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The DHT11 is a basic, ultra low-cost digital temperature and humidity sensor. It uses a capacitive humidity sensor and a thermistor to measure the surrounding air and spits out a digital signal on the data pin (no analog input pins needed). It's fairly simple to use but requires careful timing to grab data. The only real downside of this sensor is you can only get new data from it once every 2 seconds, so when using our library, sensor readings can be up to 2 seconds old.
Compared to the DHT22, this sensor is less precise, less accurate, and works in a smaller range of temperature/humidity, but its smaller and less expensive
Comes with a 4.7K or 10K resistor, which you will want to use as a pullup from the data pin to VCC.