will/baby-mobile
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
Files
57df0a1293cee8637ca0ebce86924bc2e64c2f63
baby-mobile/DESIGN.md
T
zyphlar 57df0a1293 Initial commit
2026-07-04 03:28:11 -07:00

12 KiB
Raw Blame History

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)
Reference in New Issue View Git Blame Copy Permalink