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2.2.3 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. You will learn how to set up the sensor, read temperature in both Celsius and Fahrenheit, and obtain humidity readings. This project introduces you to working with external sensors, handling real-time data, and basic exception handling in Python.

Required Components

In this project, we need the following components.

../_images/list_2.2.3_dht-11.png

Schematic Diagram

../_images/image326.png

Experimental Procedures

Step 1: Build the circuit.

../_images/image207.png

Step 2: Go to the folder of the code.

cd ~/davinci-kit-for-raspberry-pi/python_pi5/

Step 3: Run the executable file.

sudo python3 2.2.3_DHT.py

After the code runs, the program will print the temperature and humidity detected by DHT11 on the computer screen.

Warning

If there is an error prompt RuntimeError: Cannot determine SOC peripheral base address, please refer to If gpiozero doesn’t work.

Code

Note

You can Modify/Reset/Copy/Run/Stop the code below. But before that, you need to go to source code path like davinci-kit-for-raspberry-pi/python_pi5.

from gpiozero import OutputDevice, InputDevice
import time


class DHT11():
   MAX_DELAY_COUINT = 100
   BIT_1_DELAY_COUNT = 10
   BITS_LEN = 40

   def __init__(self, pin, pull_up=False):
      self._pin = pin
      self._pull_up = pull_up


   def read_data(self):
      bit_count = 0
      delay_count = 0
      bits = ""

      # -------------- send start --------------
      gpio = OutputDevice(self._pin)
      gpio.off()
      time.sleep(0.02)

      gpio.close()
      gpio = InputDevice(self._pin, pull_up=self._pull_up)

      # -------------- wait response --------------
      while gpio.value == 1:
            pass

      # -------------- read data --------------
      while bit_count < self.BITS_LEN:
            while gpio.value == 0:
               pass

            # st = time.time()
            while gpio.value == 1:
               delay_count += 1
               # break
               if delay_count > self.MAX_DELAY_COUINT:
                  break
            if delay_count > self.BIT_1_DELAY_COUNT:
               bits += "1"
            else:
               bits += "0"

            delay_count = 0
            bit_count += 1

      # -------------- verify --------------
      humidity_integer = int(bits[0:8], 2)
      humidity_decimal = int(bits[8:16], 2)
      temperature_integer = int(bits[16:24], 2)
      temperature_decimal = int(bits[24:32], 2)
      check_sum = int(bits[32:40], 2)

      _sum = humidity_integer + humidity_decimal + temperature_integer + temperature_decimal

      # print(bits)
      # print(humidity_integer, humidity_decimal, temperature_integer, temperature_decimal)
      # print(f'sum:{_sum}, check_sum:{check_sum}')
      # print()

      if check_sum != _sum:
            humidity = 0.0
            temperature = 0.0
      else:
            humidity = float(f'{humidity_integer}.{humidity_decimal}')
            temperature = float(f'{temperature_integer}.{temperature_decimal}')

      # -------------- return --------------
      return humidity, temperature


if __name__ == '__main__':
   dht11 = DHT11(17)
   while True:
      humidity, temperature = dht11.read_data()
      print(f"{time.time():.3f}  temperature:{temperature}°C  humidity: {humidity}%")
      time.sleep(2)

Code Explanation

def read_data(self):
   bit_count = 0
   delay_count = 0
   bits = ""

   # -------------- send start --------------
   gpio = OutputDevice(self._pin)
   gpio.off()
   time.sleep(0.02)

   gpio.close()
   gpio = InputDevice(self._pin, pull_up=self._pull_up)
   #...

This function is used to implement the functions of DHT11. It stores the detected data in the bits[] array. DHT11 transmits data of 40 bits at a time. The first 16 bits are related to humidity, the middle 16 bits are related to temperature, and the last eight bits are used for verification. The data format is:

8bit humidity integer data +8bit humidity decimal data +8bit temperature integer data + 8bit temperature decimal data + 8bit check bit.

When the validity is detected via the check bit, the function returns two results: 1. error; 2. humidity and temperature.

_sum = humidity_integer + humidity_decimal + temperature_integer + temperature_decimal

if check_sum != _sum:
      humidity = 0.0
      temperature = 0.0
else:
      humidity = float(f'{humidity_integer}.{humidity_decimal}')
      temperature = float(f'{temperature_integer}.{temperature_decimal}')

For example, if the received date is 00101011(8-bit value of humidity integer) 00000000 (8-bit value of humidity decimal) 00111100 (8-bit value of temperature integer) 00000000 (8-bit value of temperature decimal) 01100111 (check bit)

Calculation:

00101011+00000000+00111100+00000000=01100111.

If the final result is equal to the check bit data, the data transmission is abnormal: return False.

If the final result is equal to the check bit data, the received data is correct, then there will return humidity and temperature and output "Humidity =43%，Temperature =60C".