C-DHT22 - DHT22 / AM2302 humidity & temperature sensor/module

Digital-output relative humidity & temperature sensor/module
DHT22 (DHT22 also named as AM2302)

Capacitive-type humidity and temperature module/sensor

• Model DHT22
• Power supply 3.3-6V DC
• Output signal digital signal via single-bus
• Sensing element Polymer capacitor
• Operating range humidity 0-100%RH; temperature -40~80Celsius
• Accuracy humidity +-2%RH(Max +-5%RH); temperature <+-0.5Celsius
• Resolution or sensitivity humidity 0.1%RH; temperature 0.1Celsius
• Repeatability humidity +-1%RH; temperature +-0.2Celsius
• Humidity hysteresis +-0.3%RH
• Long-term Stability +-0.5%RH/year
• Sensing period Average: 2s
• Interchangeability fully interchangeable
• Dimensions small size 14*18*5.5mm; big size 22*28*5mm

Pin sequence number: 1 2 3 4 (from left to right direction).
Pin Function

1. VDD----power supply
2. DATA--signal
3. NULL
4. GND
   
   

(1) Power and Pins
Power's voltage should be 3.3-6V DC. When power is supplied to the sensor, don't send any instruction to the sensor within one second to pass an unstable status. One capacitor valued 100nF can be added between VDD and GND for wave filtering.

(2) Communication and signal
Single-bus data is used for communication between MCU and DHT22, it costs 5mS for single time communication.

Data is comprised of an integral and decimal part, the following is the formula for data.

DHT22 send out higher data bit firstly!
DATA=8 bit integral RH data+8 bit decimal RH data+8 bit integral T data+8 bit decimal T data+8 bit check-sum If the data transmission is right, check-sum should be the last 8 bit of "8 bit integral RH data+8 bit decimal RH data+8 bit integral T data+8 bit decimal T data".

When MCU send start signal, DHT22 change from low-power-consumption-mode to running-mode. When MCU finishes sending the start signal, DHT22 will send response signal of 40-bit data that reflect the relative humidity and temperature information to MCU. Without start signal from MCU, DHT22 will not give a response signal to MCU. One start signal for one time's response data that reflect the relative humidity and temperature information from DHT22. DHT22 will change to low-power-consumption-mode when data collecting finish if it doesn't receive start signal from MCU again.

Check bellow picture for overall communication process:

Step 1: MCU send out start signal to DHT22
Databus free status is high voltage level. When communication between MCU and DHT22 begin, a program of MCU will transform data-buses voltage level from high to low level and this process must beyond at least 1ms to ensure DHT22 could detect MCU's signal, then MCU will wait 20-40us for DHT22's response.

Step 2: DHT22 sends a response signal to MCU
When DHT22 detect the start signal, DHT22 will send out a low-voltage-level signal and this signal last 80us as response signal, the program of DHT22 transform data-buses voltage level from low to a high level and last 80us for DHT22's preparation to send data.

Check bellow picture for step 2:

Step 3: DHT22 send data to MCU
When DHT22 is sending data to MCU, every bit's transmission begins with low-voltage-level that last 50us, the
following high-voltage-level signal's length decides the bit is "1" or "0".

Check bellow picture for step 3:

If the signal from DHT22 is always high-voltage-level, it means DHT22 is not working properly, please check the electrical connection status.

Electrical Characteristics:

Power supply: DC min 3.3V, max 5.6V
Current supply: Measuring min 1mA, max 1.5 mA
                       Stand-by min 40 uA, max 50 uA
Collecting period: 2 Seconds

Collecting period should be longer than 2 seconds

Attentions of application:

(1) Operating and storage conditions
We don't recommend the applying RH-range beyond the range stated in this specification. The DHT22 sensor
can recover after working in non-normal operating condition to calibrated status but will accelerate sensors aging.
(2) Attention to chemical materials
Vapor from chemical materials may interfere with DHT22's sensitive-elements and debase DHT22's sensitivity.
(3) Disposal when (1) & (2) happens
Step one: Keep the DHT22 sensor at the condition of Temperature 50~60Celsius, humidity <10%RH for 2 hours;
Step two: After step one, keep the DHT22 sensor at the condition of Temperature 20~30Celsius, humidity
>70%RH for 5 hours.
(4) Attention to temperature's affection
Relative humidity strongly depends on temperature, that is why we use temperature compensation technology to
ensure accurate measurement of RH. But it's still much better to keep the sensor at the same temperature when
sensing.
DHT22 should be mounted at the place as far as possible from parts that may cause change to temperature.
(5) Attentions to light
Long time exposure to strong light and ultraviolet may debase DHT22's performance.
(6) Attention to connection wires
The connection wires' quality will affect communication's quality and distance, high-quality shielding-wire is
recommended.
(7) Other attentions
* Welding temperature should be bellow 260Celsius.
* Avoid using the sensor under dew condition.
* Don't use this product in safety or emergency stop devices or any other occasion that failure of DHT22 may
cause personal injury.