GPIO - Measure Temperature and Humidity

Preparation

  • AmebaD [AMB21 / AMB22 / AMB23 / BW16] x 1

  • DHT11 or DHT22 or DHT21

Example

DHT11 is a temperature and humidity sensor which operates at voltage 3.3V~5V. At room temperature, the measurable range of the humidity is 20% ~ 90%RH with ±5%RH precision, the measurable range of the temperature is 0 ~ 50℃ with ±2℃ precision.

Another choice of temperature and humidity sensor is DHT22 sensor, which has better precision. Its measurable range of the humidity is 0%~100%RH with ±5%RH precision, the measurable range of the temperature is -40~125 ℃ with ±0.2℃ precision. There are 4 pins on the sensor:

../../../_images/image150.png

Since one of the 4 pins has no function, there are temperature/humidity sensors with only 3 pins on the market:

../../../_images/image247.png

DHT is normally in the sleeping mode. To get the temperature/humidity data, please follow the steps:

  1. Awake DHT: Ameba toggles low its DATA pin of GPIO. Now the DATA pin of GPIO serves as digital out to Ameba.

  2. DHT response: DHT also toggle low its DATA pin of GPIO. Now the DATA pin of GPIO serves as digital in for Ameba.

  3. DHT sends data: DHT sends out the temperature/humidity data (which has size 5 bytes) in a bit by bit manner. To represent each bit, DHT first pull low the DATA GPIO pin for a while and then pull high. If the duration of high is smaller than low, it stands for bit 0. Otherwise it stands for bit 1.

../../../_images/image332.png

AMB21 / AMB22 Wiring Diagram:

../../../_images/image424.png

AMB23 Wiring Diagram:

../../../_images/image4-11.png

BW16 Wiring Diagram:

../../../_images/image4-31.png

BW16-TypeC Wiring Diagram:

../../../_images/image4-41.png

Open the sample code in “Files” → “Examples” → “AmebaGPIO” → “DHT_Tester”. Compile and upload to Ameba, then press the reset button. The result would be shown on the Serial Monitor.

../../../_images/image517.png

Code Reference

Use dht.readHumidity() read the humidity value, and use dht.readTemperature() to read the temperature value.

Every time we read the temperature/humidity data, Ameba uses the buffered temperature/humidity data unless it found the data has expired (i.e., has not been updated for over 2 seconds). If the data is expired, Ameba issues a request to DHT to read the latest data.