Python Program 3: Interfacing DHT11 Sensor with Raspberry Pi

OBJECTIVE:

To interface the DHT11 sensor with the Raspberry Pi and write a Python program to print the temperature and humidity readings in real-time.

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RESOURCES REQUIRED:

• Raspberry Pi (any model with GPIO support)

• DHT11 Sensor

• Breadboard

• Jumper wires

• Python Libraries: adafruit_dht, psutil

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THEORY:

DHT11 Sensor Basics:

• The DHT11 sensor is a low-cost digital sensor used for measuring temperature and humidity.

• It provides readings for relative humidity (range: 20% to 90% RH) and temperature (range: 0°C to 50°C).

• The sensor communicates with the Raspberry Pi through serial communication (one-wire protocol).

• The DHT11 sensor has 4 pins:

  • VCC (power)

  • GND (ground)

  • Data (for data communication)

  • NC (not connected, not used)

Data from the sensor is sent in a digital format where the sensor sends pulses of different durations to represent 1s and 0s. These pulses are decoded by the Raspberry Pi to retrieve temperature and humidity readings.

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Circuit Diagram:

• VCC of DHT11 connects to +5V on Raspberry Pi (or 3.3V, depending on model).

• GND of DHT11 connects to Raspberry Pi GND.

• DATA pin of DHT11 connects to GPIO4 (pin 7 on Raspberry Pi) for data communication.

• NC is unused in the DHT11.

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Python Code: Read and Print Temperature and Humidity

import time
import board
import adafruit_dht
import psutil

# Kill any previous process that might block GPIO access
for proc in psutil.process_iter():
    if proc.name() == 'libgpiod_pulsein' or proc.name() == 'libgpiod_pulsei':
        proc.kill()

# Setup DHT11 sensor on GPIO4 (D4)
sensor = adafruit_dht.DHT11(board.D4)

# Main loop to read temperature and humidity
while True:
    try:
        # Reading temperature and humidity
        temp = sensor.temperature
        humidity = sensor.humidity
        
        # Print the results
        print("Temperature: {}°C    Humidity: {}%".format(temp, humidity))
    
    except RuntimeError as error:
        print(error.args[0])  # Print errors if any
        time.sleep(2.0)       # Wait before trying again
        continue
    
    except Exception as error:
        sensor.exit()         # Clean up sensor if there is any issue
        raise error
    
    time.sleep(2.0)           # Delay before the next reading

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Explanation of the Program:

Import Libraries:

• time → Provides sleep() function for pauses between readings.
• board → Defines the GPIO pin assignments.
• adafruit_dht → Provides the library to interact with DHT11 sensor.
• psutil → Monitors and terminates processes that might block GPIO access (specifically, the libgpiod_pulsein or libgpiod_pulsei process).

Sensor Setup:

• The DHT11 sensor is initialized with adafruit_dht.DHT11(board.D4) — using GPIO4 (D4) as the data input pin.

Reading Temperature and Humidity:

• sensor.temperature reads the temperature in °C.
• sensor.humidity reads the relative humidity in %.

Error Handling:

• The program attempts to fetch the sensor readings in a try block. If an error occurs (like a communication issue with the sensor), a RuntimeError is caught and printed.

• If a more serious exception occurs (e.g., a hardware problem), the program exits cleanly, resetting the sensor with sensor.exit().

Delay:

• After each reading, time.sleep(2.0) is used to introduce a 2-second delay before the next reading to avoid overload and allow the sensor to stabilize.

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