A design proposal for a baby monitor accessory. You feel the crying instead of hearing it, so you can sleep with earplugs in.
A pearl-white mic sits next to the monitor and listens to its speaker. When the baby fusses, the watch on your wrist or ankle buzzes: gently for a whimper, harder for a full cry. It keeps going until you tap "I'm awake".
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 can block out the room and still be the first to know when the baby actually needs them, and how urgently.
A beeping monitor wakes everyone in the room. A buzz on one wrist wakes one person. Swap the watch at the next feed, or strap it to an ankle if wrists are busy holding a baby.
"Bzz. Bzz-bzz. BZZZ."3:12 am. The monitor is at normal volume across the room, the earplugs are in, and the wrist feels the crying ramp up before anyone else in the house hears it.
Pick what the monitor is playing. Every 250 ms the sensor measures how loud it is and sorts that into one of four levels. The watch buzzes to match, a little harder with each round, until someone taps "I'm awake".
All quietThe green link light blinks with the sensor's heartbeat.
This is a simulation of the first-pass firmware plan, not real audio. The thresholds are guesses, and a real sensor would be calibrated to your monitor's volume. The phone buzz only works in browsers that support vibration (most Android phones, not iPhones).
Both devices are in one first-pass CAD model, sized around real parts: the cells, the coin motor and the two routed boards. 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. The sensor's 52 × 24 mm board hangs in its crown, and the watch's 30 × 30 mm board hangs under the lens. Both carry parts on both faces.
ESP-NOW links two ESP32s directly in about a millisecond, with no Wi-Fi network, no app and no account. The sensor sends a heartbeat every few seconds, and if it stops, the watch gives a "lost link" buzz.
A digital I2S MEMS mic, ported through the board to the grille. The sensor measures loudness every 250 ms and sorts it into quiet, fussing, crying or screaming.
A Ø10 × 2.7 mm coin motor, switched by a MOSFET at 20 kHz with a flyback diode. Intensity is set by the PWM duty, capped at about 70 % because the motor is rated for 3 V.
A 2 mm RGB LED under the frosted lens. Its colour follows the level, and it blinks green when the "I'm awake" tap arrives.
Two spring pins press on gold contacts in the back plate. A Schottky diode blocks reversed polarity and keeps the cell's voltage off the exposed contacts, so sweat can't drain it.
USB-C runs the sensor beside the monitor and charges a 2000 mAh cell for nights away from home. A power-path switch stops it float-charging all night, and a slide switch turns it fully off.
From a roll of "haptic motor, new parent, wearable, Y2K" to two routed boards in one episode.
This is a speedrun first pass. Here's what we'd answer before anyone orders a board or cuts a mould.
The watch has to keep listening for the sensor all night. That only works if the radio sleeps between heartbeats, which is firmware work. Measure it on a dev board before fixing the case thickness. A bigger cell makes the watch thicker.
Both boards were re-routed (DRC 0 / 0) after the hosts dropped the watch's button and USB port and gave the sensor a bigger cell. The board-in-enclosure fit checks were last run before those changes.
An ERM through an elastic band is gentle. If it isn't enough, the upgrade is an LRA with a DRV2605L driver: crisper, stronger and able to play patterns.
Loudness alone can't. Lots of nurseries run white noise all night, so the sensor needs calibrating to the room, and probably a filter tuned to the pitch of a cry.
Real metal plating would shield the 2.4 GHz antennas. The Y2K chrome look has to be NCVM (non-conductive vacuum metallising) or pearl paint.
With no button, you can only stop the buzz from the sensor. A tap gesture would need an accelerometer, and it's still an open question whether the watch should be able to switch itself off.
Everything behind Baby Monitor Monitor 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.
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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.
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.