# Baby Mobile Audio Board v2 — Design Document

## Overview

Drop-in replacement for baby crib mobile electronics. Custom audio from
SPI flash, DC motor control, 8 buttons, and long runtime on 2×AA batteries.
No programmer needed — upload audio via USB drag-and-drop or Bluetooth.

## Architecture

```
2×AA (1.8-3.2V) → SW9 → D1(BAT54S) → TPS61220 boost → 3.3V ──┐
                                                                 ├── MDBT50Q
USB-C VBUS (5V) → D3(BAT54S) → MCP1700 LDO → 3.3V ────────────┘   (nRF52840)
                                                                     │
    ┌────────────── SPI ── IS25LP128F (16MB flash, 2.3-3.6V)        │
    │    ┌───────── PWM ── R+C LPF ── PAM8302A ── 8Ω speaker       │
    │    │   ┌───── GPIO ── AO3400A MOSFET ── DC motor              │
    │    │   │ ┌─── GPIO ── 8× buttons (internal pull-ups)          │
    │    │   │ │ ┌─ USB ── USB-C D+/D- (mass storage)              │
    │    │   │ │ │  BLE ── phone (wireless upload)                  │
    └────┴───┴─┴─┴──────────────────────────────────────────────────┘
```

## Power Design — Diode OR-ing (Safe for ALL Battery Types)

```
                BAT54S              TPS61220
  2×AA ──SW9──┤►├──────┐        ┌─ VIN  VOUT ─── 3.3V rail
                        ├── node ┤
  USB VBUS ───┤►├── LDO ┘       └─ EN   GND ─── GND
              BAT54S  MCP1700         L ─── L1(4.7µH)
```

**Why this is safe:** The Schottky diodes only pass current in one direction.
When USB provides 3.3V via the LDO, the boost converter stops switching and
the battery diode is reverse-biased. Zero current flows into the batteries
regardless of chemistry (alkaline, NiMH, lithium).

**Diode drop:** BAT54S drops ~0.25V. TPS61220 accepts inputs down to 0.7V.
Depleted 2×AA = 1.8V → boost sees 1.55V → still operational.


## Pin Mapping — MDBT50Q (nRF52840)

| Module Pin | nRF52840 Port | Function     | Connected To                  |
|------------|---------------|--------------|-------------------------------|
| 4          | P0.02/AIN0    | Button 1     | SW1 → GND (active LOW)        |
| 5          | P0.03/AIN1    | Button 2     | SW2 → GND                     |
| 6          | P0.04/AIN2    | Button 3     | SW3 → GND                     |
| 7          | P0.05/AIN3    | Button 4     | SW4 → GND                     |
| 8          | P0.06         | Button 5     | SW5 → GND                     |
| 9          | P0.07         | Button 6     | SW6 → GND                     |
| 10         | P0.08         | Button 7     | SW7 → GND                     |
| 11         | P0.09         | Button 8     | SW8 → GND                     |
| 12         | P0.10         | Motor PWM    | R4(100Ω) → Q1 gate            |
| 13         | P0.11         | Audio PWM    | R2(4.7K) → C7(10nF) → C6 → U3|
| 14         | P0.12         | Flash ~CS    | U2 pin 1                      |
| 15         | P0.13         | SPI MOSI     | U2 pin 5                      |
| 16         | P0.14         | SPI MISO     | U2 pin 2                      |
| 17         | P0.15         | SPI SCK      | U2 pin 6                      |
| 18         | P0.16         | LED output   | R6(1K) → D2 → GND             |
| 19         | P0.17         | Amp ~SD      | U3 pin 1, R3(100K) → GND      |
| 20         | P0.18/~RESET  | Reset        | (internal pull-up)             |
| 21         | P0.19         | TXD (serial) | J5 pin 2                      |
| 22         | P0.20         | RXD (serial) | J5 pin 3                      |
| 28         | D-            | USB Data -   | J6 D-                         |
| 29         | D+            | USB Data +   | J6 D+                         |
| 33         | SWDIO         | Debug        | J4 pin 2 (SWD header)         |
| 34         | SWDCLK        | Debug        | J4 pin 3 (SWD header)         |
| 32         | VCC           | Power        | 3V3 rail                      |
| 1          | GND           | Ground       | GND rail                      |


