Introduction

The digital temperature and humidity sensor DHT11 inside contains a chip that does analog to digital conversion and spits out a digital signal with the temperature and humidity, compatible with any MCUs, ideal for those who want some basic data logging stuffs. It’s very popular for electronics hobbyists because it is very cheap but still providing great performance.

In this lesson, we will first go into a little background about humidity, then we will explain how the DHT11 measures humidity. After that, we will show you how to connect the DHT11 to the micro bit and give you some example code so you can use the DHT11 in your own projects.

Parts Needed You will need the following parts:

• 1x micro:bit
• 1x Micro B USB Cable
• 1x micro:bit Breakout (with Headers)
• 5x Jumper Wires
• 1x  DHT11 Module

The DHT11 is a basic, ultra low-cost digital temperature and humidity sensor. It uses a capacitive humidity sensor and a thermistor to measure the surrounding air, and spits out a digital signal on the data pin (no analog input pins needed). Its fairly simple to use, but requires careful timing to grab data.

Only three pins are available for use: VCC, GND, and DATA. The communication process begins with the DATA line sending start signals to DHT11, and DHT11 receives the signals and returns an answer signal. Then the host receives the answer signal and begins to receive 40-bit humiture data (8-bit humidity integer + 8-bit humidity decimal + 8-bit temperature integer + 8-bit temperature decimal + 8-bit checksum).

WHAT IS RELATIVE HUMIDITY?

The DHT11 measures relative humidity. Relative humidity is the amount of water vapor in air vs. the saturation point of water vapor in air. At the saturation point, water vapor starts to condense and accumulate on surfaces forming dew.

The saturation point changes with air temperature. Cold air can hold less water vapor before it becomes saturated, and hot air can hold more water vapor before it becomes saturated.

The formula to calculate relative humidity is:

$RH = (\frac{\rho_{w}}{\rho_{s}}) \ x \ 100 \% \\ \\ RH: \ Relative \ Humidity \\ \rho_{w}: \ Density \ of \ water \ vapor\\ \rho_{s}: \ Density \ of \ water \ vapor \ at \ saturation$

Relative humidity is expressed as a percentage. At 100% RH, condensation occurs, and at 0% RH, the air is completely dry.

HOW THE DHT11 MEASURES HUMIDITY AND TEMPERATURE

The DHT11 detects water vapor by measuring the electrical resistance between two electrodes. The humidity sensing component is a moisture holding substrate with electrodes applied to the surface. When water vapor is absorbed by the substrate, ions are released by the substrate which increases the conductivity between the electrodes. The change in resistance between the two electrodes is proportional to the relative humidity. Higher relative humidity decreases the resistance between the electrodes, while lower relative humidity increases the resistance between the electrodes.

The DHT11 measures temperature with a surface mounted NTC temperature sensor (thermistor) built into the unit.

With the plastic housing removed, you can see the electrodes applied to the substrate, an IC mounted on the back of the unit converts the resistance measurement to relative humidity. It also stores the calibration coefficients, and controls the data signal transmission between the DHT11 and the Arduino:

WIRING

 Micro bit DHT11 sensor 3.3v VCC P0 DATA GND Ground