An open build, made with a kid

Two buttons.
No screen.

A small aluminium box for a child, and an iPhone app for the parent who lives somewhere else. Press Record, talk, press again, and the parent hears it. When an answer arrives, Play glows.

Build one Source on GitHub

Drawn to scale from the box as built. Record is dim blue: ready. Play breathes green: a message is waiting.

01 · What it is

Presence, not a phone call.

A call needs both people free at once, and a child on a video call often just goes quiet. A box that holds a voice until someone is ready to hear it fits how a child stays close to someone who isn't there.

For the child: the lights are the whole display

In the order a child meets them · then two that say “not now”

ReadyRecord · steady dim blue

RecordingRecord · red · mic on

Got itRecord · one green pulse

Message waitsPlay · breathing green

PlayingPlay · steady green

Not readyRecord · slow blue blink

Travel lockPlay · white blink

Box online · checked in 1 min ago

1:12 · from the box · 18:40

0:48 · sent → on the box → played 19:12

0:21 · from the box · 19:15

For the parent: an iPhone app

One conversation, both directions, the whole archive. Each message you send shows when it reached the box and when it was played.
The box screen shows signal, last check-in, quiet hours and volume, and explains any fault in words. The child never has to.
A push when the child sends something, and when the box hasn't checked in for hours.

No red light, no micThe mic is powered from the pin that lights Record. That's wiring, not software.

Nothing is ever lostAudio is on disk while the child talks, and is only deleted after the server confirms a checked copy.

Private by designEnd-to-end encrypted. No transcription, no analytics.

Built for a school bagIts own LTE, so no Wi-Fi to set up. Short bumps are ignored; a travel lock stops the rest.

02 · From the bench

The real thing, so far.

Built at a kitchen table over a few evenings in autumn 2026. Tap a photo to enlarge it.

The finished box standing on its edge on a table: two stainless buttons on the left, a speaker grille in the middle, a USB-C socket and a black antenna on top — Closed up, it stands like a little radio

Parts spread on a wooden table: the LTE modem board, a step drill, a header strip, a soldering iron, and the open aluminium box with the speaker inside — Day one: everything on the table

The aluminium lid with two stainless push buttons fitted and a drilled hexagonal speaker grille — Buttons fitted, grille drilled

The Pi Zero on the modem board with ribbon USB cables, beside the open box with the speaker on a foam block and the antenna pigtail — First fit: the Pi on the modem HAT

Close-up of the lid with 19 small holes drilled in a hexagon, with scribed centre lines — 19 holes, scribed and drilled

A busy workbench: the lid with buttons and loose wires, red pliers, and the open box with the speaker and Pi inside — A wiring evening

The back of a 16 mm button showing six tabs labelled C minus, R, G and B plus two gold switch tabs — A button from behind: four LED tabs, two switch tabs

Inside the box fully wired: coloured wires from the Pi to the amplifier and buttons, the speaker on the left — All wires on

Close-up of the small black amplifier board with purple, yellow and green signal wires and red and black power wires soldered on — The amp, soldered

The closed box seen from above with an R scratched next to the Record button — The scratched “R” marks Record

03 · Drawn to scale

Every hole, every wire.

Drawn from the build photos and hardware/cad/layout.json. The box is a 1590DD-size die-cast clone, 188 × 119 × 37.5 mm. Clones differ by a millimetre or two, so measure yours before drilling.

Two 16 mm button holes (step drill to 15, file to 16), a 2 mm mic hole and a 19-hole grille over the speaker.

Physical pin numbers, pin 1 at the SD-card end; dot colours are the wire colours used. Must match box/dadbox/hw/pi.py. 3.3 V logic, ≤16 mA per pin.

Pin map as a table

Pin	Signal	Goes to	Wire
2	5 V	Amp VIN	red
4 / 6	5 V / GND	Power in from the USB-C tail · HAT joined from below	red / black, 0.5 mm²
7	GPIO 4	Modem power key (HAT P4), from below	pin
8 / 10 / 14	UART	Debug probe (bench only)	probe cable
9	GND	Mic GND and L/R	black
11	GPIO 17	Record red LED → mic VDD from its tab	orange
12	GPIO 18 · BCLK	Amp BCLK → mic SCK	yellow
13 / 15	GPIO 27 / 22	Record green / blue	green / blue
16 / 18 / 22	GPIO 23 / 24 / 25	Play red / green / blue	orange / green / blue
20 / 30	GND	Play / Record: switch + C−	black
29 / 31	GPIO 5 / 6	Record / Play switch (pull-up, pressed = low)	white
35	GPIO 19 · LRCLK	Amp LRC → mic WS	purple
36	GPIO 16	Amp SD (the sound driver owns it)	grey
38	GPIO 20 · DIN	Mic data	brown
39	GND	Amp GND	black
40	GPIO 21 · DOUT	Amp DIN	green

04 · How it works

Box, server, phone.

