instead of a beeper, eyes that flash and a name plate on the back. Rev B also got the first firmware.
Verdict: keep building. See the Rev A page
Nothing here has been built yet. The next step is a prototype: order the board, print the case, flash the firmware and work through the list below.
The design is complete on paper. Here's what we'd still answer before anyone orders a board or cuts a mould.
The board is DRC-clean and fits the case, and the firmware compiles, but nothing has been made or flashed. Still to check on the bench: waking from deep sleep on the PIR and button, the sleep current (the boost converter's own draw is the unknown), and the setup page on iPhone and Android.
The previews above come from a Mac. On the device, expect less bass and sparkle. The foam seal under the speaker is assumed, not modelled, and the engine's CPU use on the C3 is an estimate (20–35 % with all 16 voices) until the firmware logs the real number.
White LEDs need about 2.9 V and the board runs at 3.3 V, so there's little headroom and brightness will vary by batch. The LEDs and light pipes need a daylight test together.
The switch lever ends flush with the wall, in a 0.6 mm pocket. If it's too hard to slide, it needs a press-on cap.
Through clear plastic you can see the black switch and the board. Glitter hides them best; a dichroic finish would need a white disc underneath.
The lid and base aren't fastened yet, the battery door has no latch, and the wires to the PIR, speaker and cells need routing and strain relief.
The part numbers for the white LEDs, the 47 Ω resistor and the low-profile 12 mm switch need checking against real parts before ordering.
About 18 mm is possible with a flat LiPo and USB-C charging, but that gives up the AAA cells teachers can pull. The power switch now covers part of that.
Everything behind Backpack Defender Rev B is open: the 3D design, the KiCad boards, the firmware 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 GitHubRev A (the episode)The build tools
Enclosure, ready to edit or print
KiCad project, schematic and BOM. Saved with KiCad 10: open it in 10 or newer.
PlatformIO, first pass
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.
Hardware under CERN-OHL-P v2, words and images under CC BY 4.0, code under MIT.
A short diary of every episode: the roll, the turns the idea took, and the board at the end. One email per episode.