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James Cronin

Jot, a handheld voice-note device

Hold the button and speak. Jot saves the recording to its own flash, sends it to your iPhone over Bluetooth, and the app files it as a note, a task, a reminder or an event.

The app works today, on the phone's own microphone, and a breadboard prototype already streams a recording into it. The custom board, Rev A, is laid out on four layers and goes to fab once Raytac, who make the radio module, have checked the antenna.

Read the build log

Now: PCB + fab. Updated 6 Nov 2026.

How it works

  1. Press

    Hold the button and speak. Release to finish the note.

  2. Save

    Jot writes the clip to its own flash first, so a recording never waits for a connection.

  3. Send

    Over Bluetooth Low Energy to the iPhone, whenever the phone is in range.

  4. File

    The phone transcribes it, Apple's on-device model decides what it is, and it lands as a note, a reminder or a calendar event.

The board

Rev A: 37 by 60 mm, four layers, 1.0 mm thick, every part on the top side so the battery can sit underneath. Layer 2 is a solid ground plane and layer 3 carries 3.3 V and battery power. The radio module sits on ground on every layer with a clear antenna strip across the top edge.

The schematic is nine KiCad sheets, through two independent design reviews and a fault-by-fault pass that added battery and USB protection. The layout has been through its own review: the design-rule check is clean and the board matches the schematic. JLCPCB can assemble every part.

Read the Rev A notes
Rev A, 37 by 60 mm, 4 layers. Radio module at the top edge, USB-C on the tab.
  • Top. Test pads left, motor pads right, battery wires bottom left.
  • Bottom. No parts; the battery sits under this side.
  • Top copper: parts, short connections and the ground pour.
  • Bottom copper, seen from below: the long runs, in a stitched ground pour.

Mistakes I caught before ordering

Ten things found in two design reviews and a fault-by-fault pass, before any board was made. Each would have meant a second revision.

  1. Datasheet review

    Flash chip-select on the reset pin

    P0.18 doubles as the nRF52840's reset. The flash moved to the pins Nordic's own kit uses.

  2. Schematic check

    5 V on a 3.6 V flash

    Its supply was wired straight to USB power, which would have killed it the first time USB went in. It now runs from the 3.3 V rail.

  3. Schematic check

    Two QSPI data lines swapped

    IO0 and IO2 were crossed between the flash and the MCU. In quad mode every byte would come back scrambled.

  4. Assembly check

    A charger too small to assemble

    The first charger was a 0.4 mm-pitch ball-grid chip, finer than JLCPCB's economy line can place and impossible to rework by hand. Swapped for a SOT-23 part before footprints.

  5. Architecture

    A clock line that powers a switched-off mic

    The mic rail switches off when idle, but a driven PDM clock would feed it through its pins. Firmware releases that pin first.

  6. Fault review

    No fuse below the battery's own cut-off

    The pack only protects itself above 2 to 5 A. A resettable fuse (0.5 A hold, 1 A trip) and a reverse-battery switch now sit at the battery input.

  7. Layout review

    A battery connector taller than the case

    The JST header stands 5.5 mm off the board, which would have ended 0.8 mm outside the 12.8 mm case. The battery leads now solder into two marked holes and take no height.

  8. Layout review

    Vias drilled through solder pads

    Seven vias sat inside pads, where an open hole can drain solder from the joint. Each moved beside its pad, and the LED pins were re-ordered so their lines no longer had to cross.

  9. Fault review

    I2C pull-ups too weak for the fuel gauge

    The gauge alone can add 60 pF and needs a 300 ns rise. 4.7 kΩ left about 15 pF of margin for everything else; 2.2 kΩ leaves 100 pF.

  10. Cost review

    A $36 flash chip

    128 MB was four times what Jot needs between syncs. A 32 MB part in the same package costs $4.34: about $160 saved on the first five boards.

Proved on a breadboard

Before the custom board, the whole loop ran on parts you can buy: an nRF52840 board, a battery and a button. Hold it, speak, let go, and the words land in the app, sorted. It uses the same chip family as Rev A, so the firmware and the app's Bluetooth code carry straight over.

  1. Plan for lost packets

    Sent flat out, about half the packets never arrived. Confirming each one was reliable but slow. Numbering them and asking again for only the gaps is both fast and lossless.

  2. Stream while recording

    Sending while the button is still held leaves only a fraction of a second of audio to send when you let go: about 60 ms.

  3. The library matters as much as the chip

    The first Bluetooth stack silently dropped packets and connections. Bluefruit waits for a free buffer instead, and the link became fast and stable.

The app came first

The software half of Jot works end to end, with the phone's own microphone standing in for the device: hold the orb, speak, release. The words are transcribed on the phone, Apple's on-device model sorts them into a note, task, reminder or event, and the item drops into place on the day's timeline.

  1. Holding the orb: words appear live while you speak
  2. On release it's filed as a reminder, with Undo and Edit
  3. Inbox: everything captured, filtered and searchable
  4. An event with its follow-ups, recorded after the dinner
  5. Permissions, each explained before iOS asks

  1. Design the seam before the hardware exists

    Everything downstream reads a stream of audio chunks from one AudioSource protocol. The phone mic implements it today; the Bluetooth link from the device will implement the same one, so nothing else changes when the hardware arrives.

  2. Never lose what was said

    If sorting fails or is switched off, the words are kept as a note and sorted later. If transcription fails, the recording itself is kept and can be played back. Nothing the user said is thrown away.

  3. Sort on the phone, compute dates in code

    Apple's on-device model does the sorting, so nothing leaves the phone. It got weekday maths wrong in testing, so it only extracts what was said and the app works out the date, with tests. Claude stays an explicit choice in Settings that says what is sent.

  4. A recording must always end

    A phone call, Siri, pulling AirPods out or a cancelled touch can all stop the audio without a finger being lifted. A wall-clock watchdog ends every recording on time limits and interruptions, so the mic can never stay on by accident.

Where it stands

  1. iPhone app

    Reliability + privacy pass

    Done

  2. Prototype One

    XIAO breadboard, BLE audio to app

    Done

  3. Architecture

    JOT-HW-001

    Done

  4. Parts + pin map

    Checked against datasheets

    Done

  5. Schematic

    Final, senior-reviewed

    Done

  6. PCB + fab

    Layout final; ordering after Raytac's check

    Now

  7. Firmware + BLE

    Rev A, from Prototype One

    Next

  8. Case + V1

    Case arrived; V1 waits for the boards

    Next

Updated 6 Nov 2026

  • Case front: three status dots and the jot logo
  • Case from the side: the grooved side button and USB-C on the bottom edge
  • Case back: four recessed screws, numbered 1 of 5 to 5 of 5

The case

68 by 42 by 12.8 mm, two halves held by four screws from the back, which also clamp the board. A flatter battery took it from 16 mm down to 12.8. Five SLS nylon sets, numbered on the back, arrived in October and are waiting for the boards.

Build log

  1. Order files and a first quote
  2. Layout finished
  3. Routing the hard parts first
  4. Laying out the board
  5. A case for five
All 16 entries

For engineers

Block diagram, requirements, power budget, bring-up plan, key parts and open items, on one page, with the schematic and BOM.

Read the Rev A notes