Initial commit: project plan for breath-synchronized fan system
Biofeedback system that detects breathing via sensors and synchronizes PC tower fans to inflate a bag during inhalation phases. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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# Breath-Synchronized Fan & Bag Inflation System
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## Project Overview
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A biofeedback system that detects a user's breathing pattern via sensors and synchronizes computer tower fans to inflate a bag during inhalation phases.
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---
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## System Architecture
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```
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┌─────────────────┐ ┌──────────────────┐ ┌─────────────────┐
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│ Breath Sensor │────▶│ Microcontroller │────▶│ Fan Controller │
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│ (on chest/nose)│ │ (Arduino/ESP32) │ │ (PWM/MOSFET) │
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└─────────────────┘ └──────────────────┘ └─────────────────┘
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│
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▼
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┌─────────────────┐
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│ PC Tower Fans │
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│ (12V, 3-4 pin) │
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└────────┬────────┘
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│
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▼
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┌─────────────────┐
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│ Inflation Bag │
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│ (sealed inlet) │
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└─────────────────┘
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```
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---
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## Component Options
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### 1. Breath Sensors (Choose One)
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| Sensor Type | Pros | Cons | Est. Cost |
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|-------------|------|------|-----------|
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| **Chest Strap Stretch Sensor** | Non-invasive, comfortable | Requires calibration per user | $15-30 |
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| **Nasal Airflow Thermistor** | Very accurate, medical-grade | Intrusive, needs cleaning | $5-15 |
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| **Piezoelectric Belt** | Simple, reliable | Can slip, position-sensitive | $10-20 |
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| **BioImpedance (AD8232)** | Multi-signal (breath + heart) | Complex setup, needs electrodes | $10-25 |
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| **Microphone/Sound Sensor** | No contact required | Ambient noise interference | $5-10 |
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| **Pressure Sensor (BMP280)** | Very sensitive | Needs sealed enclosure | $5-10 |
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**Recommendation:** Chest strap with conductive rubber stretch sensor or piezoelectric element - balances accuracy with comfort for seated use.
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### 2. Microcontroller
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| Option | Pros | Cons | Est. Cost |
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|--------|------|------|-----------|
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| **Arduino Nano** | Simple, lots of tutorials | Limited processing | $5-15 |
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| **ESP32** | WiFi/BLE, dual-core, more pins | Overkill for basic version | $8-15 |
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| **Raspberry Pi Pico** | Powerful, cheap, PIO for precise timing | Less community support | $4-8 |
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**Recommendation:** Arduino Nano for simplicity, ESP32 if you want wireless monitoring/control later.
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### 3. Fan Control
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| Method | Description | Components Needed |
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|--------|-------------|-------------------|
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| **PWM via MOSFET** | Speed control 0-100% | IRLZ44N MOSFET, flyback diode |
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| **Relay (On/Off)** | Simple binary control | 5V relay module |
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| **Fan Controller IC** | Dedicated chip | EMC2301 or similar |
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**Recommendation:** PWM via MOSFET - allows smooth ramping that mirrors breath intensity.
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### 4. Fans
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- Standard 120mm PC tower fans (12V, 0.2-0.5A each)
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- 4-pin PWM fans allow direct speed control
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- 3-pin fans need external PWM circuit
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- Consider 2-4 fans for adequate airflow
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### 5. Inflation Bag
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- Medical ventilator bags (Ambu-style) - designed for this
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- Weather balloon material with one-way valve
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- Custom sewn ripstop nylon with inlet fitting
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- Volume: 2-5 liters typical for breath visualization
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---
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## Bill of Materials (Estimated)
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| Component | Quantity | Est. Cost |
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|-----------|----------|-----------|
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| Stretch sensor (chest strap) | 1 | $20 |
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| Arduino Nano | 1 | $10 |
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| IRLZ44N MOSFET | 2-4 | $5 |
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| 1N4007 Diodes (flyback) | 2-4 | $2 |
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| 120mm PC Fans | 2-4 | $20-40 |
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| 12V 2A Power Supply | 1 | $10 |
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| Breadboard + Jumpers | 1 set | $10 |
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| Inflation bag + fitting | 1 | $15-30 |
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| Enclosure/mounting | - | $10-20 |
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| **Total** | | **$100-150** |
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---
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## Software Architecture
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```
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┌─────────────────────────────────────────────────────────────┐
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│ Main Loop │
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├─────────────────────────────────────────────────────────────┤
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│ │
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│ 1. READ sensor value (analog 0-1023) │
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│ │ │
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│ ▼ │
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│ 2. FILTER noise (moving average, 5-10 samples) │
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│ │ │
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│ ▼ │
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│ 3. DETECT breath phase │
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│ ├── Rising signal = INHALE │
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│ ├── Falling signal = EXHALE │
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│ └── Stable = PAUSE │
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│ │ │
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│ ▼ │
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│ 4. MAP breath intensity to fan speed (0-255 PWM) │
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│ │ │
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│ ▼ │
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│ 5. RAMP fan speed (smooth transitions) │
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│ │ │
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│ ▼ │
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│ 6. OUTPUT PWM to MOSFET gate │
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│ │
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│ Loop rate: 50-100 Hz │
