Using Analog Humidity Sensors

Example

I am currently developing a project that uses a resistive humidity sensor. The resistive humidity sensor that I am using is CM-R. For this, I chose to convert the resistance of the sensor to frequency. Therefore, I need to build an oscillator circuit and I have chosen 74HC14 based oscillator because it is easy to implement.Please note that the 74HC14 based oscillator may not be the best but I chose this because it is easy to implement. 

I looked at the electrical characteristics of the sensor. According to the datasheet, the rated voltage of the sensor is 5VAC max and sine wave is recommended. I will be using 3.3V volts in my project and the sensor will be subjected to, at most, square wave due to the oscillator circuit that I chose. Also, the operating frequency is 100 Hz ~ 10 kHz. If a 74HC14 based oscillator is used, it is difficult to keep the oscillator’s output frequency within 100Hz to 10 kHz but I will be trying to make sure that the output frequency at normal room relative humidity is within the 100Hz to 10 kHz range.

CM-R standard characteristics

So, I built an oscillator using 74HC14 and some passive components (33kΩ precision resistor and some capacitors with different values). I did not use any sensor yet.  My goal was to determine the value of the capacitor that can produce a 1 kHz output frequency using a fixed 33kΩ precision resistor. I chose 33kΩ because it is close to 31kΩ which is the resistance of the sensor at 25°C and 60%RH. After some trial and error (Yes, I decided not to use the formula presented in the previous section of this article), I decided to use 33nF capacitor.

 The type of capacitor to be used is critical. Therefore, ceramic and electrolytic capacitors are not recommended because of poor stability and accuracy. For my implementation, I chose 33nF Mylar capacitor because this was the only capacitor with good precision and acceptable stability that I can get easily. If you have a better capacitor, use it.

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