Three pieces that follow one written contract, docs/PROTOCOL.md. Only the box and the phone hold the key.

Recording16 kHz audio goes to disk as the child talks. On stop: Opus at 16 kbps, encrypted, checked and saved. Only then the green pulse.

SendingResumable uploads in 32 KB pieces. Without a signal it waits, and the child never hears about a network.

ReceivingPlugged in, the box checks in every minute. Oldest message first, one per press of Play.

05 · Where it stands

Mostly working, not finished.

As of the last bench session, 2 October 2026. The details are in docs/BUILD-LOG.md.

SimulatorThe whole box service runs on a Mac, with virtual buttons and lights

ServerLive on Supabase; every protocol endpoint tested end to end

Power and Pi5 V on the header pins, no undervoltage; pulled plugs lose nothing

Buttons and lightsBoth switches and all six light channels checked

LTEA 3-minute note uploaded in 8.5 s on a CHF 6/month SIM

iPhone appConversation, recorder and Box screen

EnclosureLid drilled; buttons, grille and mic hole done

Tests70 automated tests on the box software

Speaker ampThe first one was damaged by a wiring slip; a new one is going in

MicrophoneWired, not yet tested

Push notificationsWaiting for an Apple push key

BatteryDesigned, deferred: the first box is mains only

WorksIn progressNot yet

06 · Parts

Off the shelf, in three rounds.

No 3D printing, no custom board. Buy one round, get it working, then buy the next. About CHF 395 for all three, tools included, from Swiss shops; any electronics shop has equivalents.

Round 1 · Bench

It talks on the table

Pi Zero 2 W kit
Two 16 mm RGB buttons
I2S mic, I2S amp, speaker
Debug probe, soldering iron

Round 2 · Link

It reaches the server alone

SIM7670G LTE HAT
SMA pigtail and antenna
Data SIM, ~CHF 6/month

Round 3 · Box

It survives a school bag

Die-cast aluminium box
Step drill and hand tools
USB-C panel socket
Endurance microSD, wire, foam

Later · Battery

About 2 days unplugged

UPS module, three 18650 cells
Designed, not built

Every part, with shop and price

Qty	Part	Why	Shop	CHF
Phase 1 · The bench
1	Raspberry Pi Zero 2 W starter kitboard, 16 GB microSD, OTG cable, header to solder	The computer	pi-shop.ch	42.90
2	16 mm stainless button, RGB ring, 5 Vcommon cathode, resistors built in, fine at 3.3 V	The whole UI	bastelgarage.ch	39.80
1	DFRobot I2S MEMS micMSM261S4030H0; alt: INMP441	Microphone	bastelgarage.ch	8.90
1–2	MAX98357A I2S amplifierget a spare	Drives the speaker	galaxus.ch	9.95
1	Seeed 5 W 4 Ω speaker, enclosed	Its own back volume	bastelgarage.ch	7.90
1	Raspberry Pi Debug Probe	Serial console	galaxus.ch	12.70
1	USB soldering iron, solder, flux · 5 V 2.5 A supply	Bench	any	~12
Phase 2 · The link
1	Waveshare SIM7670G LTE Cat-1 HATa USB network interface; antenna included	Its own internet	bastelgarage.ch	55.90
1	SMA bulkhead → U.FL pigtail	Antenna through metal	reichelt.com	7.32
1	LTE antenna, SMA	Aluminium blocks radio	reichelt.com	~9
1	Data SIMDigital Republic Flat 1: unlimited at 1 Mbit/s	Plenty for voice	digitalrepublic.ch	6/mo
Phase 3 · The box
1	1590DD-size die-cast aluminium enclosure188 × 119 × 37.5 mm; a clone, so measure it	Survives a school bag	any	~13
1	Step drill 5–23 mm, 2 / 2.7 mm drills, centre punch, deburrer, file	Every hole	hardware store	~32
1	Exsys EX-49222 USB-C panel socket, 30 cm tail	Power in	exsys.ch	16.90
1	Short micro-USB → USB-C lead	Pi to modem	galaxus.ch	~12
1	SanDisk High Endurance 32 GB	The card is the machine	brack.ch	22.95
1	Standoffs, VHB tape, EVA foam, tie mounts, silicone wire, heat-shrink, threadlocker, Kapton, feet	A harness that lasts	hardware store	~72
Optional · battery (designed, not built)
1	Waveshare UPS Module 3S + 3× protected 18650 + 12.6 V charger + barrel jack	~2 days unplugged	bastelgarage.ch	~84
Phases 1–3, tools included				~395

Also needed: a Mac with Xcode and a multimeter. Running costs: the SIM, a Supabase project (we run Pro, about USD 25/month) and an Apple Developer account (USD 99/year). Outside Switzerland, only the SIM changes: pick one on a network the SIM7670G supports.

