zyphlar
230 lines
11 KiB
Markdown
230 lines
11 KiB
Markdown
# Baby Mobile Audio Board v2 — Design Document
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## Overview
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Drop-in replacement for baby crib mobile electronics. Custom audio from
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SPI flash, DC motor control, 8 buttons, and long runtime on 2×AA batteries.
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No programmer needed — upload audio via USB drag-and-drop or Bluetooth.
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## Architecture
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```
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2×AA (1.8-3.2V) → SW9 → D1(BAT54S) → TPS61220 boost → 3.3V ──┐
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├── MDBT50Q
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USB-C VBUS (5V) → D3(BAT54S) → MCP1700 LDO → 3.3V ────────────┘ (nRF52840)
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│
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┌────────────── SPI ── IS25LP128F (16MB flash, 2.3-3.6V) │
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│ ┌───────── PWM ── R+C LPF ── PAM8302A ── 8Ω speaker │
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│ │ ┌───── GPIO ── AO3400A MOSFET ── DC motor │
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│ │ │ ┌─── GPIO ── 8× buttons (internal pull-ups) │
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│ │ │ │ ┌─ USB ── USB-C D+/D- (mass storage) │
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│ │ │ │ │ BLE ── phone (wireless upload) │
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└────┴───┴─┴─┴──────────────────────────────────────────────────┘
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```
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## Power Design — Diode OR-ing (Safe for ALL Battery Types)
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```
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BAT54S TPS61220
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2×AA ──SW9──┤►├──────┐ ┌─ VIN VOUT ─── 3.3V rail
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├── node ┤
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USB VBUS ───┤►├── LDO ┘ └─ EN GND ─── GND
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BAT54S MCP1700 L ─── L1(4.7µH)
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```
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**Why this is safe:** The Schottky diodes only pass current in one direction.
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When USB provides 3.3V via the LDO, the boost converter stops switching and
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the battery diode is reverse-biased. Zero current flows into the batteries
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regardless of chemistry (alkaline, NiMH, lithium).
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**Diode drop:** BAT54S drops ~0.25V. TPS61220 accepts inputs down to 0.7V.
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Depleted 2×AA = 1.8V → boost sees 1.55V → still operational.
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## Pin Mapping — MDBT50Q (nRF52840)
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| Module Pin | nRF52840 Port | Function | Connected To |
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|------------|---------------|--------------|-------------------------------|
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| 4 | P0.02/AIN0 | Button 1 | SW1 → GND (active LOW) |
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| 5 | P0.03/AIN1 | Button 2 | SW2 → GND |
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| 6 | P0.04/AIN2 | Button 3 | SW3 → GND |
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| 7 | P0.05/AIN3 | Button 4 | SW4 → GND |
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| 8 | P0.06 | Button 5 | SW5 → GND |
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| 9 | P0.07 | Button 6 | SW6 → GND |
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| 10 | P0.08 | Button 7 | SW7 → GND |
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| 11 | P0.09 | Button 8 | SW8 → GND |
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| 12 | P0.10 | Motor PWM | R4(100Ω) → Q1 gate |
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| 13 | P0.11 | Audio PWM | R2(4.7K) → C7(10nF) → C6 → U3|
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| 14 | P0.12 | Flash ~CS | U2 pin 1 |
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| 15 | P0.13 | SPI MOSI | U2 pin 5 |
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| 16 | P0.14 | SPI MISO | U2 pin 2 |
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| 17 | P0.15 | SPI SCK | U2 pin 6 |
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| 18 | P0.16 | LED output | R6(1K) → D2 → GND |
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| 19 | P0.17 | Amp ~SD | U3 pin 1, R3(100K) → GND |
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| 20 | P0.18/~RESET | Reset | (internal pull-up) |
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| 21 | P0.19 | TXD (serial) | J5 pin 2 |
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| 22 | P0.20 | RXD (serial) | J5 pin 3 |
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| 28 | D- | USB Data - | J6 D- |
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| 29 | D+ | USB Data + | J6 D+ |
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| 33 | SWDIO | Debug | J4 pin 2 (SWD header) |
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| 34 | SWDCLK | Debug | J4 pin 3 (SWD header) |
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| 32 | VCC | Power | 3V3 rail |
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| 1 | GND | Ground | GND rail |
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## SPI Flash: U2 — IS25LP128F (SOIC-8)
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| Pin | Name | Connected To |
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|-----|-----------|-------------------|
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| 1 | ~CS | P0.12 (FLASH_CS) |
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| 2 | SO (MISO) | P0.14 (SPI_MISO) |
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| 3 | ~WP | 3V3 (tie high) |
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| 4 | GND | GND |
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| 5 | SI (MOSI) | P0.13 (SPI_MOSI) |
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| 6 | SCLK | P0.15 (SPI_SCK) |
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| 7 | ~HOLD | 3V3 (tie high) |
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| 8 | VCC | 3V3 |
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**Operating voltage: 2.3V–3.6V** — works across entire boost output range.
