zyphlar
11 KiB
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)
- Plug USB-C cable into board
- Board appears as USB mass storage drive
- Drag .wav or .mp3 files onto drive
- Eject and unplug — music ready to play
Upload Audio (Bluetooth)
- Open Web Bluetooth page on phone (no app install)
- Connect to "BabyMobile" device
- Select audio files → transfer
- ~15 min for 30 minutes of audio at BLE speeds
Update Firmware
- Double-tap reset button (or hold BTN1+BTN2 on power-on)
- Board appears as UF2 drive
- Drag .uf2 firmware file onto drive
- 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.