07 · Build it

Step by step, with your kid.

Each stage ends with something that works. Tick steps as you go; the exact commands live in the repo files named in each stage.

0 of 0 steps donesaved in this browser

Try it with no hardware

An evening. Proves the idea and your setup.

⌄

Clone the repo and set up Python (the venv lives outside the repo).

git clone https://github.com/mararnx/DadBox.git && cd DadBox
python3 -m venv ~/.venvs/dadbox
~/.venvs/dadbox/bin/pip install cryptography requests pytest

Run the tests, start the virtual box, open http://127.0.0.1:8765 and press its buttons.

cd box
~/.venvs/dadbox/bin/python -m pytest -q
~/.venvs/dadbox/bin/python -m dadbox.sim

Record the very first message into the simulator and watch the lights change.

Read docs/BRIEF.md and skim docs/decisions/: each record explains why a part or rule is what it is.

The Pi on the bench

docs/DEV-PROCESS.md · box/setup/README.md

⌄

Flash Raspberry Pi OS Lite 64-bit with Raspberry Pi Imager: hostname dadbox, SSH key, home Wi-Fi. Before the first boot: remove resize from cmdline.txt; add growpart: {mode: "off"} and resize_rootfs: false to user-data. This keeps room for /data.

Only pins 4, 6 and 7 get header pins, soldered from below for the modem; every other lead is soldered straight into the Pi's holes later. Boot, then ssh dadbox.

Soldering is a great first skill: little shiny volcanoes. You hold the iron, they feed the solder.

Install packages and Tailscale; create /data (8 GiB root, the rest ext4 labelled data). Exact sfdisk lines are in box/setup/README.md.

sudo apt install ffmpeg alsa-utils python3-venv python3-lgpio i2c-tools
curl -fsSL https://tailscale.com/install.sh | sh

In /boot/firmware/config.txt set dtparam=audio=off and append:

dtoverlay=googlevoicehat-soundcard
dtparam=i2c_arm=on
dtparam=watchdog=on
enable_uart=1
dtoverlay=disable-bt
gpio=4=op,dl

Buttons, mic and amp

Pin map below · hardware/schematics/WIRING.md

⌄

Find the tabs: four silver for the LED (C−, R, G, B), two gold for the switch. Use the meter's beep: gold pair silent at rest, beeps when pressed.

"Find the two tabs that only beep when you press": a perfect detective job.

Wire both buttons alike: switch to a GPIO with the internal pull-up, LED channels straight to GPIO. One wire per header pin.

The privacy wire (see the drawing): mic VDD comes from the Record button's R tab, never from a 3.3 V pin.

A mic on a 3.3 V pin would be always-on and break the box's central promise.

Amp: VIN → pin 2, GND → 39, BCLK/LRC/DIN → 12/35/40, SD → 36. Chain the mic's clocks from the amp. GAIN unconnected.

Check the amp's ground wire is on GND, not the neighbouring GAIN pin. That exact slip cost us an amp.

Test each light channel with pinctrl, each button, then arecord a WAV and aplay it back.

The box software

box/README.md · box/DESIGN.md · box/deploy.sh

⌄

Create the service user, deploy with box/deploy.sh, install into a venv that sees apt's lgpio:

sudo useradd --system --home /data --shell /usr/sbin/nologin \
     --groups gpio,audio,i2c,dialout dadbox
python3 -m venv --system-site-packages /opt/dadbox/.venv
/opt/dadbox/.venv/bin/pip install -e .[pi]
sudo systemctl enable --now dadbox

Drive it: dadboxctl state, led test, record start, play. Then the real buttons. Record records, Play plays.

First real message: something for the other parent. It waits safely in the outbox.

Your own server

server/README.md · docs/SERVER-CONCEPT.md

⌄

Create your own Supabase project, link server/supabase, push the schema, deploy the one function:

cd server && npm test
npm run supabase -- db push
npm run supabase -- functions deploy api --use-api
npm run supabase -- secrets set TICK_SECRET=<random>

Then the two Vault lines from server/README.md.

Mint one token per identity (prints it once, stores only the hash):

python3 tools/mint_token.py box
python3 tools/mint_token.py parent-a

Make the family key: 32 random bytes. It goes on the box and in the phone, never on the server or in git. Keep a paper copy, because losing the key means losing the archive.

python3 -c "import secrets; print(secrets.token_hex(32))"

Exercise it with the fake box (tools/fakebox/.env.example → .env):

~/.venvs/dadbox/bin/python tools/fakebox/fakebox.py send --seconds 60 --drop-after 1
~/.venvs/dadbox/bin/python tools/fakebox/fakebox.py resume
~/.venvs/dadbox/bin/python tools/fakebox/fakebox.py --as parent-a send

On the box, /data/config.env (DADBOX_URL, DADBOX_TOKEN) and /data/keys/1.key, owned by dadbox with mode 0600. Over Wi-Fi it can already send.

