# Baby Mobile Audio Board — Design Document

## Overview

Drop-in replacement board for a baby crib mobile. Plays custom audio from
SPI flash memory, drives a DC motor, handles 8 button inputs, and runs
for days on two AA batteries.

**Key design decisions for long battery life:**
- SPI flash instead of microSD (4mA vs 30–100mA read current)
- Class-D amplifier at 90% efficiency (vs ~50% for class AB)
- ATmega328P internal 8MHz oscillator (no crystal, lower BOM)
- Logic-level MOSFET motor driver (no gate driver IC needed)
- Aggressive sleep with pin-change interrupt wake


## Schematic Netlist (Logical Connections)

### Power
```
2xAA Battery → SW9(PWR) → VCC rail
VCC → C1(100µF) → GND          bulk decoupling
VCC → C2(100nF) → GND          MCU VCC decoupling
VCC → C3(100nF) → GND          MCU AVCC decoupling
VCC → C5(100nF) → GND          Flash decoupling
VCC → C8(10µF)  → GND          Amp supply bypass
```

### MCU: U1 — ATmega328P-AU (TQFP-32)

| MCU Pin | Port     | Function      | Connected To              |
|---------|----------|---------------|---------------------------|
| 30      | PD0/RXD  | Serial RX     | J5 pin 3 (SERIAL header)  |
| 31      | PD1/TXD  | Serial TX     | J5 pin 2 (SERIAL header)  |
| 32      | PD2/INT0 | Button 3      | SW3 → GND (active LOW)    |
| 1       | PD3/INT1 | Button 4      | SW4 → GND (active LOW)    |
| 2       | PD4      | Button 5      | SW5 → GND (active LOW)    |
| 9       | PD5/OC0B | Motor PWM     | R4(100Ω) → Q1 gate       |
| 10      | PD6/OC0A | Button 6      | SW6 → GND (active LOW)    |
| 11      | PD7      | Button 7      | SW7 → GND (active LOW)    |
| 12      | PB0      | Button 8      | SW8 → GND (active LOW)    |
| 13      | PB1/OC1A | Audio PWM     | R2(4.7K) → C7(10nF) → U3 |
| 14      | PB2/SS   | Flash ~CS     | U2 pin 1                  |
| 15      | PB3/MOSI | SPI MOSI      | U2 pin 5, J4 MOSI         |
| 16      | PB4/MISO | SPI MISO      | U2 pin 2, J4 MISO         |
| 17      | PB5/SCK  | SPI SCK       | U2 pin 6, J4 SCK          |
| 23      | PC0/ADC0 | Button 1      | SW1 → GND (active LOW)    |
| 24      | PC1/ADC1 | Button 2      | SW2 → GND (active LOW)    |
| 25      | PC2/ADC2 | Amp ~SD       | U3 pin 1 + R3(100K)→GND   |
| 26      | PC3/ADC3 | Status LED    | R6(1K) → D2(LED) → GND    |
| 27      | PC4/SDA  | (spare/I2C)   | —                         |
| 28      | PC5/SCL  | (spare/I2C)   | —                         |
| 29      | PC6/~RST | Reset         | R1(10K)→VCC, C4(100nF)→GND|
| 4       | VCC      | Power         | VCC rail                  |
| 6       | AVCC     | Analog power  | VCC rail (via ferrite opt) |
| 21      | AREF     | Reference     | C3(100nF) → GND           |
| 3, 5    | GND      | Ground        | GND rail                  |
| 7       | XTAL1    | (unused)      | leave floating             |
| 8       | XTAL2    | (unused)      | leave floating             |


### SPI Flash: U2 — W25Q128JVSIQ (SOIC-8)

| Pin | Name      | Connected To    |
|-----|-----------|-----------------|
| 1   | ~CS       | PB2 (FLASH_CS)  |
| 2   | DO (MISO) | PB4 (SPI_MISO)  |
| 3   | ~WP       | VCC (tie high)  |
| 4   | GND       | GND             |
| 5   | DI (MOSI) | PB3 (SPI_MOSI)  |
| 6   | CLK       | PB5 (SPI_SCK)   |
| 7   | ~HOLD     | VCC (tie high)  |
| 8   | VCC       | VCC             |


### Audio Amplifier: U3 — PAM8302AASCR (SOIC-8)

| Pin | Name | Connected To                              |
|-----|------|-------------------------------------------|
| 1   | ~SD  | PC2 (AMP_SD) + R3(100K) pulldown to GND   |
| 2   | VDD  | VCC + C8(10µF) bypass                     |
| 3   | GND  | GND                                       |
| 4   | A+   | Audio signal from RC filter               |
| 5   | A-   | GND (single-ended input)                  |
| 6   | PAD  | GND (thermal pad)                         |
| 7   | VO-  | Speaker -                                 |
| 8   | VO+  | Speaker +                                 |

