Bathroom Fan Humidity Automation

The Problem

Bathroom exhaust fans are often forgotten, leading to excessive humidity that causes mold growth and paint damage. Manual operation is inconvenient, and leaving the fan running wastes energy. A smart solution was needed to automate fan control based on real-time humidity levels.

Hardware Components

Core System

• Arduino Uno: Main microcontroller for logic and control
• DHT22 Temperature & Humidity Sensor: Measures ambient humidity with ±2% accuracy
• Standard Servo Motor: Mechanically actuates the fan switch
• 5V Power Supply: Powers the Arduino and servo

Why DHT22?

The DHT22 (also known as AM2302) was chosen for several key advantages:

• ±2% humidity accuracy vs ±5% for the cheaper DHT11
• 0-100% humidity range covers all conditions
• -40°C to 80°C temperature range handles bathroom temperature swings
• Simple digital interface requires only one GPIO pin
• Low cost (~$5) makes it accessible for DIY projects

Technical Approach

1. Humidity Monitoring

The DHT22 sensor provides both temperature and humidity readings:

#include <DHT.h>

#define DHTPIN 2
#define DHTTYPE DHT22
DHT dht(DHTPIN, DHTTYPE);

void loop() {
  float humidity = dht.readHumidity();
  float temperature = dht.readTemperature();

  if (isnan(humidity) || isnan(temperature)) {
    Serial.println("Sensor read error!");
    return;
  }

  controlFan(humidity);
  delay(2000); // DHT22 requires 2s between reads
}

2. Hysteresis Control

To prevent the fan from rapidly cycling on and off when humidity hovers near the threshold, I implemented hysteresis:

#define HUMIDITY_HIGH 70  // Turn fan ON above 70%
#define HUMIDITY_LOW 60   // Turn fan OFF below 60%

bool fanRunning = false;

void controlFan(float humidity) {
  if (!fanRunning && humidity > HUMIDITY_HIGH) {
    // Turn fan ON
    servo.write(90);  // Press switch
    fanRunning = true;
    Serial.println("Fan activated");
  }
  else if (fanRunning && humidity < HUMIDITY_LOW) {
    // Turn fan OFF
    servo.write(0);   // Release switch
    fanRunning = false;
    Serial.println("Fan deactivated");
  }
}

This 10% hysteresis band prevents oscillation and reduces mechanical wear.

3. Servo Calibration

The servo physically presses the wall switch. Calibration was critical:

1. Position Testing: Found optimal angles (0° = OFF, 90° = ON)
2. Mounting Bracket: 3D-printed bracket holds servo aligned with switch
3. Travel Limits: Limited servo rotation to prevent mechanical stress
4. Hold Time: Brief 500ms actuation ensures reliable contact

4. Error Handling

Added robustness for sensor failures:

int consecutiveErrors = 0;
#define MAX_ERRORS 5

void loop() {
  float humidity = dht.readHumidity();

  if (isnan(humidity)) {
    consecutiveErrors++;
    if (consecutiveErrors >= MAX_ERRORS) {
      // Safe mode: turn fan ON to prevent humidity buildup
      servo.write(90);
      Serial.println("Sensor failure - entering safe mode");
    }
    return;
  }

  consecutiveErrors = 0;  // Reset on successful read
  controlFan(humidity);
}

System Operation

Normal Cycle

1. Baseline: Humidity around 40-50% (fan OFF)
2. Shower Event: Humidity rises above 70%
3. Fan Activation: Servo presses switch, fan turns ON
4. Ventilation: Fan runs until humidity drops below 60%
5. Fan Deactivation: Servo releases switch, returns to idle

Real-World Performance

• Response Time: Fan activates within 5 seconds of threshold breach
• Runtime: Typically 10-15 minutes after shower completion
• Accuracy: ±2% humidity measurement from DHT22
• Reliability: Zero false activations over 3-month deployment
• Power Consumption: Less than 1W when idle

Results

✅ Automated Operation: No manual intervention required ✅ Energy Efficient: Fan only runs when needed ✅ Mold Prevention: Maintains humidity below 60% ✅ Simple Installation: No electrical wiring modification ✅ Cost Effective: Total build cost under $20

Video Demonstration

Watch the system in action, including humidity sensor readings, servo actuation, and real-time fan control:

Future Enhancements

Potential improvements for v2:

• WiFi Connectivity: Remote monitoring via ESP32
• Data Logging: Track humidity trends over time
• Smart Scheduling: Learn typical shower times
• IFTTT Integration: Trigger other smart home devices
• Battery Backup: Maintain operation during power outages

Technical Resources

DHT22 Sensor Specifications

• Humidity Range: 0-100% RH
• Humidity Accuracy: ±2% RH
• Temperature Range: -40°C to 80°C
• Temperature Accuracy: ±0.5°C
• Sampling Rate: 0.5 Hz (one reading every 2 seconds)
• Operating Voltage: 3.3-6V DC
• Signal Type: Single-wire digital

Key Libraries Used

• DHT Sensor Library by Adafruit: For DHT22 communication
• Servo Library: For servo motor control (Arduino standard library)

Lessons Learned

1. Sensor Selection Matters: The DHT22’s superior accuracy justified the slight cost increase over DHT11
2. Mechanical Reliability: Servo calibration and proper mounting are critical for long-term operation
3. Hysteresis is Essential: Prevents rapid cycling and extends component lifespan
4. Fail-Safe Design: Safe mode activation prevents humidity damage during sensor failures
5. Testing Takes Time: Real-world environmental testing revealed edge cases not apparent in bench testing