HM2302 Digital-output humidity and temperature sensor

Feature & Application:

Description:

DHT22 output calibrated digital signal. It utilizes exclusive digital-signal-collecting-technique and humidity sensing technology, assuring its reliability and stability.Its sensing elements is connected with 8-bit single-chip computer.
Every sensor of this model is temperature compensated and calibrated in accurate calibration chamber and the calibration-coefficient is saved in type of programme in OTP memory, when the sensor is detecting, it will cite coefficient from memory.
Small size & low consumption & long transmission distance(20m) enable HM2302 to be suited in all kinds of harsh application occasions.
Single-row packaged with four pins, making the connection very convenient.

Technical Specification:

Dimensions: (unit----mm)

Electrical connection diagram:

Operating specifications:

(1) Power and Pins
Power's voltage should be 3.3-6V DC. When power is supplied to sensor, don't send any instruction to the sensor within one second to pass 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 HM2302 it costs 5mS for single time communication.
Data is comprised of integral and decimal part, the following is the formula for data.
HM2302 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, HM2302change from low-power-consumption-mode to running-mode. When MCU finishs sending the start signal, HM2302 will send response signal of 40-bit data that reflect the relative humidity and temperature information to MCU. Without start signal from MCU, HM2302 will not give response signal to MCU. One start signal for one time's response data that reflect the relative humidity and temperature information from HM2302. HM2302will change to low-power-consumption-mode when data collecting finish if it don't receive start signal from MCU again.
1)Check bellow picture for overall communication process:

2)Step 1: MCU send out start signal to HM2302
Data-bus's free status is high voltage level. When communication between MCU and DHT22 begin, program of MCU will transform data-bus's voltage level from high to low level and this process must beyond at least 1ms to ensure HM2302 could detect MCU's signal, then MCU will wait 20-40us for HM2302's response.
Check bellow picture for step 1:

3)Step 2: HM2302 send response signal to MCU
When HM2302 detect the start signal, HM2302 will send out low-voltage-level signal and this signal last 80us as response signal, then program of HM2302 transform data-bus's voltage level from low to high level and last 80us for HM2302's preparation to send data.
Check bellow picture for step 2:

4)Step 3: HM2302 send data to MCU
When HM2302 is sending data to MCU, every bit's transmission begin with low-voltage-level that last 50us, the following high-voltage-level signal's length decide the bit is "1" or "0".
Check bellow picture for step 3:

If signal from HM2302 is always high-voltage-level, it means HM2302 is not working properly, please check the electrical connection status.

Electrical Characteristics:

- Collecting period should be : >2 second.
Attentions of application:

(1) Operating and storage conditions
We don't recommend the applying RH-range beyond the range stated in this specification. The HM2302 sensor can recover after working in non-normal operating condition to calibrated status, but will accelerate sensors' aging.
(2) Attentions to chemical materials
Vapor from chemical materials may interfereHM2302's sensitive-elements and debase HM2302's sensitivity.
(3)Disposal when (1) & (2) happens
Step one: Keep the HM2302sensor at condition of Temperature 50~60Celsius, humidity <10%RH for 2 hours;
Step two: After step one, keep the HM2302 sensor at condition of Temperature 20~30Celsius, humidity >70%RH for 5 hours.
(4)Attention to temperature's affection
Relative humidity strongly depend on temperature, that is why we use temperature compensation technology to ensure accurate measurement of RH. But it's still be much better to keep the sensor at same temperature when sensing.
HM2302 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 debaseHM2302's performance.
(6)Attentions to connection wires
The connection wires' quality will effect 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 HM2302 may cause personal injury

HM2302-Digital-output-humidity-and-temperature-sensor.pdf