Humidity Sensor Design: Figaro Type NH-02
The Figaro Type NH-02 humidity sensor incorporates a capacitive humidity sensor, Zs, and a thermistor, Z, arranged in series on an aluminum substrate. The thermistor’s parameters are carefully chosen to compensate for the temperature-dependence of the sensor’s dielectric, offering nearly complete temperature compensation. Across the temperature range of 15–35 °C, the NH-02 exhibits a temperature dependence of approximately 0.3% K-1.
Energizing the Sensor: Generating Alternating Voltage
Before the sensor can operate, it requires activation by an alternating voltage with a frequency falling within the range of 50–1000 Hz. The circuit achieves this by employing a Wien oscillator based on IC1a, which sets its frequency to around 1 kHz through the R3-R4-C3-C4 bridge. The two diodes within the feedback loop contribute to amplitude stability. The level of the alternating signal, and consequently the maximum output voltage, can be extensively adjusted using P2. This preset is linked to the sensor through buffer IC1b, preventing the sensor’s impedance from influencing the set voltage. The output voltage from the sensor undergoes buffering by IC1 and is subsequently directed to the peak rectifier IC1d-D3-D4. The rectifier’s output voltage undergoes smoothing with R10-C6. After which, the direct voltage undergoes amplification by IC2b (a = 1+R12/R11).
Non-linear Characteristics of Relative Humidity vs. Output Voltage
The graph illustrates that the relationship between relative humidity and output voltage is non-linear. While this non-linearity is inconsequential for basic on-off switching applications, it may be imperative to achieve linearity for other applications. The most effective approach to achieving linearity is through the utilization of a computer and appropriate software.
Power Supply Configuration and Symmetry Adjustment
The power supply for the circuit can take the form of an asymmetrically stabilized 8–15 V type. The opamp plays a crucial role by generating an artificial earth at half the supply voltage, resulting in a circuit powered by a symmetric 4-7.5 V supply. It’s important to note that the supply voltage must exceed the desired output voltage. Aligning the circuit is most effectively accomplished with the assistance of an oscilloscope. Using P1, the amplitude of the oscillator can be set to a peak value of 2 V (measured between pin 1 of IC1 and earth). Subsequently, with P2, the desired peak value of the supply to the sensor can be adjusted. In many instances, a peak value of 1 V proves to be suitable, resulting in an output voltage variation between 0 V and 5 V.