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DHT11 is an inexpensive sensor that provides calibrated digital outputs for ambient temperature and relative humidity. The relative humidity is the amount of water vapor in air vs. the saturation point of water vapor in the air. At the saturation point, water vapor starts to condense and accumulate on surfaces forming dew.
It comes in a single row 4-pin package and operates from 3.5 to 5.5V power supply. The humidity sensing component is a moisture holding substrate with electrodes applied to the surface. It can measure temperature from 0-50 °C with an accuracy of ±2°C and relative humidity ranging from 20-95% with an accuracy of ±5%. It has got its own proprietary 1-wire protocol, therefore, the communication between the sensor and a microcontroller is not possible with any of its peripherals through a direct interface. The protocol must be implemented in the firmware of the MCU through bit-banging of an I/O pin.
The DHT11 detects water vapor by measuring the electrical resistance between two electrodes. When water vapor is absorbed by the substrate, ions are released by the substrate which increases the conductivity between the electrodes. The change in resistance between the two electrodes is proportional to the relative humidity. Higher relative humidity decreases the resistance between the electrodes, while lower relative humidity increases the resistance between the electrodes.
The DHT11 uses just one signal wire to transmit data to the Arduino. Power comes from separate 5V and ground wires. A 10K Ohm pull-up resistor is needed between the signal line and 5V line to make sure the signal level stays high by default.
There are two different versions of the DHT11 you might come across. One type has four pins, and the other type has three pins and is mounted to a small PCB. The PCB mounted version is nice because it includes a surface mounted 10K Ohm pull up resistor for the signal line.

Model: DHT11
Output Type: Digital
Input voltage: 3.5V ~ 5.5V
Resolution: 8 bit
Humidity Accuracy: +/- 5% RH (at 25°C)
Temperature Accuracy: +/- 2°C (at 25°C)
Humidity Range: 20% ~ 95% RH
Temperature Range: 0°C ~ 50°C
Maximum Response Time: 15s (Humidity), 30s (Temperature)
Interface: 1 Wire
Pin Number: 4 Pins
Size:12.5mm x 16mm x 6mm

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