## SPI Flash: U2 — IS25LP128F (SOIC-8)

| Pin | Name      | Connected To      |
|-----|-----------|-------------------|
| 1   | ~CS       | P0.12 (FLASH_CS)  |
| 2   | SO (MISO) | P0.14 (SPI_MISO)  |
| 3   | ~WP       | 3V3 (tie high)    |
| 4   | GND       | GND               |
| 5   | SI (MOSI) | P0.13 (SPI_MOSI)  |
| 6   | SCLK      | P0.15 (SPI_SCK)   |
| 7   | ~HOLD     | 3V3 (tie high)    |
| 8   | VCC       | 3V3               |

**Operating voltage: 2.3V–3.6V** — works across entire boost output range.


## Audio Signal Path

```
P0.11 (PWM) → R2(4.7K) → node → C6(1µF coupling) → PAM8302A A+
                           │
                       C7(10nF) → GND

RC LPF cutoff: 1/(2π × 4700 × 10e-9) ≈ 3.4 kHz
```


## USB-C Wiring

```
USB-C Pin → Board
────────────────────
VBUS (5V) → D3 anode → MCP1700 VIN
GND       → GND
D+        → nRF52840 D+ (pin 29)
D-        → nRF52840 D- (pin 28)
CC1       → R7(5.1K) → GND
CC2       → R8(5.1K) → GND
```


## Bill of Materials

| Ref    | Part                 | Package       | Qty | ~Cost  |
|--------|----------------------|---------------|-----|--------|
| U1     | MDBT50Q-P1MV2        | Module        | 1   | $6.00  |
| U2     | IS25LP128F-JBLE      | SOIC-8        | 1   | $2.00  |
| U3     | PAM8302AASCR         | SOIC-8        | 1   | $0.80  |
| U4     | TPS61220DCKR         | SC-70-5       | 1   | $0.80  |
| U5     | MCP1700-3302E/TT     | SOT-23        | 1   | $0.20  |
| Q1     | AO3400A              | SOT-23        | 1   | $0.15  |
| D1     | BAT54S               | SOT-23        | 1   | $0.05  |
| D3     | BAT54S               | SOT-23        | 1   | $0.05  |
| D4     | SS14                 | SMA           | 1   | $0.10  |
| D2     | LED green            | 0805          | 1   | $0.05  |
| L1     | 4.7µH inductor       | 0805          | 1   | $0.15  |
| R2     | 4.7KΩ                | 0805          | 1   | $0.01  |
| R3     | 100KΩ                | 0805          | 1   | $0.01  |
| R4     | 100Ω                 | 0805          | 1   | $0.01  |
| R5     | 100KΩ                | 0805          | 1   | $0.01  |
| R6     | 1KΩ                  | 0805          | 1   | $0.01  |
| R7,R8  | 5.1KΩ                | 0805          | 2   | $0.02  |
| C1     | 100µF/10V            | Radial 5mm    | 1   | $0.15  |
| C2     | 100nF                | 0805          | 1   | $0.01  |
| C3     | 10µF                 | 0805          | 1   | $0.05  |
| C5     | 100nF                | 0805          | 1   | $0.01  |
| C6     | 1µF                  | 0805          | 1   | $0.02  |
| C7     | 10nF                 | 0805          | 1   | $0.01  |
| C8     | 10µF                 | 0805          | 1   | $0.05  |
| C9,C10 | 10µF                 | 0805          | 2   | $0.10  |
| C11,C12| 1µF                  | 0805          | 2   | $0.04  |
| SW1-8  | Tactile switch       | 6mm SMD       | 8   | $0.80  |
| SW9    | SPDT slide           | THT           | 1   | $0.20  |
| J1     | JST-PH 2-pin         | THT           | 1   | $0.15  |
| J2     | JST-PH 2-pin         | THT           | 1   | $0.15  |
| J3     | JST-PH 2-pin         | THT           | 1   | $0.15  |
| J4     | 1×4 pin header       | 2.54mm        | 1   | $0.10  |
| J5     | 1×3 pin header       | 2.54mm        | 1   | $0.05  |
| J6     | USB-C receptacle     | SMD           | 1   | $0.30  |
|        | **TOTAL**            |               |     |**~$12**|


