Temperature and humidity monitoring plays a crucial role in various applications, from smart home climate control to agricultural automation. The DHT11 and DHT22 sensors are among the most widely used temperature and humidity sensors due to their ease of integration, affordability, and accurate readings. These sensors provide a simple yet effective way to collect environmental data for IoT, weather stations, and other DIY electronics projects.

Overview of DHT11 and DHT22 Sensors

DHT11 and DHT22 belong to the DHTxx series of sensors, which measure both temperature and humidity using a combination of a humidity-sensing element and an NTC thermistor. Though they share a similar appearance and functionality, there are significant differences in their accuracy, range, and performance.

DHT11: A basic and cost-effective sensor that is suitable for general-purpose applications where high precision is not a requirement.

DHT22: A more advanced sensor with higher accuracy and a wider measurement range, making it suitable for professional and industrial applications.

Specifications Comparison

DHT11 vs. DHT22: Key Differences

While both sensors perform similar functions, their key differences lie in accuracy, range, and response time:

DHT22 is more precise and can measure extreme temperatures, making it ideal for weather stations and climate-sensitive applications.

DHT11 is a budget-friendly option suitable for applications that do not require extreme accuracy.

Interchangeability: The sensors can be swapped based on project needs without major hardware modifications.

Working Principle of DHT11 and DHT22 Sensors

These sensors operate based on two fundamental sensing components:

1. Humidity Measurement

Both sensors use a humidity-sensing component, typically a polymer capacitor, which consists of two electrodes separated by a moisture-absorbing substrate. When ambient humidity changes, the electrical capacitance of the polymer changes, altering the resistance between the electrodes. This change in resistance is converted into a digital signal by the sensor’s internal microcontroller.

2. Temperature Measurement

Temperature sensing is achieved using an NTC (Negative Temperature Coefficient) thermistor. The thermistor's resistance decreases as temperature increases, and this resistance variation is processed by the internal microcontroller to determine the ambient temperature.

NTC Thermistor Characteristic Curve: The resistance-to-temperature relationship follows an exponential pattern, meaning small temperature changes cause noticeable resistance shifts.

3. Data Processing and Communication

The internal 8-bit microcontroller in both sensors processes the analog signals from the humidity and temperature sensing elements. It then converts this data into a digital signal and transmits it to the microcontroller (such as Arduino) using a one-wire communication protocol.

Communication Protocol: Aosong One-Wire Protocol

DHT11 and DHT22 sensors communicate with microcontrollers using the Aosong One-Wire Protocol, a simplified data transmission method requiring only a single data pin. The transmission occurs in the following sequence:

Start Signal: The microcontroller pulls the data line low for at least 18ms.

Response Signal: The sensor pulls the line low for 80µs and then high for another 80µs.

Data Transmission: The sensor sends 40 bits of data—16 bits for humidity, 16 bits for temperature, and 8 bits for a checksum.

Checksum Validation: The microcontroller verifies the data integrity using the checksum.

Pinout

The pin configuration of both sensors is similar:

VCC- Power supply (3.3V to 5.5V for DHT11, 3.3V to 6V for DHT22)

Data- One-wire communication pin to transmit sensor readings

N/C- Not connected

GND- Ground connection

Interfacing DHT11 Sensor to an Arduino