First Electronic Sensor: The DHT-22
This gadget requires a manufactured electronic sensor, since there is not much point to building a device to measure warmth, coldness, and humidity unless it returns data in standard, precisely calibrated units of measurement.
The sensor we selected is the DHT-22 (Figure 7-1), manufactured by Aosong Electronics of China. The DHT-22 uses a polymer capacitor to sense the temperature and humidity, measuring the temperature of the air between –40 and 80 degrees Centigrade (which Arduino can convert to Fahrenheit), and the relative humidity between 0 and 100%.
Figure 7-1. The DHT-22 sensor
The necessary calibration information is stored in a tiny 8-bit computer inside the DHT-22, and each unit is tested in the factory. In other words, this sensor is ready to use right out of the box.
The DHT-22 has four pins, but only three are used. When looking at the sensor face-on, the leftmost pin is for voltage to power the sensor (anywhere from 3.3 to 6 volts; we’ll use the 3.3 volt pin on Arduino); the second pin outputs data from the sensor to the Arduino; the third pin is null (not connected to anything); and the rightmost pin is GND.