LTE: its own internet

box/setup/README.md §6 · ADR 0026

⌄

HAT DIP switches: 1, 2 off; 3 (PWR) on; 4 off. SIM in before power. Antenna on: never transmit without one.

Join the HAT under the Pi by three pins only: 4 (5 V), 6 (GND), 7 (GPIO 4 → power key). Data over the short USB lead.

Once per modem on /dev/ttyACM0: your APN, AT+DIALMODE=0, AT$MYCONFIG="usbnetmode",0, AT+CRESET. Then check:

sudo python3 box/setup/modem_check.py --apn <your-apn>

Carry the box to the rooms it will live in and read signal with AT+CSQ.

Signal hunting: walk room to room calling out the number.

The parent's iPhone app

ios/README.md · ios/DESIGN.md

⌄

Test the protocol library, open the project, run DadBox in the simulator with -demo.

cd ios/DadBoxKit && swift test
open ios/DadBox.xcodeproj

Set your team and your own bundle identifier in Signing. On your iPhone, enter the setup code (fakebox.py --as parent-a setup-code) and the family key.

For push: an APNs key (.p8) from your Apple Developer account, given to the server as secrets. The .p8 never enters the repo.

Into the aluminium box

See the drawings · hardware/LAYOUT.md

⌄

Measure first: inside length, width, depth, the screw bosses, the stack height. Lay the parts in, close the lid, mark each hole over its part.

Drill the lid: two 16 mm button holes (step drill to 15, file to 16), a 2 mm mic hole, a 19-hole grille. Top edge: 22.3 mm USB-C, 6.5 mm antenna. Deburr everything.

The step drill in a clamped box at low speed, safety glasses on, your hands over theirs: the best job of the build.

Replace jumpers with soldered 26 AWG silicone wire and heat-shrink; tie down loops; foam block under the speaker; threadlocker on the nuts.

Power: the USB-C tail ends in two leads, red → pin 4, black → pin 6. The tail must carry the 5.1 kΩ CC resistors or a USB-C supply won't switch on.

No protection on this input. Meter the polarity, both plug orientations, before it touches the Pi. Never power the micro-USB port at the same time.

Seal it for the field

Working unattended, for a month

⌄

Enable the read-only root (raspi-config nonint enable_overlayfs), then set overlayroot=tmpfs:recurse=0 in cmdline.txt. findmnt /data must say ext4 rw.

Without recurse=0, /data lives in RAM too and recordings vanish at the next power cut.

Move to the High Endurance card; keep an image of the bench card. Pull the plug a few times and check every file in /data.

Set quiet hours and volume in the app. Hand it over.

Deciding where the box lives, and sending the first real message.

08 · Lessons

What went wrong, so you can skip it.

Power

USB-C couplers work one way up

A socket-to-socket coupler passes only one CC wire, so the plug worked one way round. Use a socket with a cable tail.

Audio

A wire on GAIN killed an amp

Its ground landed on the neighbouring pin; afterwards it scratched even on silence. Check grounds before power.

Linux

overlayroot puts /data in RAM

The stock read-only overlay covers every mount. Without recurse=0, the “safe” partition lives in memory.

Modem

It isn't who it says it is

The SIM7670G shows up as Qualcomm 05c6:9330, and at DIALMODE=1 forwards nothing. Use 0.

Protocol

Re-flashing reused a number

A fresh card restarted message numbering and the upload stuck. The server now tells the box where to continue.

Boot

Hold the power key low

GPIO 4 floats high at boot, and a long press turns the modem off. gpio=4=op,dl holds it low.

09 · The source

All of it, open.

DadBox/
├── docs/          brief · architecture · protocol · build log
│   └── decisions/ why each choice was made
├── hardware/      parts · sourcing · layout
│   ├── schematics/ wiring
│   └── cad/       layout and 1:1 drill templates
├── box/           Python service and simulator
├── server/        Supabase: SQL and one Edge Function
├── ios/           SwiftUI app
├── tools/         fake box, token minting, serial
└── site/          this page

Read in this order

docs/BRIEF.md — what and why
docs/PROTOCOL.md — the contract
hardware/schematics/WIRING.md
box/setup/README.md — the Pi, exactly
docs/BUILD-LOG.md — what actually happened

github.com/mararnx/DadBox

Built with Claude Code as a co-builder: it drives the box over SSH and asks a person only for what it can't do itself, like listening, looking at an LED or reading a meter.