**Audio signal path:**
```
PB1 (OC1A PWM) → R2 (4.7KΩ) → node → C6 (1µF coupling) → U3 A+ (pin 4)
                                  |
                              C7 (10nF) → GND

RC low-pass filter: fc = 1/(2π × 4700 × 10e-9) ≈ 3.4 kHz
This removes the PWM carrier while passing audio baseband.
```

**Shutdown control:**
- R3 (100K) pulls ~SD LOW by default = amplifier OFF
- MCU drives PC2 HIGH to enable amplifier
- Saves ~2mA quiescent current when not playing


### Motor Driver

```
PD5 (OC0B) → R4 (100Ω gate resistor) → Q1 Gate
                                          |
                                     R5 (100K) → GND  (pulldown, ensures OFF at boot)

Q1: AO3400A N-ch MOSFET (SOT-23)
  Drain → Motor terminal 1
  Source → GND
  (Motor terminal 2 → VCC)

D1: SS14 Schottky flyback diode across motor (Cathode → VCC, Anode → Drain)
```

**Motor specs:**
- PWM on Timer0 (OC0B) for variable speed
- AO3400A: Vgs(th) = 0.65V typ, fully enhanced at 1.8V → works at depleted AA voltage
- R4 limits gate ringing; R5 ensures MOSFET OFF during MCU reset/programming


### Button Matrix

All 8 buttons connect between their MCU pin and GND (active LOW).
MCU internal pull-ups enabled (~30–50KΩ). No external pull-up resistors needed.

Buttons are on PCINT-capable pins for wake-from-sleep:
- SW1 (PC0) → PCINT8
- SW2 (PC1) → PCINT9
- SW3 (PD2) → INT0 (hardware interrupt)
- SW4 (PD3) → INT1 (hardware interrupt)
- SW5 (PD4) → PCINT20
- SW6 (PD6) → PCINT22
- SW7 (PD7) → PCINT23
- SW8 (PB0) → PCINT0


### Headers

**J4 — ISP (2×3, standard Atmel AVR ISP pinout):**
```
  MISO  1 ● 2  VCC
  SCK   3   4  MOSI
  ~RST  5   6  GND
```

**J5 — Serial (1×3, for bootloader programming / debug):**
```
  GND  1
  TXD  2  (MCU TX out)
  RXD  3  (MCU RX in)
```


## Bill of Materials

| Ref  | Value/Part       | Package           | Qty | ~Cost  | Notes                         |
|------|------------------|-------------------|-----|--------|-------------------------------|
| U1   | ATmega328P-AU    | TQFP-32           | 1   | $2.50  | Pre-burn Arduino bootloader   |
| U2   | W25Q128JVSIQ     | SOIC-8            | 1   | $1.20  | 16MB, 33min @ 8kHz/8bit      |
| U3   | PAM8302AASCR     | SOIC-8            | 1   | $0.80  | 2.5W class-D, Vmin=2.0V      |
| Q1   | AO3400A          | SOT-23            | 1   | $0.15  | Logic-level N-ch MOSFET       |
| D1   | SS14             | SMA               | 1   | $0.10  | Motor flyback protection      |
| D2   | LED (green)      | 0805              | 1   | $0.05  | Status indicator              |
| R1   | 10KΩ             | 0805              | 1   | $0.01  | Reset pull-up                 |
| R2   | 4.7KΩ            | 0805              | 1   | $0.01  | Audio LPF resistor            |
| R3   | 100KΩ            | 0805              | 1   | $0.01  | Amp shutdown pull-down        |
| R4   | 100Ω             | 0805              | 1   | $0.01  | MOSFET gate resistor          |
| R5   | 100KΩ            | 0805              | 1   | $0.01  | MOSFET gate pull-down         |
| R6   | 1KΩ              | 0805              | 1   | $0.01  | LED current limit             |
| C1   | 100µF/10V        | Radial 5mm        | 1   | $0.15  | Bulk power decoupling         |
| C2   | 100nF            | 0805              | 1   | $0.01  | MCU VCC bypass                |
| C3   | 100nF            | 0805              | 1   | $0.01  | MCU AVCC/AREF bypass          |
| C4   | 100nF            | 0805              | 1   | $0.01  | Reset noise filter            |
| C5   | 100nF            | 0805              | 1   | $0.01  | Flash bypass                  |
| C6   | 1µF              | 0805              | 1   | $0.02  | Audio AC coupling             |
| C7   | 10nF             | 0805              | 1   | $0.01  | Audio LPF capacitor           |
| C8   | 10µF             | 0805              | 1   | $0.05  | Amp supply bypass             |
| SW1-8| Tactile switch   | 6mm SMD           | 8   | $0.80  | Or reuse existing buttons     |
| SW9  | SPDT slide       | THT               | 1   | $0.20  | Power switch                  |
| J1   | JST-PH 2-pin     | THT               | 1   | $0.15  | Battery connector             |
| J2   | JST-PH 2-pin     | THT               | 1   | $0.15  | Speaker connector             |
| J3   | JST-PH 2-pin     | THT               | 1   | $0.15  | Motor connector               |
| J4   | 2×3 pin header   | 2.54mm            | 1   | $0.10  | ISP programming               |
| J5   | 1×3 pin header   | 2.54mm            | 1   | $0.05  | Serial debug                  |
| —    | AA battery holder| 2×AA              | 1   | $0.50  | Reuse existing                |
|      |                  |                   |     |        |                               |
|      | **TOTAL**        |                   |     |**~$7** |                               |


