Practical case: Rack Thermal Control with Raspberry Pi 5

#!/usr/bin/env python3
"""
Server Rack Thermal Watchdog
Monitors ambient temperature and controls a cooling fan via a relay.
Implements hysteresis to prevent relay chatter.
"""

import time
import logging
import sys

logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s - %(levelname)s - %(message)s',
    handlers=[logging.StreamHandler(sys.stdout)]
)

# ---------------------------------------------------------
# Hardware Abstraction Layer
# ---------------------------------------------------------
try:
    import smbus2
    import bme280
    from gpiozero import OutputDevice
    HARDWARE_AVAILABLE = True
    logging.info("Hardware libraries detected. Running in PHYSICAL mode.")
except ImportError:
    HARDWARE_AVAILABLE = False
    logging.warning("Hardware libraries missing. Running in MOCK mode.")

class BME280Adapter:
    """Adapter for the BME280 I2C sensor."""
    def __init__(self, port=1, address=0x76):
        self.mock_temp = 25.0
        self.mock_direction = 1.0 # 1.0 for heating, -1.0 for cooling

        if HARDWARE_AVAILABLE:
            try:
                self.bus = smbus2.SMBus(port)
                self.address = address
                # Load calibration parameters
                self.calibration_params = bme280.load_calibration_params(self.bus, self.address)
                self.is_mock = False
            except Exception as e:
                logging.error(f"Failed to initialize physical BME280: {e}. Falling back to mock.")
                self.is_mock = True
        else:
            self.is_mock = True

    def read_temperature(self) -> float:
        """Returns temperature in Celsius."""
        if not self.is_mock:
            try:
                data = bme280.sample(self.bus, self.address, self.calibration_params)
                return data.temperature
            except Exception as e:
                logging.error(f"Sensor read error: {e}")
                return 999.0 # Fail-safe high temperature to trigger cooling
        else:
            # Simulate temperature rising to 32C then falling to 24C
            self.mock_temp += (0.5 * self.mock_direction)
            if self.mock_temp >= 32.0:
                self.mock_direction = -1.0
            elif self.mock_temp <= 24.0:
                self.mock_direction = 1.0
            return self.mock_temp

class RelayAdapter:
    """Adapter for the GPIO Relay module."""
    def __init__(self, pin=17):
        self.state = False
        if HARDWARE_AVAILABLE:
            try:
                # active_high=True means setting to True outputs 3.3V
                self.relay = OutputDevice(pin, active_high=True, initial_value=False)
                self.is_mock = False
            except Exception as e:
                logging.error(f"Failed to initialize physical relay on GPIO {pin}: {e}")
                self.is_mock = True
        else:
            self.is_mock = True

    def turn_on(self):
        if not self.state:
            logging.info("ACTION: Energizing relay (Fan ON)")
            self.state = True
            if not self.is_mock:
                self.relay.on()

    def turn_off(self):
        if self.state:
            logging.info("ACTION: De-energizing relay (Fan OFF)")
            self.state = False
            if not self.is_mock:
                self.relay.off()

# ---------------------------------------------------------
# Main Watchdog Logic
# ---------------------------------------------------------
def main():
    # Configuration thresholds
    TEMP_THRESHOLD_ON = 30.0  # Turn on fan above 30.0 C
    TEMP_THRESHOLD_OFF = 27.0 # Turn off fan below 27.0 C
    POLL_INTERVAL = 2.0       # Seconds between readings

    logging.info(f"Starting Server Rack Thermal Watchdog.")
    logging.info(f"Thresholds -> ON: >{TEMP_THRESHOLD_ON}°C, OFF: <{TEMP_THRESHOLD_OFF}°C")

    sensor = BME280Adapter(port=1, address=0x76)
    fan_relay = RelayAdapter(pin=17)

    try:
        while True:
            current_temp = sensor.read_temperature()
            logging.info(f"Current Temperature: {current_temp:.2f}°C")

            # Hysteresis Logic
            if current_temp > TEMP_THRESHOLD_ON:
                fan_relay.turn_on()
            elif current_temp < TEMP_THRESHOLD_OFF:
                fan_relay.turn_off()
            # If temperature is between OFF and ON thresholds, do nothing (maintain current state)

            time.sleep(POLL_INTERVAL)

    except KeyboardInterrupt:
        logging.info("Watchdog terminated by user. Shutting down safely.")
    finally:
        # Ensure fan is turned off on script exit (optional, depends on fail-safe preference)
        fan_relay.turn_off()
        logging.info("Cleanup complete.")

if __name__ == "__main__":
    main()