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# 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)