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## Audio Signal Path
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```
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P0.11 (PWM) → R2(4.7K) → node → C6(1µF coupling) → PAM8302A A+
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│
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C7(10nF) → GND
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RC LPF cutoff: 1/(2π × 4700 × 10e-9) ≈ 3.4 kHz
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```
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## USB-C Wiring
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```
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USB-C Pin → Board
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────────────────────
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VBUS (5V) → D3 anode → MCP1700 VIN
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GND → GND
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D+ → nRF52840 D+ (pin 29)
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D- → nRF52840 D- (pin 28)
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CC1 → R7(5.1K) → GND
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CC2 → R8(5.1K) → GND
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```
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## Bill of Materials
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| Ref | Part | Package | Qty | ~Cost |
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|--------|----------------------|---------------|-----|--------|
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| U1 | MDBT50Q-P1MV2 | Module | 1 | $6.00 |
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| U2 | IS25LP128F-JBLE | SOIC-8 | 1 | $2.00 |
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| U3 | PAM8302AASCR | SOIC-8 | 1 | $0.80 |
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| U4 | TPS61220DCKR | SC-70-5 | 1 | $0.80 |
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| U5 | MCP1700-3302E/TT | SOT-23 | 1 | $0.20 |
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| Q1 | AO3400A | SOT-23 | 1 | $0.15 |
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| D1 | BAT54S | SOT-23 | 1 | $0.05 |
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| D3 | BAT54S | SOT-23 | 1 | $0.05 |
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| D4 | SS14 | SMA | 1 | $0.10 |
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| D2 | LED green | 0805 | 1 | $0.05 |
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| L1 | 4.7µH inductor | 0805 | 1 | $0.15 |
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| R2 | 4.7KΩ | 0805 | 1 | $0.01 |
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| R3 | 100KΩ | 0805 | 1 | $0.01 |
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| R4 | 100Ω | 0805 | 1 | $0.01 |
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| R5 | 100KΩ | 0805 | 1 | $0.01 |
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| R6 | 1KΩ | 0805 | 1 | $0.01 |
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| R7,R8 | 5.1KΩ | 0805 | 2 | $0.02 |
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| C1 | 100µF/10V | Radial 5mm | 1 | $0.15 |
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| C2 | 100nF | 0805 | 1 | $0.01 |
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| C3 | 10µF | 0805 | 1 | $0.05 |
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| C5 | 100nF | 0805 | 1 | $0.01 |
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| C6 | 1µF | 0805 | 1 | $0.02 |
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| C7 | 10nF | 0805 | 1 | $0.01 |
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| C8 | 10µF | 0805 | 1 | $0.05 |
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| C9,C10 | 10µF | 0805 | 2 | $0.10 |
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| C11,C12| 1µF | 0805 | 2 | $0.04 |
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| SW1-8 | Tactile switch | 6mm SMD | 8 | $0.80 |
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| SW9 | SPDT slide | THT | 1 | $0.20 |
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| J1 | JST-PH 2-pin | THT | 1 | $0.15 |
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| J2 | JST-PH 2-pin | THT | 1 | $0.15 |
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| J3 | JST-PH 2-pin | THT | 1 | $0.15 |
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| J4 | 1×4 pin header | 2.54mm | 1 | $0.10 |
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| J5 | 1×3 pin header | 2.54mm | 1 | $0.05 |
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| J6 | USB-C receptacle | SMD | 1 | $0.30 |
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| | **TOTAL** | | |**~$12**|
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## Power Analysis
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| State | MCU | Flash | Amp | Motor | Boost η | Battery mA |
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|--------------------|--------|--------|--------|--------|---------|------------|
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| Audio + motor | 4 mA | 8 mA | 10 mA | 80 mA | 90% | ~113 mA |
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| Audio only | 4 mA | 8 mA | 10 mA | 0 | 90% | ~24 mA |
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| BLE idle | 0.5 mA | 0.001 | 0.001 | 0 | 85% | ~0.7 mA |
