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# Toy PCB Brain Transplant Guide
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## Overview
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Instead of building a whole new board, desolder the original MCU from the
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toy's PCB and replace it with a breakout connector wired to the XIAO
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nRF52840. The original PCB keeps doing everything it's good at (power,
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buttons, motor, speaker routing) — you just swap the brain.
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## What You Need
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| Item | ~Cost | Notes |
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|------|-------|-------|
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| SOP-16 solder-down DIP adapter | $2 | Logical Systems PA-SOF-D420-16, or generic SOIC-16→DIP breakout |
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| Seeed XIAO nRF52840 | $10 | The new brain |
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| IS25LP128F (SPI flash) | $2 | Audio storage (solder to a small perfboard with the XIAO) |
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| 16-pin ribbon cable or Dupont jumpers | $1 | Connect adapter to XIAO |
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| Solder, flux, solder wick | — | For desoldering and rework |
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| Multimeter | — | For reverse-engineering the pinout |
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**Total added cost: ~$15** (and you reuse 90% of the original toy)
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## Step 1: Document the Original Board
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Before desoldering anything:
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1. **Photograph both sides** of the PCB in good light
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2. **Identify the MCU** — look for markings like TRSF1602A, chip-on-board
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(black blob), or other SOP-8/SOP-16 packages
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3. **Note the pin 1 indicator** — dot, notch, or beveled corner
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4. **Count pins** — this guide assumes SOP-16, but adapt for SOP-8
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5. **Trace visible connections** by eye:
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- Which pins have fat traces? → probably power (VCC/GND)
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- Which pins connect to the speaker? → audio output
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- Which pins go to the motor area? → motor drive
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- Which pins go to buttons? → GPIO inputs
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## Step 2: Desolder the Original MCU
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**Hot air method (recommended):**
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1. Apply flux to all pins
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2. Set hot air to ~350°C, low airflow
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3. Heat evenly around the chip until solder reflows
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4. Lift chip with tweezers
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5. Clean pads with wick and flux
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**Soldering iron method:**
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1. Apply flux generously
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2. Add fresh leaded solder to all pins (lower melting point helps)
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3. Drag iron back and forth across pins on each side
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4. Use solder wick to remove excess
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5. Gently pry chip up while heating one side, then the other
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6. Clean pads with wick
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**If it's a chip-on-board (black blob):**
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Unfortunately, these can't be desoldered. You'll need to cut the traces
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instead and jumper to the XIAO. Identify the traces leading to the blob
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and cut them with an X-acto knife, then solder wires directly to the
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button/speaker/motor pads on the board.
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## Step 3: Solder the SOP-16 Breakout Adapter
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1. Place the SOP-16 → DIP adapter on the now-empty pads
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2. Align pin 1 with the original pin 1 marking
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3. Tack-solder two corner pins
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4. Check alignment under magnification
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5. Solder remaining pins
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6. Now you have 16 through-hole pins you can plug jumper wires into
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## Step 4: Reverse-Engineer the Pinout
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This is the detective work. You need to figure out what the original
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MCU was connected to on each pin.
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### 4a: Power pins (multimeter, board OFF)
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Set multimeter to continuity/beep mode.
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1. Touch one probe to the battery negative terminal (GND)
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2. Touch the other probe to each of the 16 breakout pins
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3. **Mark any pins that beep as GND**
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4. Now touch one probe to battery positive (after the power switch)
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5. Touch other probe to each remaining pin
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6. **Mark any pins that beep as VCC**
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Typical SOP-16 MCUs have 1-2 GND pins and 1-2 VCC pins.
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### 4b: Speaker pins (visual trace + multimeter)
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Follow the traces from the speaker solder pads back to the breakout.
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Often the speaker connects through a small capacitor or resistor, then
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to one or two MCU pins.
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- **One speaker pin** = single-ended PWM audio (most common)
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- **Two speaker pins** = bridge-tied-load (BTL) direct drive (no amp chip)
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### 4c: Motor pin (visual trace)
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Follow the motor wires. In cheap toys, the motor often connects to a
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single transistor (TO-92 or SOT-23 package near the motor area). The
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transistor's base connects (through a resistor) to an MCU pin.
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If there's no transistor and the motor connects directly to an MCU pin
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(rare but possible on very cheap toys), you'll need to add a MOSFET —
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MCU pins can't drive motors directly.
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### 4d: Button pins (multimeter, board OFF)
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For each physical button on the toy:
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1. Press the button while testing continuity between its pads
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2. Trace one pad to GND (common) and the other to an MCU pin
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3. Label that breakout pin as a button input
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Most cheap toys wire buttons between the MCU pin and GND, relying on
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the MCU's internal pull-up resistor — same as our design.
