Temperature and Humidity sensor Experiment

RELATIVE HUMIDITY?

The DHT11 humidity and temperature sensor measures relative humidity (RH) and temperature. Relative humidity is the ratio of water vapor in air vs. the saturation point of water vapor in the air. The saturation point of water vapor in air changes with temperature. Cold air can hold less water vapor before it is saturated, and hot air can hold more water vapor before it is saturated. The formula for relative humidity is as follows:

Relative Humidity = (density of water vapor / density of water vapor at saturation) x 100%

Basically, relative humidity is the amount of water in the air compared to the amount of water that air can hold before condensation occurs. It’s expressed as a percentage.  For example, at 100% RH condensation (or rain) occurs, and at 0% RH, the air is completely dry.

HOW THE DHT11 MEASURES HUMIDITY AND TEMPERATURE?

The DHT11 calculates relative humidity by measuring the electrical resistance between two electrodes. The humidity sensing component of the DHT11 is a moisture holding substrate (usually a salt or conductive plastic polymer) with the electrodes applied to the surface. 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. Inside the DHT11 you can see electrodes applied to a substrate on the front of the chip:

The DHT11 converts the resistance measurement to relative humidity on an IC mounted to the back of the unit and transmits the humidity and temperature readings directly to the Arduino. This IC also stores the calibration coefficients and controls the data signal transmission between the DHT11 and the Arduino:

The temperature readings from the DHT11 come from a surface mounted NTC temperature sensor (thermistor) built into the unit. To learn more about thermistors and how to use them on the Arduino, check out our Arduino Thermistor Temperature Sensor Tutorial.

The DHT11 uses one signal wire to transmit sensor readings to the Arduino digitally. The power comes from separate 5V and ground wires. A 5K – 10K Ohm pull-up resistor is connected from the signal line to 5V to make sure the signal level stays high by default (see the data sheet for specifics on how the signal is sent).

There are two different variations of the DHT11 sensor you might come across. One type has four pins, and the other type is mounted to a small PCB that has three pins. The PCB mounted version with three pins is nice since it includes a surface mounted 10K Ohm pull-up resistor for the signal line:

   

The circuit Looks like this:

Here is the video of the output:

Here is the link to the code of this experiment: https://github.com/pschragger/Adafruit_Python_DHT

References :

1. https://cdn-learn.adafruit.com/downloads/pdf/dht-humidity-sensing-on-raspberry-pi-with-gdocs-logging.pdf
2. https://github.com/adafruit/Adafruit_Python_DHT
3. https://learn.sparkfun.com/tutorials/serial-peripheral-interface-spi
4. http://www.rototron.info/dht22-tutorial-for-raspberry-pi/