## Power Analysis

| State              | MCU    | Flash  | Amp    | Motor  | Boost η | Battery mA |
|--------------------|--------|--------|--------|--------|---------|------------|
| Audio + motor      | 4 mA   | 8 mA   | 10 mA  | 80 mA  | 90%     | ~113 mA    |
| Audio only         | 4 mA   | 8 mA   | 10 mA  | 0      | 90%     | ~24 mA     |
| BLE idle           | 0.5 mA | 0.001  | 0.001  | 0      | 85%     | ~0.7 mA    |
| Deep sleep         | 0.002  | 0.001  | 0.001  | 0      | —       | ~0.01 mA   |

**Estimated runtimes on 2×AA alkaline (2500 mAh usable with boost):**

| Mode             | Runtime      |
|------------------|--------------|
| Audio + Motor    | ~22 hours    |
| Audio only       | ~104 hours   |
| BLE standby      | ~149 days    |
| Deep sleep       | ~28 years    |
| 30 min/day use   | **~44 days** |


## Module Options (Soldering Difficulty)

The schematic is designed around the MDBT50Q module, but the firmware
works on any nRF52840 board. Here are your options from easiest to hardest:

| Module                      | ~Cost | USB-C | Bootloader | Soldering        |
|-----------------------------|-------|-------|------------|------------------|
| **Seeed XIAO nRF52840**     | $10   | Yes   | UF2 preloaded | **Through-hole headers** — solder like any DIP. Has castellated pads too. |
| **Adafruit Feather nRF52840** | $25 | Yes   | UF2 preloaded | Through-hole headers |
| **SparkFun Pro nRF52840**   | $20   | Yes   | UF2 preloaded | Through-hole headers |
| **Ebyte E73-2G4M08S1C**     | $3    | No    | None       | Castellated edge pads — hand-solderable with regular iron |
| **MDBT50Q-P1MV2 (Raytac)**  | $6    | No    | None       | SMD pads — needs reflow or hot air |

**Recommended for beginners: Seeed XIAO nRF52840.** It plugs into standard
2.54mm headers like an Arduino Nano. USB-C and UF2 bootloader are built in,
so you never need a programmer. Just drag-and-drop firmware and audio files.

**If using a XIAO or Feather:** you can skip the TPS61220 boost converter,
MCP1700 LDO, USB-C connector, and CC resistors from the BOM. The dev board
handles all of that. Feed the boost converter's 3.3V output to the board's
3V3 pin. Total BOM drops to ~$15 including the XIAO.

**If using MDBT50Q or E73:** you need the full schematic as designed. The
E73 is the budget option — castellated pads are solderable with flux, a
fine tip, and patience. Pre-tin both the pads and the board footprint,
then tack one corner and reflow the rest.


## User Experience

### Upload Audio (USB)
1. Plug USB-C cable into board
2. Board appears as USB mass storage drive
3. Drag .wav or .mp3 files onto drive
4. Eject and unplug — music ready to play

### Upload Audio (Bluetooth)
1. Open Web Bluetooth page on phone (no app install)
2. Connect to "BabyMobile" device
3. Select audio files → transfer
4. ~15 min for 30 minutes of audio at BLE speeds

### Update Firmware
1. Double-tap reset button (or hold BTN1+BTN2 on power-on)
2. Board appears as UF2 drive
3. Drag .uf2 firmware file onto drive
4. Board reboots with new firmware

### No-Programmer Setup
The MDBT50Q ships with the nRF52840 bootloader ROM. Flash the Adafruit
nRF52 UF2 bootloader once via SWD (J4 header) using any $3 CMSIS-DAP
probe, J-Link, or even a Raspberry Pi. After that, all future updates
are drag-and-drop over USB. The SWD header is only needed once.