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### 4e: Automated pin discovery (firmware)
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Upload the pin_discovery sketch (included below) to the XIAO. Connect
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each breakout pin one at a time to a XIAO GPIO and the sketch will
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tell you what it sees:
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- High/low/floating state
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- Whether pressing a button changes it
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- Whether it shows PWM activity when the toy was running
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- Approximate voltage level
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## Step 5: Create the Pin Map
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Fill out a table like this for your specific toy:
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| SOP-16 Pin | Connects To | XIAO Pin | Notes |
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|------------|-------------|----------|-------|
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| 1 | VCC | 3V3 | Power supply |
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| 2 | GND | GND | Ground |
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| 3 | Button 1 | D0 | Play/Pause (active LOW) |
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| 4 | Button 2 | D1 | Next track |
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| 5 | Button 3 | D2 | Motor on/off |
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| 6 | Button 4 | D3 | Volume/speed |
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| 7 | Speaker + | D10 | PWM audio output |
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| 8 | Speaker - | GND | Or second PWM for BTL |
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| 9 | Motor ctrl | D6 | Through transistor on board |
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| 10 | NC | — | Not connected |
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| 11-16 | ... | ... | (varies by toy) |
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## Step 6: Wire It Up
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1. Solder header pins to the XIAO if not already done
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2. Solder the IS25LP128F flash chip to a small piece of perfboard
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next to the XIAO (VCC, GND, and 4 SPI wires)
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3. Run jumper wires from each SOP-16 breakout pin to the
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corresponding XIAO pin per your pin map
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4. Connect VCC breakout pin → XIAO 3V3 (to power the XIAO from toy batteries)
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5. Connect GND breakout pin → XIAO GND
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**IMPORTANT voltage check:** Before connecting power, measure the toy's
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VCC with batteries installed. If it's 2×AA (3V) or 3×AA (4.5V), you
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can connect directly to the XIAO's 3V3 pin (which accepts 1.7-3.6V on
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the nRF52840). If the toy uses more than 3 batteries (>5V), you MUST
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use the XIAO's 5V pin or add a regulator.
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For 2×AA: Connect through the TPS61220 boost to get stable 3.3V,
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then feed that to XIAO 3V3. Or if you're fine with the limited voltage
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range, connect the battery VCC directly to XIAO 3V3 — the nRF52840
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runs down to 1.7V, but the IS25LP128F needs 2.3V minimum, so you'll
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lose the bottom of the battery capacity.
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## Step 7: Update the Firmware Pin Mapping
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In baby_mobile_v2.ino, update the pin definitions to match your wiring:
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```cpp
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// Adjust these to match YOUR toy's pin map
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#define PIN_AUDIO_PWM D10 // connected to speaker trace
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#define PIN_AMP_SD -1 // set to -1 if toy has no separate amp
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#define PIN_MOTOR_PWM D6 // connected to motor transistor base
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#define PIN_BTN1 D0 // play/pause button
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#define PIN_BTN2 D1 // next
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#define PIN_BTN3 D2 // motor toggle
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#define PIN_BTN4 D3 // fourth button
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// ... set unused buttons to -1
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```
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If the toy's original circuit drives the speaker directly from the MCU
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(no amp chip), the XIAO's PWM output through the RC filter should work
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similarly. The original PWM was likely at a higher carrier frequency
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(the TRSF1602A runs at up to 13MHz), but 8kHz audio content will
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sound the same.
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## Common Toy PCB Patterns
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### Pattern A: Direct-drive speaker (most common in cheap toys)
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```
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MCU pin → small cap (100nF-1µF) → Speaker+
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Speaker- → GND
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```
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The MCU generates PWM, the cap blocks DC. No amplifier. Maximum volume
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is limited by the MCU's output current (~20mA). This works fine with
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the XIAO — just set the PWM pin and skip the PAM8302A entirely.
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### Pattern B: Transistor amplifier
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```
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MCU pin → resistor (1K-10K) → transistor base
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collector → Speaker+
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emitter → GND
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Speaker- → VCC
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```
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Again, the XIAO's PWM drives this directly. The transistor provides
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current amplification.
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### Pattern C: Dedicated amp IC (higher-end toys)
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```
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MCU pin → coupling cap → amp IC input
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amp IC output → Speaker ±
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```
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Here you're feeding the amp's input from the XIAO PWM through the
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existing coupling cap. Should work as-is. If there's a shutdown pin
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on the amp, trace it back to the MCU and control it from the XIAO.
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### Motor drive patterns:
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```
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Common: MCU pin → resistor → NPN base → motor → VCC
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emitter → GND
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diode across motor
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Simple: MCU pin → motor → VCC (tiny motors only, <20mA)
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```
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## Troubleshooting
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**No sound:** Check speaker connections. Measure voltage on the audio pin
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with a multimeter while playing — you should see ~1.5V average. If 0V,
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wrong pin. If VCC, the pin is driving high but not PWMing.
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**Distorted sound:** The RC filter values may need adjustment. Try
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increasing R2 or C7 to lower the cutoff frequency. Also check that
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analogWriteResolution(8) is set in setup().
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**Motor doesn't spin:** The XIAO pin may not source enough current to
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drive the transistor. Check if the base resistor is reasonable (1K-10K).
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If the original design was a darlington or MOSFET, the XIAO should
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drive it fine.
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**Buttons don't work:** Verify the button wires go to GND when pressed.
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Some toys use a resistor ladder (all buttons on one ADC pin with
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different resistor values) — this requires reading analogRead() instead
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of digitalRead(). The pin_discovery sketch will detect this.
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**Board doesn't power up:** The original MCU may have been pulling
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certain pins to specific states that other components depend on. Check
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if any traces from the MCU footprint go to enable pins on regulators
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or other ICs on the board.
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