## Power Analysis

### Current draw by state:

| State              | MCU    | Flash  | Amp    | Motor  | Total   |
|--------------------|--------|--------|--------|--------|---------|
| Playing + spinning | 4 mA   | 4 mA   | 3 mA   | 80 mA  | ~91 mA  |
| Playing only       | 4 mA   | 4 mA   | 3 mA   | 0      | ~11 mA  |
| Idle (awake)       | 4 mA   | 0.001  | 0.001  | 0      | ~4 mA   |
| Deep sleep         | 0.0001 | 0.001  | 0.001  | 0      | <0.01mA |

### Estimated runtime on 2× AA Alkaline (2500 mAh):

| Mode               | Current | Runtime     |
|--------------------|---------|-------------|
| Audio + Motor      | 91 mA   | ~27 hours   |
| Audio only         | 11 mA   | ~227 hours  |
| Sleep              | <10 µA  | ~28 years   |

With typical use of 30 min/day (motor + audio), batteries last roughly **54 days**.
With 1 hour/day, roughly **27 days** — well within AA change interval for a toy.


## PCB Layout Guidelines

**Board size target:** 40mm × 30mm (fits inside most mobile housings)

**Layer stackup:** 2-layer (plenty for this design)

**Key layout rules:**
1. Place C2 and C3 as close to U1 pins 4/6 as physically possible
2. Place C5 directly adjacent to U2 pin 8
3. Keep SPI traces short and parallel-routed
4. Audio RC filter (R2, C7) close to U3 input
5. Wide traces for motor path (VCC → motor → Q1 drain): 0.5mm minimum
6. Ground pour on bottom layer
7. Thermal relief on U3 pad (pin 6) connected to ground pour
8. Keep battery/motor power traces away from audio section

**Recommended trace widths:**
- Power (VCC, GND, motor): 0.5mm+
- Signal (SPI, buttons): 0.25mm
- Audio: 0.25mm (keep short, away from digital noise)


## Programming

### Bootloader
Burn the Arduino bootloader for "ATmega328P 8MHz internal" using the ISP
header (J4) and an Arduino-as-ISP or USBasp programmer.

In Arduino IDE:
- Board: "ATmega328P"
- Clock: "Internal 8 MHz"
- BOD: "BOD 1.8V" (for low-voltage operation)

### Loading audio onto W25Q128
Use the serial header (J5) with a USB-serial adapter. The firmware
includes a serial protocol to receive audio data and write it to flash.
Alternatively, program the flash chip directly with a CH341A SPI
programmer before soldering (~$3 on Amazon).

### Audio format
Raw unsigned 8-bit PCM, 8000 Hz sample rate, mono.
Convert with ffmpeg:
```
ffmpeg -i input.mp3 -ar 8000 -ac 1 -f u8 -acodec pcm_u8 output.raw
```

At 8KB/s, the 16MB flash holds about **34 minutes** of audio.


## Possible Enhancements

- **Volume control:** Use a digital pot on amp input, or just use
  the PWM duty cycle adjustment in software
- **Battery voltage monitor:** Read VCC via bandgap reference trick
  on ATmega328P (no extra pin needed) — blink LED when low
- **Touch sensing:** Replace buttons with copper pads; use ADC-based
  capacitive sensing on PC0/PC1 (ATmega328P supports this with
  careful firmware)
- **More audio:** Swap W25Q128 (16MB) for W25Q256 (32MB) → 68 min
- **Bluetooth:** Add HC-05 module on serial pins for wireless audio
  upload from phone (adds ~30mA when active)