2.2.3 DHT-11¶

Introduction¶

The digital temperature and humidity sensor DHT11 is a composite sensor that contains a calibrated digital signal output of temperature and humidity. The technology of a dedicated digital modules collection and the technology of the temperature and humidity sensing are applied to ensure that the product has high reliability and excellent stability.

The sensors include a wet element resistive sensor and a NTC temperature sensor and they are connected to a high performance 8-bit microcontroller.

Components¶

Schematic Diagram¶

Experimental Procedures¶

Step 1: Build the circuit.

Step 2: Go to the folder of the code.

cd /home/pi/raphael-kit/python/

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.

Code

Note

You can Modify/Reset/Copy/Run/Stop the code below. But before that, you need to go to source code path like raphael-kit/python. After modifying the code, you can run it directly to see the effect.

import RPi.GPIO as GPIO
import time

dhtPin = 17

GPIO.setmode(GPIO.BCM)

MAX_UNCHANGE_COUNT = 100

STATE_INIT_PULL_DOWN = 1
STATE_INIT_PULL_UP = 2
STATE_DATA_FIRST_PULL_DOWN = 3
STATE_DATA_PULL_UP = 4
STATE_DATA_PULL_DOWN = 5

def readDht11():
    GPIO.setup(dhtPin, GPIO.OUT)
    GPIO.output(dhtPin, GPIO.HIGH)
    time.sleep(0.05)
    GPIO.output(dhtPin, GPIO.LOW)
    time.sleep(0.02)
    GPIO.setup(dhtPin, GPIO.IN, GPIO.PUD_UP)

    unchanged_count = 0
    last = -1
    data = []
    while True:
        current = GPIO.input(dhtPin)
        data.append(current)
        if last != current:
            unchanged_count = 0
            last = current
        else:
            unchanged_count += 1
            if unchanged_count > MAX_UNCHANGE_COUNT:
                break

    state = STATE_INIT_PULL_DOWN

    lengths = []
    current_length = 0

    for current in data:
        current_length += 1

        if state == STATE_INIT_PULL_DOWN:
            if current == GPIO.LOW:
                state = STATE_INIT_PULL_UP
            else:
                continue
        if state == STATE_INIT_PULL_UP:
            if current == GPIO.HIGH:
                state = STATE_DATA_FIRST_PULL_DOWN
            else:
                continue
        if state == STATE_DATA_FIRST_PULL_DOWN:
            if current == GPIO.LOW:
                state = STATE_DATA_PULL_UP
            else:
                continue
        if state == STATE_DATA_PULL_UP:
            if current == GPIO.HIGH:
                current_length = 0
                state = STATE_DATA_PULL_DOWN
            else:
                continue
        if state == STATE_DATA_PULL_DOWN:
            if current == GPIO.LOW:
                lengths.append(current_length)
                state = STATE_DATA_PULL_UP
            else:
                continue
    if len(lengths) != 40:
        #print ("Data not good, skip")
        return False

    shortest_pull_up = min(lengths)
    longest_pull_up = max(lengths)
    halfway = (longest_pull_up + shortest_pull_up) / 2
    bits = []
    the_bytes = []
    byte = 0

    for length in lengths:
        bit = 0
        if length > halfway:
            bit = 1
        bits.append(bit)
    #print ("bits: %s, length: %d" % (bits, len(bits)))
    for i in range(0, len(bits)):
        byte = byte << 1
        if (bits[i]):
            byte = byte | 1
        else:
            byte = byte | 0
        if ((i + 1) % 8 == 0):
            the_bytes.append(byte)
            byte = 0
    #print (the_bytes)
    checksum = (the_bytes[0] + the_bytes[1] + the_bytes[2] + the_bytes[3]) & 0xFF
    if the_bytes[4] != checksum:
        #print ("Data not good, skip")
        return False

    return the_bytes[0], the_bytes[2]

def main():

    while True:
        result = readDht11()
        if result:
            humidity, temperature = result
            print ("humidity: %s %%,  Temperature: %s ℃" % (humidity, temperature))
        time.sleep(1)

def destroy():
    GPIO.cleanup()

if __name__ == '__main__':
    try:
        main()
    except KeyboardInterrupt:
        destroy()

Code Explanation

def readDht11():
    GPIO.setup(dhtPin, GPIO.OUT)
    GPIO.output(dhtPin, GPIO.HIGH)
    time.sleep(0.05)
    GPIO.output(dhtPin, GPIO.LOW)
    time.sleep(0.02)
    GPIO.setup(dhtPin, GPIO.IN, GPIO.PUD_UP)
    unchanged_count = 0
    last = -1
    data = []
    #...

This function is used to implement the functions of DHT11. It stores the detected data in the the_bytes[] 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.

checksum = (the_bytes[0] + the_bytes[1] + the_bytes[2] + the_bytes[3]) & 0xFF
if the_bytes[4] != checksum:
    #print ("Data not good, skip")
    return False

return the_bytes[0], the_bytes[2]

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 the_bytes[0] and the_bytes[2] and output “Humidity =43%，Temperature =60C”.