A design proposal for a baby monitor accessory. You feel the crying instead of hearing it, so you can sleep with earplugs in.
The idea: a pearl-white sensor sits next to your monitor and listens to its speaker. When the baby whimpers, the watch on your wrist glows. When the baby cries, it buzzes, and buzzes harder the longer nobody gets up. To tell it you're on your way, tap the glowing bar on top of the sensor, like the snooze bar on an alarm clock.
The one on duty wears it. The one off duty gets to sleep.
The monitor's hiss, a snoring partner or the street outside keeps them awake, but earplugs feel like neglect. With the watch on, they could block out the room and still be the first to know when the baby actually needs them. A whimper would only make the watch glow. Crying would buzz it, harder the longer nobody gets up.
A small button is hard to find at 3 am. So the design gives the sensor a snooze bar, the one thing on an alarm clock you can hit without opening your eyes. It's 30 mm long, works wherever you press it, and always glows faintly so you can see where it is. Tap it and the watch goes quiet.
"Bzz-bzz-bzz. … BZZZ."3:12 am. Three firm pulses, then a full buzz. A hand finds the amber glow on the nightstand, taps the bar, and the watch flashes green and goes quiet.
Try it here. This runs the devices' real firmware rules, ported line for line to your browser. Pick what the monitor is playing and watch the sensor sort it into quiet, fussing, crying or screaming. Then tap the snooze bar on the sensor to stop the watch, or hold it for 3 seconds to send a test buzz.
Last 20 s on screen. Line: what the sensor hears (dB SPL). Dashed: its noise floor. Faint lines: the fussing, crying and screaming thresholds. Bottom strip: the watch motor.
All quiet
This is a simulation in your browser, not the device, and the sounds are made up. The thresholds, attack and release, 45 s escalation, 3 min snooze and the buzz patterns are the firmware's real numbers, all still waiting for bench tuning. The light colours are brightened for the screen; the real bar's "armed" glow is a dim night-light. The phone in your hand doesn't buzz: there's no ERM in it. The white-noise machine shows a bug we found while building this page, now fixed in the firmware: switch it on, then let 20 s pass (see what we still don't know).
Both devices, modelled from the concept art and built around the real circuit boards inside. Drag to spin it.
Each device has its own routed two-layer board with an ESP32-C6 module, and the two talk directly over ESP-NOW, with no Wi-Fi network in between.
A long bar would rock on one switch, so two switches 12 mm apart sit in parallel on the same pin. A press at either end registers, and either one wakes the chip.
A 2 mm RGB LED between the switches, under the bar's light leg. It runs from the power switch's output through a Schottky diode, so 3.3 V data still reads high, and it draws nothing when the sensor is off.
ESP-NOW links the two ESP32s directly, with no Wi-Fi network, no app and no account. The sensor decides everything and the watch just follows, so the two can never disagree about whether to buzz.
A digital I2S MEMS mic behind the grille. The firmware filters it to roughly the band a cry occupies through a small speaker (~330 Hz to 3.7 kHz) and measures the level every 250 ms.
A Ø10 mm coin motor on a 20 kHz PWM. The firmware divides by the battery voltage, so a buzz feels the same on a full cell as on a tired one and the 3 V motor is never over-driven.
USB-C runs the sensor beside the monitor and charges a 2000 mAh cell for nights away. The hosts kept the long cell on purpose: about three nights unplugged is a bonus for parents.
One PlatformIO project per device, sharing a radio protocol. Both build cleanly, but neither has run on real hardware yet.
The sensor learns the room's noise floor and needs sound well above it before it calls fussing, crying or screaming. It needs 1 s of sound within 2 s, so a door slam doesn't count, and it steps down one level per 6 s, so the breaths between cries don't end an alert. A sound that stays within 4 dB for 20 s, like a white-noise machine, becomes the new background.
Fussing only glows soft pink (the hosts' call). Crying gives three firm pulses every 2.5 s. Screaming gives an almost continuous full buzz. A crying alert nobody answers steps up to screaming after 45 s. A tap on the bar stops it for 3 min, unless the crying gets louder.
Not Arduino: the pinned platform's Arduino core can't target the ESP32-C6. Each device builds to ~815 KB of a 1.875 MB slot, with room for over-the-air updates later. A host test runs the real detector and buzz code on a computer (26 checks pass).
pins.h is generated from each board's KiCad design script before every build, and the build stops if a net or pull-up the code relies on disappears.
Baby Monitor Monitor started as a random roll on Product Speedrun, a show where two hosts take a randomly rolled product idea from brainstorm to a showcase-ready concept. Episode 003 rolled "haptic motor, new parent, wearable, Y2K", and Jordan Bunker and Donald Bell talked it into a monitor for the baby monitor.
The episode ended with a first version, Rev A: concept art, a CAD model and two routed boards. The hosts voted to keep building, and set one goal for the second version, Rev B (this page): make the enclosures match the concept art, with its curves, asymmetry and details. Along the way, the sensor's small button became the snooze bar, and both devices got their first firmware.
Verdict: keep building. See the Rev A page
Nothing has been made yet. The next step is two boards on the bench, the firmware flashed and a real monitor playing real crying. On the CAD side, a second pass with a steeper fall to the back is written but not built.
The design is complete on paper. Here's what we'd still answer before anyone orders a board or cuts a mould.
Both boards are DRC-clean and fit their cases, and the firmware compiles, but nothing has been made or flashed. Every sound threshold and buzz strength is a first guess, waiting for a real board, a real monitor and recorded crying.
While building the demo above we found that a white-noise machine switched on after the sensor was never learned as background: the watch glowed or buzzed all night. That's fixed. A sound that stays within 4 dB for 20 s now becomes the background, up to 62 dB, so an unbroken scream never does. It rests on crying through a monitor swinging 10–20 dB with breaths and sobs, which still needs checking against recordings of real crying. It has two costs: when a machine switches on, the watch alerts for about 20 s before the sensor learns it, and over the noise, crying has to be 10 dB louder to glow and 18 dB louder to buzz.
The watch listens 50 ms in every 200 ms, so the estimate is ~11 mA, or about 13 h. That's untested. The glow LED also draws up to 1 mA when dark. A switch to cut its power is posted as a request for the board, along with a charger sense, so a full watch on its clip stops buzzing "link lost".
The sensor reads longer and more tube-like than the concept, because its 50 mm cell sets the length (the hosts chose to keep it), and the inset back panel is too big. A second pass with a steeper fall to the back is written but not built: it ran over an hour without finishing.
The concept's watch is about 10 mm thick. Ours is set by the cell and board stack. A 1.0 mm board and a 3 mm cell would win back about 1.5 mm.
The status bean moved to the cheek, but its LED didn't move on the board, so the light travels about 22 mm to get there. If it looks dim on a real part, the LED moves toward the front of the board.
ESP-NOW packets aren't encrypted, so a nearby device could pretend to be the sensor and say "all quiet". A key set at pairing is the next step. Over-the-air updates for the sealed watch are planned in the partition table, but not written.
Real metal plating on the bezel and trim would shield the 2.4 GHz antennas, so the chrome look has to be non-conductive vacuum metallising or pearl paint.
Everything behind Baby Monitor Monitor 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.
KiCad project, schematic and BOM. Saved with KiCad 10: open it in 10 or newer.
PlatformIO, first pass
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.