Relaxation Oscillator

Both the resistive and capacitive humidity sensors may be used in RC relaxation oscillators. If you use a capacitive humidity sensor, you must use a fixed value precision resistor to complete the charge-discharge section of the RC oscillator. If you use a resistive humidity sensor, you must use a very accurate and stable capacitor to complete the RC part of the oscillator.

Relative humidity can be determined by measuring the output frequency of the relaxation oscillator. For the capacitive humidity sensors, capacitance is converted to frequency. For the resistive humidity sensor, resistance is converted to frequency. Don’t worry, I will be providing examples later.

RC relaxation oscillators can be implemented in several ways. I will present you some of the ways.

Using a 555 Timer

The 555 timer IC is a very popular device and it is commonly used in RC relaxation oscillators. Just look inside the datasheet of a 555 timer IC and you will find an example of a RC oscillator.

If you use a capacitive humidity sensor for the above sample circuit, keep both R_A and R_B constant and replace C with the sensor.  If you use a resistive humidity sensor, keep C and R_A constant and replace R_B with the sensor.

To measure the relative humidity using this circuit, just measure the output frequency and compute the resistance (if resistive sensor is used) or capacitance (if capacitive sensor is used) using the formula above. The computed resistance or capacitance can, then, be translated to the equivalent relative humidity measurement. Just consult the sensor’s datasheet to know the relationship of the sensor’s resistance or capacitance to relative humdity.

An alternative RC oscillator implementation using a 555 timer IC and a resistive humidity sensor is discussed in this forum.

Using a Schmitt-trigger Inverting Buffers

Using a Schmitt-trigger inverting buffer is one of the simplest ways to implement a RC oscillator. To implement this, I prefer to use 74HC14. If you look inside the application information section of the 74HC14 datasheet provided by NXP, you will find the circuit and formula shown below:

For the capacitive humidity sensor, replace C with the sensor and keep R constant. For the resistive humidity sensor, replace R with the sensor and keep C constant.

To measure the relative humidity using this circuit, just measure the output frequency and compute the resistance (if resistive sensor is used) or capacitance (if capacitive sensor is used) using the formula above. The computed resistance or capacitance can, then, be translated to the equivalent relative humidity measurement. Just consult the sensor’s datasheet to know the relationship of the sensor’s resistance or capacitance to relative humdity.

Using a Schmitt-trigger inverting buffer as the oscillator requires less components than using the 555 timer IC as the oscillator.

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