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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.

Required Components

In this project, we need the following components.

It’s definitely convenient to buy a whole kit, here’s the link:

Name	ITEMS IN THIS KIT	LINK
Raphael Kit	337	Raphael Kit

You can also buy them separately from the links below.

COMPONENT INTRODUCTION	PURCHASE LINK
GPIO Extension Board	BUY
Breadboard	BUY
Jumper Wires	BUY
Resistor	BUY
Humiture Sensor Module	BUY

Schematic Diagram

Experimental Procedures

Step 1: Build the circuit.

Step 2: Go to the folder of the code.

cd ~/raphael-kit/c/2.2.3/

Step 3: Compile the code.

gcc 2.2.3_DHT.c -lwiringPi

Step 4: Run the executable file.

sudo ./a.out

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

Note

If it does not work after running, or there is an error prompt: "wiringPi.h: No such file or directory", please refer to Install and Check the WiringPi.

Code

#include <wiringPi.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

#define MAXTIMINGS 85  // Maximum number of timing transitions

int dht11_dat[5] = {0, 0, 0, 0, 0};  // Data array to hold sensor values

// Function to read data from DHT11 sensor
void read_dht11_dat(int GPIOPIN)
{
    uint8_t currState;
    uint8_t laststate = HIGH;
    uint8_t counter = 0;
    uint8_t j = 0;
    uint8_t i;
    float f; // Temperature in Fahrenheit

    // Reset data array before each read
    dht11_dat[0] = dht11_dat[1] = dht11_dat[2] = dht11_dat[3] = dht11_dat[4] = 0;

    // Pull pin down for 18 milliseconds to initiate communication
    pinMode(GPIOPIN, OUTPUT);
    digitalWrite(GPIOPIN, LOW);
    delay(18);

    // Then pull it up for 40 microseconds
    digitalWrite(GPIOPIN, HIGH);
    delayMicroseconds(40);

    // Prepare to read the pin
    pinMode(GPIOPIN, INPUT);

    // Detect change and read data
    for (i = 0; i < MAXTIMINGS; i++)
    {
        counter = 0;

        // Count how long each state lasts
        while (digitalRead(GPIOPIN) == laststate)
        {
            counter++;
            delayMicroseconds(2);
            if (counter == 255)
            {
                break;
            }
        }

        // Save the current state
        laststate = digitalRead(GPIOPIN);

        if (counter == 255) break;

        // Ignore first 3 transitions (DHT11 response signal)
        if ((i >= 4) && (i % 2 == 0))
        {
            // Shift bits and store data
            dht11_dat[j/8] <<= 1;
            if (counter > 16)
            {
                dht11_dat[j/8] |= 1;
            }

            j++;
        }
    }

    // Check if we received 40 bits (5 bytes) and verify checksum
    if ((j >= 40) && (dht11_dat[4] == ((dht11_dat[0] + dht11_dat[1] + dht11_dat[2] + dht11_dat[3]) & 0xFF)) )
    {
        // Convert temperature to Fahrenheit
        f = dht11_dat[2] * 9.0 / 5.0 + 32;
        printf("Humidity = %d.%d %% Temperature = %d.%d °C (%.1f °F)\n",
               dht11_dat[0], dht11_dat[1], dht11_dat[2], dht11_dat[3], f);
    }
    else
    {
        printf("Data not good, skip\n");
    }
}

int main (void)
{
    printf("Raspberry Pi wiringPi DHT11 Temperature test program\n");

    // Initialize wiringPi using BCM GPIO pin numbering
    if (wiringPiSetupGpio() == -1)
    {
        exit(1);
    }

    while(1)
    {
        // Read data from DHT11 connected to GPIO pin 17
        read_dht11_dat(17);
        delay(1000); // Wait 1 second before next read
    }

    return 0;
}

Code Explanation

Include Headers: The code includes necessary headers for wiringPi functions and standard input/output.
```
#include <wiringPi.h>
#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>
```
Define Constants:
- MAXTIMINGS: The maximum number of timing transitions expected from the DHT11 sensor (85).
```
#define MAXTIMINGS 85  // Maximum number of timing transitions
```
Global Data Array:
- dht11_dat[5]: An array to hold the 5 bytes of data received from the DHT11 sensor.
```
int dht11_dat[5] = {0, 0, 0, 0, 0};  // Data array to hold sensor values
```

Function read_dht11_dat(int GPIOPIN): Reads data from the DHT11 sensor connected to the specified GPIO pin.

Initialization: Reset the dht11_dat array to zero before each read.

dht11_dat[0] = dht11_dat[1] = dht11_dat[2] = dht11_dat[3] = dht11_dat[4] = 0;

Start Signal: Pull the GPIO pin low for at least 18 milliseconds to signal the DHT11 to start sending data.
```
pinMode(GPIOPIN, OUTPUT);
digitalWrite(GPIOPIN, LOW);
delay(18);  // 18 milliseconds
```

Pull the GPIO pin high for 40 microseconds.

digitalWrite(GPIOPIN, HIGH);
delayMicroseconds(40);  // 40 microseconds

Set the GPIO pin to input mode to read data from the sensor.
```
pinMode(GPIOPIN, INPUT);
```

Data Reading Loop: The loop runs up to MAXTIMINGS times to read the data bits.

For each transition (from high to low or low to high), it measures how long the pin stays in each state.

for (i = 0; i < MAXTIMINGS; i++)
{
    counter = 0;
    while (digitalRead(GPIOPIN) == laststate)
    {
        counter++;
        delayMicroseconds(2);
        if (counter == 255)
        {
            break;
        }
    }
    laststate = digitalRead(GPIOPIN);
    // ... rest of the loop
}

Data Bit Extraction: The first 3 transitions are ignored as they are part of the DHT11’s initial response.

For each data bit, it determines if the bit is 0 or 1 based on the duration the pin stays high.
```
if ((i >= 4) && (i % 2 == 0))
{
    dht11_dat[j/8] <<= 1;
    if (counter > 16)
    {
        dht11_dat[j/8] |= 1;
    }
    j++;
}
```

Checksum Verification: After receiving all bits, the code verifies the checksum to ensure data integrity.

if ((j >= 40) && (dht11_dat[4] == ((dht11_dat[0] + dht11_dat[1] + dht11_dat[2] + dht11_dat[3]) & 0xFF)) )

If the checksum is correct, it prints out the humidity and temperature values.

f = dht11_dat[2] * 9.0 / 5.0 + 32;
printf("Humidity = %d.%d %% Temperature = %d.%d °C (%.1f °F)\n",
       dht11_dat[0], dht11_dat[1], dht11_dat[2], dht11_dat[3], f);

If the checksum fails, it prints an error message.

else
{
    printf("Data not good, skip\n");
}

Main Function:

Prints a starting message.

printf("Raspberry Pi wiringPi DHT11 Temperature test program\n");

Initializes wiringPi using BCM GPIO pin numbering.

if (wiringPiSetupGpio() == -1)
{
    exit(1);
}

Enters an infinite loop to read data from the DHT11 sensor every second.

while(1)
{
    read_dht11_dat(17);
    delay(1000); // wait 1 second
}

2.2.3 DHT-11

Introduction

Required Components

Schematic Diagram

Experimental Procedures

Phenomenon Picture