Temperature & Humidity¶

The DHT11 is a small blue sensor that measures both temperature and humidity (how much water vapour is in the air). It's a new kind of input: instead of a voltage, it sends its readings back as a short burst of digital data down a single wire. MicroPython already knows how to decode it, so reading it takes just two lines.

You'll need:

1 × DHT11 module (3 pins, on a small board), or a DHT22
3 × jumper wires
The RGB LED from the RGB LEDs lesson (for the challenge)

Hardware¶

The DHT11 module has three pins, usually labelled on the board:

DHT11 pin	Connect to
+ or VCC	ESP32 3V3
OUT or S (data)	ESP32 GPIO 19
– or GND	ESP32 GND

Bare 4-pin sensors

If your DHT11 is just the blue sensor with four legs (no small board), it needs a 10 kΩ pull-up resistor between the data leg (leg 2) and 3V3. Leg 1 is VCC, leg 4 is GND, and leg 3 isn't used. The modules have this resistor built in.

DHT11 vs DHT22

The DHT22 (usually white) is the DHT11's more accurate big brother. It measures to a tenth of a degree and works below freezing. It wires up the same way; just change dht.DHT11 to dht.DHT22 in the code.

Software¶

dht_read.py

# Read temperature and humidity from a DHT11 sensor
import time
import dht
from machine import Pin

DHT_PIN = 19

sensor = dht.DHT11(Pin(DHT_PIN))   # use dht.DHT22 for the white DHT22 sensor

while True:
    try:
        sensor.measure()               # ask the sensor for a new reading
        temp_c = sensor.temperature()
        humidity = sensor.humidity()
        temp_f = temp_c * 9 / 5 + 32
        print(f"{temp_c} °C ({temp_f:.1f} °F)   Humidity: {humidity} %")
    except OSError:
        print("Couldn't read the sensor. Check the wiring.")
    time.sleep(2)   # DHT11: at most one reading per second. DHT22: every 2 s

import dht loads the DHT driver that comes built into MicroPython.
sensor.measure() asks the sensor to take a reading. Then sensor.temperature() (in °C) and sensor.humidity() (in %) return the results.
temp_c * 9 / 5 + 32 converts Celsius to Fahrenheit. In the f-string, {temp_f:.1f} means "show one digit after the decimal point".
Breathe gently on the sensor: the humidity should jump up within a few seconds.

Handling a failed reading¶

Every now and then, the sensor's data gets garbled on the way to the ESP32 and measure() raises an OSError. Without a plan, that would crash your program. The try: / except OSError: block catches the error, prints a message, and lets the loop carry on to the next reading.

This pattern is worth remembering. Anything that talks to the outside world, from sensors to Wi-Fi, can occasionally fail, and good programs expect it.

Don't read too often

The DHT11 can only take one reading per second, and the DHT22 one every two seconds. Ask more often and you'll get errors or the same old reading again.

Challenge¶

Turn the RGB LED into a comfort indicator: blue when the room is too cold, green when it's comfortable, and red when it's too hot.

Show a solution

dht_comfort.py

# Show whether the room is comfortable using the RGB LED:
# blue = too cold, green = just right, red = too hot
import time
import dht
from machine import Pin

TOO_COLD = 18   # °C
TOO_HOT = 26    # °C

sensor = dht.DHT11(Pin(19))
red = Pin(16, Pin.OUT)
green = Pin(17, Pin.OUT)
blue = Pin(18, Pin.OUT)


def show(r, g, b):
    red.value(r)
    green.value(g)
    blue.value(b)


while True:
    try:
        sensor.measure()
        temp = sensor.temperature()
        print("Temperature:", temp, "°C")
        if temp < TOO_COLD:
            show(0, 0, 1)
        elif temp > TOO_HOT:
            show(1, 0, 0)
        else:
            show(0, 1, 0)
    except OSError:
        print("Sensor read failed, trying again")
    time.sleep(2)

Hold the sensor between your fingers to warm it up and watch the colour change. Adjust TOO_COLD and TOO_HOT to suit your room.

Next up: Touch Sensors: buttons made out of thin air (and a wire).