Using Analog Humidity Sensors

I already have the 74HC14 and a fixed value capacitor (33nF) for my circuit but I still need to know if the oscillator that I will be using will be operating within the recommended 100Hz to 10 kHz frequency range. I am assuming that I will be operating my circuit within 50%RH to 70%RH most of the time. Therefore, I will be maximizing my design within that relative humidity range. Please note that I made many assumptions here.

At 25°C (based from the sensor’s datasheet), the sensor’s resistance is from 87kΩ down to 13kΩ if subjected to 50%RH to 70%RH. I gathered some precision resistors with resistances from 1kΩ to 100kΩ. I made another oscillator circuit (using 74HC14, 33nF capacitor, and some precision resistors) and I measured the output frequency of each resistor and I got the following results. Again, I did not use the formula given in the previous section because I am going to derive my own.

Here is the data from my measurement.

The formula of the frequency of the 74HC14 RC oscillator given in the previous section is:

Formula 1

Where:

f=frequency

R=resistance

C=capacitance

But I will be deriving my own formula. So, I replaced 0.8 with k (k is an unknown value) because I want to determine the constant factor in my implementation. Therefore, my formula is now:

Where:

k= unknown constant

Rearranging my formula, I get:

Formula 3

Using formula 3 to get k. From my data that I got previously, I get the following:

The average k is 0.921.

.

Therefore (substitute k or k_ave with 0.921), the formula to determine the output frequency of my circuit is:

Replacing R with 13kΩ (sensor’s resistance at 50%RH) and 87kΩ (sensor’s resistance at 70%RH)  and C with 33nF:

Equation 1

Remember that 13kΩ is the resistance of CM-R at 50%RH and 87kΩ is the resistance of the sensor at 50%RH. Also, remember that I am assuming that my circuit will operate within 50%RH to 70%RH most of the time. At 50%RH, the output frequency is 2.53kHz and at 70%RH, the output frequency is 378.19kHz. Therefore, the operating frequency of my circuit at normal room temperature is within the recommended operating frequency range (100 Hz ~ 10 kHz). Also, there is still room if my circuit operates beyond the target 50%RH to 70%RH and outside the normal room temperature.

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