# Hardware - Bill of Materials and wiring

<!-- SPDX-License-Identifier: CERN-OHL-S-2.0 -->

OpenScribe v1 is built from off-the-shelf modules (no custom PCB). There are two build
options: a **compact** build for a wearable, and a **breadboard/dev** build that is easier
to bring up and debug. Pick one; the firmware and case are parametric to suit either.

## Build A - Compact (recommended for the wearable)

Uses the Seeed Studio XIAO ESP32-S3 Sense, which puts the MCU, PSRAM, a microphone and a
microSD slot on a thumb-sized board. Fewer wires, smallest case.

| # | Part | Spec / why | Qty | Notes |
|---|------|-----------|-----|-------|
| 1 | **Seeed XIAO ESP32-S3 Sense** | ESP32-S3, 8 MB PSRAM, 8 MB flash, WiFi + BLE 5, onboard PDM mic + microSD slot, USB-C, LiPo charge circuit | 1 | The mic + SD + charger are already on the "Sense" expansion board |
| 2 | **microSD card** | 16-32 GB, Class 10 / A1 | 1 | FAT32 |
| 3 | **LiPo battery** | 3.7 V, 400-600 mAh, JST-PH 1.25 mm | 1 | Fits the XIAO battery pads |
| 4 | **Tactile button** | momentary, 6 mm | 1 | Record start/stop, long-press = pair/config |
| 5 | **Status LED** | XIAO onboard user LED (or add 1 external + resistor) | 1 | Optional external RGB for richer status |
| 6 | (optional) **Vibration motor** | coin ERM + driver transistor | 1 | Haptic feedback like a NotePin |
| 7 | (optional) **Slide switch** | SPDT, for hard power cut | 1 | In line with the battery |

Notes on Build A:
- The XIAO Sense onboard mic is **PDM**, not I2S. The firmware supports both; PDM keeps the
  compact build to one board. For best audio quality use Build B with an I2S MEMS mic.
- USB-C present = "on charge / hard-powered"; the firmware reads VBUS to switch to the
  independent WiFi upload mode.

## Build B - Dev / best audio (breadboard or protoboard)

Discrete modules, easiest to probe, best microphone.

| # | Part | Spec / why | Qty | Notes |
|---|------|-----------|-----|-------|
| 1 | **ESP32-S3 dev board with PSRAM** | e.g. ESP32-S3-DevKitC-1 **N16R8** (16 MB flash, 8 MB PSRAM), WiFi + BLE 5, USB | 1 | PSRAM is required for audio buffering |
| 2 | **I2S MEMS microphone** | **INMP441** (default, cheap) or **ICS-43434** (better SNR ~65 dB) | 1 | Digital I2S, clean audio, no analogue front-end |
| 3 | **microSD breakout** | SPI or SDMMC (1-bit) microSD module | 1 | SDMMC 1-bit is faster if pins allow |
| 4 | **microSD card** | 16-32 GB, Class 10 / A1 | 1 | FAT32 |
| 5 | **LiPo battery** | 3.7 V, 500-1000 mAh, JST | 1 | Bigger cell OK on the dev build |
| 6 | **LiPo charger + protection** | **TP4056 with DW01 protection** module (USB-C), or **MCP73831** | 1 | STAT pin -> GPIO to detect charging |
| 7 | **Boost/regulator** | only if the board lacks a suitable 3V3 LDO from battery | 0-1 | Most dev boards handle this |
| 8 | **Tactile button** | momentary, 6 mm | 1 | Record start/stop, long-press = pair |
| 9 | **RGB status LED** | common-cathode or WS2812 | 1 | Idle / recording / uploading / error states |
| 10 | (optional) **Vibration motor + transistor** | coin ERM + 2N2222/AO3400 + flyback diode | 1 | Haptic feedback |
| 11 | Wires / protoboard / headers | - | - | For bring-up |

## Pinout (default, Build B - ESP32-S3)

These are the firmware defaults; all are overridable in `firmware/include/pins.h` (added
in M1). Avoid strapping pins (0, 3, 45, 46) and USB pins (19, 20).

| Signal | ESP32-S3 GPIO | Notes |
|--------|---------------|-------|
| I2S mic BCLK (SCK) | 4 | to mic SCK |
| I2S mic WS (LRCL) | 5 | to mic WS |
| I2S mic SD (DOUT) | 6 | mic data out -> ESP32 in |
| I2S mic L/R select | GND | selects left channel (mono) |
| microSD CLK | 12 | SPI SCK (or SDMMC CLK) |
| microSD CMD/MOSI | 11 | |
| microSD D0/MISO | 13 | |
| microSD CS (SPI) | 10 | SPI mode only |
| Record button | 7 | active-low, internal pull-up, other side to GND |
| Status LED (WS2812 data) | 48 | or plain LED via 330R on any free GPIO |
| Charger STAT (charging?) | 14 | input, from TP4056 STAT (open-drain) |
| VBUS present (on-charge) | 15 | via divider from 5V USB, or board VBUS sense |
| Vibration motor (optional) | 16 | via transistor |

For **Build A (XIAO Sense)** the mic is PDM and the microSD uses the Sense board's fixed
pins; the firmware ships a XIAO pin profile so no manual wiring table is needed.

## Power and modes

- **Portable (battery):** low-power; record on button; BLE advertising; WiFi on demand.
- **On charge / hard-powered:** detected via VBUS/STAT; device joins configured WiFi, runs
  the REST API on the LAN, and uploads new recordings to the configured object store.

## Rough cost (indicative, GBP)

- Build A (compact): XIAO ESP32-S3 Sense ~£14, microSD ~£5, LiPo ~£5, button/bits ~£3.
  **~£27** plus a printed case.
- Build B (dev): S3 devkit ~£10, INMP441 ~£4, SD module ~£3, TP4056 ~£2, LiPo ~£6, LED +
  button + wires ~£5. **~£30** plus a printed case.

## Assembly

Wiring diagrams and assembly photos are added alongside the case in milestone M8. Until
then this table plus `firmware/include/pins.h` is the source of truth for connections.