Contents
What is DHT11?
The DHT11 is a sensor that is useful in a variety of applications. It has a highly accurate sensor that has its calibration done in a humidity calibration chamber. The sensor then stores calibration coefficients as OTP program memory, where it retrieves the 0-100% humidity readings as digital signal output. The sensor includes a thermistor and a capacitive humidity sensor for measuring temperature and humidity. The 8-bit microcontroller converts the analog signal to a digital signal for reading the temperature and humidity.
The sensors are small and have a 20-meter signal transmission range while having very low operating voltage.
a DHT11 module
Application of the DHT11 sensor
DHT11 sensors are essential in components that measure temperature & humidity. these are:
- appliances used in the home
- dehumidifiers
- HVAC systems
- temperature regulation in vehicles
- to predict temperature and humidity conditions in weather stations
- the sensor is also helpful in data loggers
- in medical equipment that requires humidity and measurement control
- the sensor plays a role in the automation of specific processes
DHT11’s Advantages over other sensors
- it has an excellent long-term stability
- DHT11 can transmit over a wide range
- The sensor works well with demanding applications and has a low energy consumption rate
- the adaptive real-time memory accelerator makes it favorable among users
- the digital humidity sensor’s 4-pin package has a single row arrangement enhancing convenience
Features/ Technical Specifications of the Sensor
The sensor’s operating voltage ranges from 3.5V to 5.5V.
DHT11 has a sampling period of more than two seconds with a standby current of 60uA and an output current of 0.3mA
The sensor also has a 4-pin single row pin package
Additional relative humidity features are:
- Output signal: digital signal via a single-bus
- Measuring range: At 50℃ 20-80% humidity readings
- sensing element: polymer resistor
- Interchangeability: fully interchangeable
- Long-term stability: <± 0.5% RH / Yr
- Accuracy: At 25℃ is ±5% RH
- Lag: <± 0.3% RH
- Resolution: 1%RH
The temperature specifications include:
- Temperature resolution: 1 degree Celsius
- Repeatability: ±1℃
- Operating range: 0-50℃
- Accuracy: +-2.0℃
DHT11’s PIN configuration
The four pins in the sensor are:
- VCC power supply of 3.5 ~ 5.5V DC – connects to the red wire
- DATA serial data, single-bus – connects to the yellow or white wire
- No connection, thus not used
- GND grounding, power negative – connects to the black wire
The difference between the humidity sensor module and the humidity sensor is that the module has an in-built filtering capacitor and pull-up resistor.
A 2-D Model of the DHT11 Sensor
(Source: creativCreative Commons
How do We use DHT11?
As shown below, since the sensor undergoes calibration at the production point, it is easy to set up.
Electrical connection diagram
You will need a high-performance 8-bit microcontroller that has microsecond timing for better efficiency.
The single-bus wire is responsible for communication between the microcontroller and the DHT11. One communication cycle lasts up to 4 microseconds, and the 5K pull resistor helps control the sensor’s status. What this means is that when the rate is high, the bus is idle.
Components in the sensor share a master-slave relationship. When the master calls, it is only when the slave can answer. Failure to adhere to this single-bus sequence will result in the device not responding to the host signal.
The sensor sends higher bits of data first, and a complete transmission comprises 40-bits of data consisting of integral and decimal parts.
The data format is as shown below:
The 8-bit humidity integer data + 8-bit the Humidity decimal data +8-bit temperature critical data +8-bit fractional temperature data +8-bit parity bit.
It is important to note that the decimal bit is always 0 in both temperature and humidity.
If the data transmission is correct, the last bit of ”8bit critical RH data + 8bit decimal RH data + 8bit integral T data + 8bit decimal T data” should be the checksum.
For instance, the microcontroller receiving 40-bits of data from the DHT11 presents as
0010 0001 0000 0000 0001 1010 0000 0000 0011 1011
High humidity 8 Low humidity 8 High temperature 8 Low temperature 8 Parity bit
The calculation is done as shown below:
0011 0101+0000 0000+0001 1000+0000 0000= 0100 1101
Incoming data is correct：
Humidity: 0011 0101=33H=33%RH
Temperature: 0001 1010=18H=26℃
When the microcontroller sends a signal to the sensor, the sensor changes from a low power consumption rate to a high consumption state.
This process happens while the MCU awaits to complete the initial signal. Completing the start signal is essential because there will be no response from the sensor without it.
The DHT11 then responds with an indication of 40-bit data and triggers further processes.
The overall communication process
(Source: creativCreative Commons
In the second process, since the voltage is high at the data bus, the MCU will lower the voltage when communication begins. For the sensor to detect the signal from MCU, this process should last for about 1 to 10ms. After noticing the call, the microcontroller pulls up and waits for a signal response for about 20-40us.
Detection of the start signal influences an 80us low voltage pull by the DHT11. As it prepares to send data, it heightens voltage to 80us.
MCU sending out start signal to DHT11 and DHT11 sending response signal to MCU
(Source: creativCreative Commons
The following step involves the sensor sending information to the microcontroller at a low voltage of 50us.
The bits can be “1” or “0,” depending on the length of the signal.
bit data “1” format
(Source: Creative Commons)
A few factors may lead to poor humidity measurement accuracy; they include;
- overexposure to ultraviolet radiation, such as the sun
- a low-quality data signal wire or a shorter wire
- exposure to smoke, acid, or oxidizing gasses may ruin the DHT11 sensor module
Alternatives to DHT11
These are a couple of equal alternative sensors to DHT11.
- DHT22
- SHT71
- AM2302
Summary
In short, the DHT11 sensor uses a simple process technology of signal conditioning for temperature and humidity sensing. The sensor is advantageous over other sensors since it is easy to maintain and acquire and has already undergone calibration. If you have an interest in making your DHT11 sensor, you can use this video for guidance. All the components necessary for learning or just even experimenting with the DHT11 data sheet are available here.
