AOSong DHT topic

Quick Notes

• Applies to AOSong modules:
  • DHT11/CHT11,
  • DHT21/AM2301, and
  • DHT22/AM2302/CM2302
• Depends on the Adafruit DHT library
• Communicate with a single-bus single wire digital signaling protocol
  • Can be connected to any digital pin on a Arduino
  • The communication with these sensors is slow and interrupts are turned off during communication.
  • See the Adafruit DHT library's DHT.cpp for details.
  • Could possibly cause issues if used in combination with other interrupt-driven sensors.
• Only 1 can be connected to a system at a time
• Requires a 3.3 - 6V power source
• Sensors should not be polled more frequently than once every 2 seconds
• Uses a Maxim DS18 sensor internally

Sensor Datasheet

Datasheet

Sensor Constructor

Example Code

The DHT is used in the menu a la carte example.

Classes

class AOSongDHT

The Sensor sub-class for the AOSong digital-output relative humidity and temperature sensor modules.

class AOSongDHT_Humidity

The Variable sub-class used for the humidity output from an AOSong DHT.

class AOSongDHT_Temp

The Variable sub-class used for the temperature output from an AOSong DHT.

class AOSongDHT_HI

The Variable sub-class used for the heat index output calculated from measurements made by an AOSong DHT.

Sensor Variable Counts

The number of variables that can be returned by an AOSong DHT

#define DHT_NUM_VARIABLES = 3

Sensor::_numReturnedValues; the DHT can report 3 values.

#define DHT_INC_CALC_VARIABLES = 0

Sensor::_incCalcValues; we don't calculate any additional values.

Sensor Timing

The sensor timing for an AOSong DHT

#define DHT_WARM_UP_TIME_MS = 1700

Sensor::_warmUpTime_ms; DHT warms up in 1700ms (1.7sec).

#define DHT_STABILIZATION_TIME_MS = 0

Sensor::_stabilizationTime_ms; We assume the sensor is stable immediately after warm-up.

#define DHT_MEASUREMENT_TIME_MS = 2000

Sensor::_measurementTime_ms; DHT takes 2000ms (2s) to complete a measurement.

Humidity

The humidity variable from an AOSong DHT

• Accuracy is ± 2 % RH for DHT22 and ± 5 % RH for DHT11
• Range is 0 to 100 % RH

#define DHT_HUMIDITY_RESOLUTION = 1

Decimals places in string representation; humidity should have 1 (0.1 % RH for DHT22 and 1 % RH for DHT11)

#define DHT_HUMIDITY_VAR_NUM = 0

Sensor variable number; humidity is stored in sensorValues[0].

#define DHT_HUMIDITY_VAR_NAME = "relativeHumidity"

Variable name in ODM2 controlled vocabulary; "relativeHumidity".

#define DHT_HUMIDITY_UNIT_NAME = "percent"

Variable unit name in ODM2 controlled vocabulary; "percent" (percent relative humidity or % RH)

#define DHT_HUMIDITY_DEFAULT_CODE = "DHTHumidity"

Default variable short code; "DHTHumidity".

Temperature

The temperature variable from an AOSong DHT

• Accuracy is ±0.5°C for DHT22 and ± ±2°C for DHT11
• Range is -40°C to +80°C

#define DHT_TEMP_RESOLUTION = 1

Decimals places in string representation; temperature should have 1 - resolution is 0.1°C.

#define DHT_TEMP_VAR_NUM = 1

Sensor variable number; temperature is stored in sensorValues[1].

#define DHT_TEMP_VAR_NAME = "temperature"

Variable name in ODM2 controlled vocabulary; "temperature".

#define DHT_TEMP_UNIT_NAME = "degreeCelsius"

Variable unit name in ODM2 controlled vocabulary; "degreeCelsius" (degrees Celsius, °C)

#define DHT_TEMP_DEFAULT_CODE = "DHTTemp"

Default variable short code; "DHTTemp".

Heat Index

The heat index variable from an AOSong DHT

• Heat index is calculated within the Adafruit library from the measured temperature and humidity.

#define DHT_HI_RESOLUTION = 1

Decimals places in string representation; heat index should have 1 - resolution is 0.1°C.

#define DHT_HI_VAR_NUM = 2

Sensor variable number; HI is stored in sensorValues[2].

#define DHT_HI_VAR_NAME = "heatIndex"

Variable name in ODM2 controlled vocabulary; "heatIndex".

#define DHT_HI_UNIT_NAME = "degreeCelsius"

Variable unit name in ODM2 controlled vocabulary; "degreeCelsius" (°C)

#define DHT_HI_DEFAULT_CODE = "DHTHI"

Default variable short code; "DHTHI".