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DHT11: Measuring temperature and humidity with the Pi Pico in MicroPython

DHT11 sensor overview

The DHT11 sensor with its characteristic blue protection

The DHT11 sensor is a favorite among makers and DIY enthusiasts and is pre-installed in MicroPython. This sensor stands out because of its affordability and convenience.

Warning

Although it is straightforward, this tool is limited in speed, as it can only measure once per second.

Getting started with the DHT11 sensor

The DHT22 is the more advanced model and is identifiable by its white case and slightly larger size than the DHT11, which has a blue case. The two models have the same user; however, if you want to explore the differences, you can check out the comparison between the DHT11 and the DHT22. If you are using the DHT22, you can follow the corresponding tutorial.

Some technical characteristics of the DHT11

It is possible to power the DHT11 directly from pin 3V3 of the Raspberry Pi Pico

Technical specifications of the DHT11

Features

DHT11

Temperature accuracy

± 2°C

Humidity accuracy

± 5%

Temperature range

0-50 °C

Humidity range

20-90%

Sampling

1/s

Supply voltage

3-5.5V

Current

~2.5mA

If you’re looking for more accurate and consistent measurements, I recommend using a sensor from the Bosh BMExxx family. Though it may be more expensive, it’s worth the investment.

DHT11 Sensor Wiring on Raspberry Pi Pico

The DHT11 sensor has between 3 and 4 pins depending on the model. One pin is redundant if your module has 4. Unfortunately, the pin-out is different depending on the manufacturer. 😕

DHT11 variant

Different pin-outs are possible (Elegoo’s is on the left)

When the sensor is placed in front of you, the pins numbering will begin from the left side. Here is a brief overview of the available options:

Pin Wiring

DHT11 Module

Raspberry Pi Pico

1 (S )

GPIO17

2

3V3

3 (- )

GND

Pin Wiring

DHT11 Module

Raspberry Pi Pico

1

GND

2

GPIO17

3

3V3

Pin Wiring

DHT11 Module

Raspberry Pi Pico

1

3V3

2

GPIO17

3

Do not connect

4

GND

In addition to the pins for providing power, the DHT11 also has one output pin to communicate the sensor readings.

Electrical circuit to use the DHT11 module with a Raspberry Pi Pico

If your module has no pullup resistor installed, you must add one between the pins 3V3 and the signal (S ). The resistor should have a value between 4.7kΩ to 10kΩ. It won’t hurt to add a resistor if desired.

A pin with PWM capacity can be used for the output. We will use GP5 in this example. The sensor we will use is the Elegoo DHT11. Be sure to adjust the circuitry if you are using a different model. This module has a 10kΩ resistor, labeled as R1, on it (above the letter S ).

dht11 pico circuit

Electrical circuit to be made

dht11 sensor microptyhon rp2040 devkit

Electrical circuit to be made

Wiring on DHT11 RP2040 DevKit breadboard

Wiring the DHT11 with a uPesy RP2040 DevKit

Measuring the temperature and humidity of the DHT11 with MicroPython

If you are a beginner, you might need help communicating with the DHT11 sensor as it uses a proprietary protocol. Luckily, the necessary library is included in recent versions of MicroPython, so you don’t have to download anything extra. The code is easy - create an object called dht using the PIN, then call the sensor.measure() function.

from machine import Pin
from time import sleep
import dht

sensor = dht.DHT11(Pin(5))

while True:
    try:
        sleep(1)
        # The DHT11 returns at most one measurement every 1s
        sensor.measure()
        # Retrieves measurements from the sensor
        print(f"Temperature : {sensor.temperature():.1f}")
        print(f"Humidity    : {sensor.humidity():.1f}")
        # Transmits the temperature to the terminal
    except OSError as e:
        print('Failed reception')
        # If the pico does not receive the measurements from the sensor

Note

Remember to add a 1-second delay between 2 measurements because the DHT11 cannot go faster.😮💨 If it is not able to keep up with the rate, an error will be returned and captured by the try: except:

In the Thonny IDE terminal, you will see the temperature expressed in °C and the humidity in percentage.

micropython demo of DHT11 measurement with a Pico on Thonny

Note

You can blow on the DHT11 sensor like a cold window to clear the fog. You should then see the humidity level fluctuate.

In Python, an f-string can be used to display variables with text. An f must be placed at the beginning of the string, and the variable must be placed in square brackets. The number of digits after the decimal point can be specified with``:.1f`` . For the DHT11, we should use 0 as the sensor is accurate to one degree. For example, f"Humidite: {sensor.humidity():.1f}" .

Optimized library using the Pi Pico PIO

https://github.com/danjperron/PicoDHT22/blob/main/PicoDHT22.py

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