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2.14 DHT-11

Introduction

In this lesson, you will learn how to connect and read data from a DHT11 temperature and humidity sensor using a Raspberry Pi. The DHT11 is a reliable, low-cost digital sensor used in various applications, such as weather monitoring and environmental control. This project introduces you to interfacing with external sensors, handling real-time data, and basic exception handling in Python. By the end, you will know how to read temperature (in both Celsius and Fahrenheit) and humidity values.

What You’ll Need

Here are the components required for this project:

COMPONENT INTRODUCTION	PURCHASE LINK
Humiture Sensor Module	BUY
Jumper Wires	BUY
Fusion HAT+	-
Raspberry Pi	-

Circuit Diagram

Below is the schematic diagram for connecting the DHT11 sensor:

Wiring Diagram

Build the circuit as shown in the wiring diagram below:

Running the Example

All example code used in this tutorial is available in the ai-lab-kit directory. Follow these steps to run the example:

cd ~/ai-lab-kit/python/
sudo python3 2.14_DHT.py

This Python script reads temperature and humidity data from a DHT11 sensor connected to a Fusion HAT+ using GPIO pin 17. When executed:

Data is printed to the console every 1 seconds, providing real-time updates.
If the data reading is not successful, the script outputs “time out” , indicating an error in data reading.

Code

Below is the Python code used for this project:

# Import the DHT11 temperature & humidity sensor module
from fusion_hat.modules import DHT11
# Import sleep function for delays
from time import sleep

# Create a DHT11 sensor object on GPIO pin 17
dht11 = DHT11(pin=17)

# Loop forever
while True:
   # Read data from the DHT11 sensor
   result = dht11.read()

   # If data was successfully read, unpack humidity and temperature
   if result:
      humidity, temperature = result

      # Print the humidity and temperature values
      print("humidity: %s %%,  Temperature: %s C" % (humidity, temperature))
   else:
      # Print timeout message if reading failed
      print("time out")

   # Wait 1 second before the next reading
   sleep(1)

Understanding the Code

Import:

# Import the DHT11 temperature & humidity sensor module
from fusion_hat.modules import DHT11
# Import sleep function for delays
from time import sleep

These lines import the necessary modules for the script.

Sensor Initialization:
```
dht11 = DHT11(pin=17)
```
This line initializes the DHT11 sensor on GPIO pin 17.

Data Reading:

while True:
   result = dht11.read()
   if result:
      humidity, temperature = result
      print ("humidity: %s %%,  Temperature: %s C`" % (humidity, temperature))
   else:
      print("time out")
   sleep(1)

The read method sends a start signal to the DHT11.

Troubleshooting

No Output or Constant 0.0°C and 0.0% Values:
- Cause: Sensor connection issues.
- Solution: Ensure the DHT11 sensor is properly connected to GPIO pin 17, power, and ground.
Script Crashes with ValueError:
- Cause: Corrupt or incomplete data from the sensor.
- Solution: Check for stable connections and ensure the DHT11 is functioning properly. Ensure the data line is not noisy or floating.
High CPU Usage:
- Cause: Continuous polling of the GPIO pin without delays.
- Solution: The code already has built-in delays (e.g., time.sleep(2)). Ensure these are not removed.
Incorrect Temperature or Humidity Values:
- Cause: Calibration issue or sensor malfunction.
- Solution: Test the DHT11 in a controlled environment to verify accuracy. Replace the sensor if the issue persists.

Script Does Not Exit Gracefully:

Cause: The script lacks proper exception handling for manual interruptions.
Solution: Wrap the main loop in a try...except KeyboardInterrupt block to allow clean termination.

try:
   while True:
         humidity, temperature = dht11.read()
         print(f"{time.time():.3f}  Temperature: {temperature}°C  Humidity: {humidity}%")
         time.sleep(2)
except KeyboardInterrupt:
   print("Exiting gracefully...")

Extendable Ideas

Data Logging: Save the temperature and humidity readings to a file for analysis:

with open("dht11_log.txt", "a") as log_file:
    log_file.write(f"{time.time():.3f}, {temperature}°C, {humidity}%\n")

Threshold-Based Alerts: Trigger an alert when the temperature or humidity exceeds specific thresholds:

if temperature > 30:
    print("Warning: High temperature!")
if humidity > 70:
    print("Warning: High humidity!")

Conclusion

This project demonstrates how to interface with the DHT11 temperature and humidity sensor using a Fusion HAT+. By understanding how to read and process sensor data, you can integrate such sensors into various practical applications, from weather stations to automated climate control systems.