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│ │
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└─────────────────────────────────────────────────────────────┘
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```
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### Key Algorithm: Breath Detection
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```cpp
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// Pseudocode
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float baseline = calibrate(); // 5-second calm breathing average
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float threshold = baseline * 0.1; // 10% deviation triggers
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void loop() {
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float current = readSensor();
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float filtered = movingAverage(current, 10);
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if (filtered > baseline + threshold) {
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// INHALING - ramp fans up proportionally
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int intensity = map(filtered, baseline, maxInhale, 0, 255);
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setFanSpeed(intensity);
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} else {
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// EXHALING or PAUSE - fans off, bag deflates passively
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setFanSpeed(0);
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}
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// Adaptive baseline (slow drift compensation)
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baseline = baseline * 0.999 + filtered * 0.001;
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}
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```
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---
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## Wiring Diagram
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```
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+12V (Fan Power)
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│
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│
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Arduino Nano ┌────┴────┐
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┌───────────┐ │ │
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│ D9 │───────┬──────────▶│G FAN │
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│ (PWM) │ │ │ 120mm │
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│ │ ┌──┴──┐ │ │
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│ GND │────┤IRLZ │ └────┬────┘
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│ │ │44N │ │
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│ A0 │◀───┤ ├─────────────┘
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│ (Sensor) │ └──┬──┘ GND
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│ │ │
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│ 5V │───────┼──────▶ Sensor VCC
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│ GND │───────┼──────▶ Sensor GND
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└───────────┘ │
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│
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┌──┴──┐
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│1N4007│ (Flyback diode)
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└──┬──┘
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│
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GND
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```
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---
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## Implementation Phases
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### Phase 1: Sensor Validation
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- [ ] Select and acquire breath sensor
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- [ ] Wire sensor to Arduino
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- [ ] Write basic serial plotter sketch
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- [ ] Verify clean breath signal on serial plotter
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- [ ] Tune filtering parameters
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### Phase 2: Fan Control
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- [ ] Wire MOSFET circuit on breadboard
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- [ ] Test PWM control with potentiometer
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- [ ] Verify fan speed varies 0-100%
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- [ ] Add flyback diode protection
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- [ ] Test with multiple fans if needed
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### Phase 3: Integration
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- [ ] Combine sensor input with fan output
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- [ ] Implement breath detection algorithm
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- [ ] Add calibration routine (button-triggered)
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- [ ] Tune response curve (linear vs. exponential)
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- [ ] Test full loop with user breathing
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### Phase 4: Bag System
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- [ ] Design or acquire inflation bag
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- [ ] Create sealed inlet with fan output
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- [ ] Add one-way valve (prevents backflow on exhale)
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- [ ] Test bag inflation matches breath cycle
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- [ ] Adjust fan power for bag volume
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### Phase 5: Enclosure & Polish
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- [ ] Design mounting for fans and bag
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- [ ] Create comfortable sensor wearable
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- [ ] Add status LEDs (power, breathing, calibrating)
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- [ ] Optional: Add OLED display for breath rate
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- [ ] Optional: Add WiFi for remote monitoring
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---
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## Testing Checklist
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- [ ] Sensor responds to chest expansion (not just movement)
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- [ ] No false triggers from ambient noise/vibration
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- [ ] Fan ramps smoothly (no jerky motion)
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- [ ] Bag inflates during inhale, deflates during exhale
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- [ ] System works for different breathing rates (8-20 breaths/min)
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- [ ] Calibration works for different users
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- [ ] Safe shutdown if sensor disconnected
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- [ ] No overheating after 30+ min operation
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---
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## Safety Considerations
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1. **Electrical**: Use proper flyback diodes - fan motors create back-EMF
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2. **Ventilation**: Ensure fans don't overheat in enclosure
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3. **Bag Material**: Don't use latex (allergy risk) - use medical silicone or nylon
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4. **Power**: Fuse the 12V supply line
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5. **User Comfort**: Sensor shouldn't restrict breathing or movement
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---
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## Optional Enhancements
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| Feature | Complexity | Description |
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|---------|------------|-------------|
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| Breath rate display | Low | Count peaks per minute, show on OLED |
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| Guided breathing mode | Medium | LEDs/display guide user to target breath rate |
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| Data logging | Medium | SD card or WiFi upload of session data |
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| Multiple bags | Medium | Different bags for inhale vs exhale |
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| Haptic feedback | Medium | Vibration motor mirrors breath |
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| Sound sync | High | Audio tones follow breath pattern |
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| VR integration | High | Breath controls virtual environment |
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