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| Deep sleep | 0.002 | 0.001 | 0.001 | 0 | — | ~0.01 mA |
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**Estimated runtimes on 2×AA alkaline (2500 mAh usable with boost):**
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| Mode | Runtime |
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|------------------|--------------|
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| Audio + Motor | ~22 hours |
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| Audio only | ~104 hours |
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| BLE standby | ~149 days |
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| Deep sleep | ~28 years |
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| 30 min/day use | **~44 days** |
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## Module Options (Soldering Difficulty)
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The schematic is designed around the MDBT50Q module, but the firmware
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works on any nRF52840 board. Here are your options from easiest to hardest:
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| Module | ~Cost | USB-C | Bootloader | Soldering |
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|-----------------------------|-------|-------|------------|------------------|
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| **Seeed XIAO nRF52840** | $10 | Yes | UF2 preloaded | **Through-hole headers** — solder like any DIP. Has castellated pads too. |
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| **Adafruit Feather nRF52840** | $25 | Yes | UF2 preloaded | Through-hole headers |
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| **SparkFun Pro nRF52840** | $20 | Yes | UF2 preloaded | Through-hole headers |
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| **Ebyte E73-2G4M08S1C** | $3 | No | None | Castellated edge pads — hand-solderable with regular iron |
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| **MDBT50Q-P1MV2 (Raytac)** | $6 | No | None | SMD pads — needs reflow or hot air |
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**Recommended for beginners: Seeed XIAO nRF52840.** It plugs into standard
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2.54mm headers like an Arduino Nano. USB-C and UF2 bootloader are built in,
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so you never need a programmer. Just drag-and-drop firmware and audio files.
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**If using a XIAO or Feather:** you can skip the TPS61220 boost converter,
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MCP1700 LDO, USB-C connector, and CC resistors from the BOM. The dev board
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handles all of that. Feed the boost converter's 3.3V output to the board's
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3V3 pin. Total BOM drops to ~$15 including the XIAO.
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**If using MDBT50Q or E73:** you need the full schematic as designed. The
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E73 is the budget option — castellated pads are solderable with flux, a
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fine tip, and patience. Pre-tin both the pads and the board footprint,
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then tack one corner and reflow the rest.
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## User Experience
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### Upload Audio (USB)
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1. Plug USB-C cable into board
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2. Board appears as USB mass storage drive
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3. Drag .wav or .mp3 files onto drive
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4. Eject and unplug — music ready to play
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### Upload Audio (Bluetooth)
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1. Open Web Bluetooth page on phone (no app install)
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2. Connect to "BabyMobile" device
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3. Select audio files → transfer
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4. ~15 min for 30 minutes of audio at BLE speeds
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### Update Firmware
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1. Double-tap reset button (or hold BTN1+BTN2 on power-on)
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2. Board appears as UF2 drive
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3. Drag .uf2 firmware file onto drive
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4. Board reboots with new firmware
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### No-Programmer Setup
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The MDBT50Q ships with the nRF52840 bootloader ROM. Flash the Adafruit
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nRF52 UF2 bootloader once via SWD (J4 header) using any $3 CMSIS-DAP
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probe, J-Link, or even a Raspberry Pi. After that, all future updates
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are drag-and-drop over USB. The SWD header is only needed once.
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