Humidity sensors measure the relative humidity of air. They come in different types – both digital and analog types. The digital types are relatively more expensive than the analog types. The measured relative humidity by digital humidity sensors are commonly sent out using digital serial interfaces like I2C and SPI. Analog humidity sensors are cheaper and they come in two types – capacitive and resistive humidity sensors.
Most capacitive humidity sensors use a polymer film that acts like a dielectric material of a capacitor. As we know, the capacitance of a capacitor is a function of the surface area of the two metal plates, the distance between the two metal plates and, of course, the dielectric material between the two plates. The polymer film used in a capacitive humidity sensor is sensitive to air moisture. Thus, any change to air moisture causes a change to the capacitance of the sensor.
A resistive humidity sensor uses a conductive polymer that changes its resistance according to the measured relative humidity.
You must remember that applying direct currents (DC) to MOST of these sensors may cause permanent damage to them. Therefore, it is advisable to use alternating currents (AC) to analog humidity sensors if you plan to use them in your future project.
In this article, I will show you some of the ways in using analog humidity sensors and I will be focusing to resistive humidity sensors. Just remember that this article only serve for learning purposes only. There is no warranty that any information from this article is correct and accurate. Therefore, use any information from this article at your own risk.
from where i will take output frequency to measure resistance,mean to say from shmit triggerr’s output pin or from input pin?
Obviously from output of shmitt trigger output..:-D
**output of trigger…:-P