Introduction¶

This page covers the low cost DHT temperature & humidity sensors. These sensors are very basic and slow, but are great for some basic data logging. The DHT sensors are made of two parts, a capacitive humidity sensor and a thermistor. There is also a very basic chip inside that does some analog to digital conversion and spits out a digital signal with the temperature and humidity. The digital signal is fairly easy to read using any microcontroller.

Product Specifications¶

There are different versions of the DHT sensor, they look a bit similar and have the same pinout, but have different characteristics.

DHT11¶

• Power Supply: 3 to 5V DC, 2.5mA max current use during conversion (while requesting data)
• Operating range: Good for 20-80% humidity readings with 5% accuracy
• Good for 0-50°C temperature readings ±2°C accuracy
• No more than 1 Hz sampling rate (once every second)
• Body size: 15.5mm x 12mm x 5.5mm

DHT22¶

• 3 to 5V power and I/O 2.5mA max current use during conversion (while requesting data)
• Good for 0-100% humidity readings with 2-5% accuracy
• Good for -40 to 125°C temperature readings ±0.5°C accuracy
• No more than 0.5 Hz sampling rate (once every 2 seconds)
• Body size: 15.1mm x 25mm x 7.7mm

The DHT22 is a little more accurate. All sensors have a low sampling rate.

Different names¶

They come in different packages, breakoutboards:

• most common : DHT11, DHT22
• SEN11301P, Grove - Temperature&Humidity Sensor (Seeed Studio)
• SEN51035P, Grove - Temperature&Humidity Sensor Pro (Seeed Studio)
• AM2302 , temperature-humidity sensor
• RHT03 , Humidity and Temperature Sensor

Wiring / Connectivity¶

Connection of the SEN11301P or SEN51035P:

• black - GND (Ground)
• red - VCC (3 to 5V power)
• white - NC (not connected)
• yellow - SIG (Signal or data)

Connection of the DHT11 or DHT22:

Don't forget to place a 5K or 4k7 resistor between VCC and the data pin, to act as a medium-strength pull up on the data line.

• pin 1 - VCC (3 to 5V power)
• pin 2 - Data
• pin 3 - NC (not connected)
• pin 4 - GND

Library¶

Test Code¶

#include "mbed.h"
#include "DHT.h"

DigitalOut myled(LED1);

DHT sensor(p23,SEN11301P); // Use the SEN11301P sensor

int main() {
    int err;
    printf("\r\nDHT Test program");
    printf("\r\n******************\r\n");
    wait(1); // wait 1 second for device stable status
    while (1) {
        myled = 1;
        err = sensor.readData();
        if (err == 0) {
            printf("Temperature is %4.2f C \r\n",sensor.ReadTemperature(CELCIUS));
            printf("Temperature is %4.2f F \r\n",sensor.ReadTemperature(FARENHEIT));
            printf("Temperature is %4.2f K \r\n",sensor.ReadTemperature(KELVIN));
            printf("Humidity is %4.2f \r\n",sensor.ReadHumidity());
            printf("Dew point is %4.2f  \r\n",sensor.CalcdewPoint(sensor.ReadTemperature(CELCIUS), sensor.ReadHumidity()));
            printf("Dew point (fast) is %4.2f  \r\n",sensor.CalcdewPointFast(sensor.ReadTemperature(CELCIUS), sensor.ReadHumidity()));
        } else
            printf("\r\nErr %i \n",err);
        myled = 0;
        wait(5);
    }
}

Output¶